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The 18" Wheel Debate

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JensenA

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:p Please don't close this topic Mod's - and please everyone no arguing :) I'm not trying to prove anything, start an argument, slag people off etc etc.........just a simple discussion on why a change to 18" should cause a drop in mpg.

Right then.....Today I drove into town with the 18" wheels on the car. Mostly m'way and the rest town traffic. Cruise control on the motorway, set at dead on 70mph (8.4 miles of the journey). Now I now it's only 25miles, but this reslut bears out exactly what I discovered over a previous 160 mile journey - a drop of about 2-3 mpg.

18" Honda Altimo wheels.
Falken ZE912 225/40/18 tyres at 38psi all round.
Overall Diameter 637mm
Weight of wheels and Tyres 22.4kg

16" Honda wheels.
Michelin Primacy 205/55x16 tyres at 33psi front, 30psi rear.
Overal Diameter 632mm
Weight of Wheels and tyres 17.2kg

Factors that are different;

DIfferent make of tyres.
Rolling radius up from 1985mm to 2002mm so the speedo reading is out by (only) 0.79%
18" wheels & Tyres are 5.2kg heavier



With 18" wheels:

18inch.jpg


I got back home, changed the wheels back to the original 16" ones, and repeated the journey exactly, driving exactly the same way (Miss Daisy mode)

With 16" Wheels

16inch.jpg


So anyone who is better than maths at me, please reassure me that I have not decreased my mpg. Personally I reclon the heavier wheels may well be the cause. 5kg heavier each corner. Thats like me strapping on 5 bags of sugar to each wheel, that has to make a difference, shirley :)

Now behave :D
 
Interesting to read.
Unrelated question to kick things off ;)
How is your speedometer surround blue and mine red ish? Prefer the blue tbh
 
Hamid92 said:
Interesting to read.
Unrelated question to kick things off ;)
How is your speedometer surround blue and mine red ish? Prefer the blue tbh
Click to expand...

I thought they were all like that :) Mine's the facelift model.
 
Ok Alan i will not close this thread as long as it keeps on topic and all posts are informative.
The other thread was just getting out of hand imo.
My input on this from experience of driving a 7th gen and now 8th gen is that you will lose a slight MPG if your driving on 18" wheels.
When Fahad had 19" wheels on his old accord he also noticed slight loss in mpg as well.This might only be the trip computer though.
With upgraded wheels other than stock there are slight differences in diameter but within spec i be leave.Hence a smaller wheel will have less mpg than a larger wheel i agree.
Im not an expert in this so i am only going on my experiences.
To get clarification on this i would advise you to sign up to http://www.wheels-inmotion.co.uk/forum/index.php?act=idx as these guys no almost everything on this subject.
Also something to bear in mind is to get a true reading of mpg always calculate this rather than going but the trip computer.
 
And regarding your post correct me anyone else if I'm wrong but I think it's due to drag and unsprung weight! Car will be heavier thus using more fuel to move (not much but it all counts) especially from a standstill. And bigger and wider tyres will cause more drag so the car will ultimately use more fuel to counteract the extra 'friction'! My six pence worth anyways sounds about right ;)
 
18" Wheels weigh alot more than 16", I've just swapped mine back.

A better test would be too work it out from brimming your tank ( upto filler neck) travelling to exactly the same point A>B (not mileage based) using your revs as a indicator of speed then working out mpg. The avg mpg readout on the trip is not very accurate.
 
Salim said:
18" Wheels weigh alot more than 16", I've just swapped mine back.

A better test would be too work it out from brimming your tank ( upto filler neck) travelling to exactly the same point A>B (not mileage based) using your revs as a indicator of speed then working out mpg. The avg mpg readout on the trip is not very accurate.
Click to expand...


Yeah Salim- I'm thinking of swapping mine back too. The whole point of a diesel is economy! They do look pretty though :)
 
Red dials are found in sport models, executive pre-facelift got them orange-brownish colour, facelift got blue. If I am not wrong that is :)
 
IMO the weight difference is about as trivial as the difference in rolling radius, as 20kg extra on a, say, 1500kg (loaded) vehicle is only just over 1% (so much less than the difference between an empty and full fuel tank). But these two factors will obviously combine to form a larger factor and then, if you also add a bit for increased friction from the wider tyres, you're starting to explain the difference in MPG... however, calculated MPG figures over these relatively short distances will be inaccurate (to several %, I'm guessing)... your 'trials' will also be affected by differences in head/tail winds as well as air pressure and temperature (again to several %). But I understand your interest in resolving this issue.

Come on Freddofrog... where are you???
 
Mine was a tad more thirsty with18s I put this down to the excess weight and you could actually feel the weight through the steering wheel. The tyres where the same 225 width but we're a 40 profile instead of a 45 profile on my old 17s. But the mpg wasn't that much worse to be honest.
 
Jon_G said:
IMO the weight difference is about as trivial as the difference in rolling radius, as 20kg extra on a, say, 1500kg (loaded) vehicle is only just over 1% (so much less than the difference between an empty and full fuel tank). But these two factors will obviously combine to form a larger factor and then, if you also add a bit for increased friction from the wider tyres, you're starting to explain the difference in MPG... however, calculated MPG figures over these relatively short distances will be inaccurate (to several %, I'm guessing)... your 'trials' will also be affected by differences in head/tail winds as well as air pressure and temperature (again to several %). But I understand your interest in resolving this issue.

Come on Freddofrog... where are you???
Click to expand...
I'm here Jon, I decided to take a break for a bit. I agree with what you say, totally.

Rolling resistance: it's basically the flexing of the tyre walls, and the action of the rubber compound on the surface in question. If you changed a "light" wheel to a "heavier" wheel of the same size, and put the same tyres onto the "heavier" wheel, the effect on rolling resistance would be the same as keeping the tyres on the "light" wheel and adding a bit of weight to the rear seats of the car. Stevearcade said this in the previous thread. Say you add a couple of young kids, one to each rear seat, you're not adding much to the 1500 kg weight of the car, so you'd hardly expect much change in mpg.

Acceleration: likewise for change in momentum, the extra weight of a couple of young kids, relative to the 1500 kg weight of the car, will hardly affect the engine.

Tyre width: this is also part of rolling resistance. You will find some people who say extra width affects rolling resisitance, some who say it won't. I can't see how extra width affects rolling resistance to such an extent that you'd notice a change in mpg.

Wall profile: this can obviously affect the rolling resistance. But one can argue that a lower profile has a stiffer wall, so it should have less rolling resistance. But I searched on the internet and saw some people say the opposite. This would be a good one for Mythbusters.

Tyre pressure: this has a massive effect on rolling resistance. Full stop. So changing a wheel size and a tyre profile and tyre compound, in combination you need to think about the relative tyre pressure from previous to replacement.

Tyre compound: as with tyre pressure, has a big effect. Changing a wheel size and a tyre profile and tyre compound, in combination you need to think about the difference in tyre compound. Ask the manufacturers for some details as to the energy efficiency of the tyre compound, this will be coming into effect soon with a green through to red rating for each make and typre of tyre. There was a thread somewhere on this forum about this new proposed rating system.

Finally ....one thing that was alluded to in the previous thread ....cornering and slip angle. If your new bigger wheel with lower profile tyres grips more, then you are likely to go faster round corners without realising. Faster cornering = hotter tyres. Hotter tyre means loss of energy. So although one is going round a corner faster and hence maintaing momentum, it's arguable that the result is actually a nett loss of energy. So on a twisty bumpy country lane, I would expect the a low profile tyre, given that it can grip better, will provoke a driver into less conservative driving style. Maybe another one for Mythbusters.

Overall the only way to compare would be to:
1. make sure that tyre pressures on each set of wheels are set properly
2. drive for several weeks on each set to get a long comparison
3. hope that in those periods the weather was similar
4. have some onboard instrumentation that could be used to prove no sub-conscious change in driving style

I hope I haven't left anything out this time, sorry if it reads like a lecture, it's the sort of email I send to my lads sometimes about certain lifestyle things I'm aware of :lol:


EDIT: I forgot ..tyre compound ... added it above
 
Weight increase is pap. You can fill the boot with a weekly food shop or have a passenger fairly often right? Because that's the kind of weight difference we're talking about, surely. Too insignificant to make a difference.

Do you have the correct size tyre (as recommended by Honda - no one else!!!)? If the tyre wall is too tall or too shallow, then your MPG will be affected.

Are your tyre pressures too low? Should probably be high 30s PSI for 18-19 inch rims. If they're low, they can seriously reduce MPG.

When did you last have a service? Could it be that coincidentally your MPG has dropped due to something unrelated to the wheel size? For example, a filter, fluid, glow plug needing replacing.

Is it significantly colder and/or wetter outside since you changed the wheels? If so (and I reckon it is), the air is denser and your MPG will drop significantly.

Are you driving in the rain? If so (and you probably have been given the last couple of weeks mad rain), the water on the road surface will affect your MPG again.

Has it been windy outside? Driving in high winds makes a difference.

Some tyre brands/designs are significantly better/worse than others for MPG. Could it be that the tyres currently on your new rims are very inefficient?

Is it probably a bit of all of the above? In my opinion yes.

If you're seriously concerned, get some very economic tyres on your rims, fill them to the correct PSI, get the car fully serviced by Honda and in truth, reserve judgement until the summer when the weather improves.

I think changing back to your old rims would be rash. I don't accept that simply the change in rim size will affect MPG (***uming you have the correct tyre size - as mentioned above).

My last two tanks of fuel have delivered me a drop of about 50 miles over the tank. It's quite a drop, but I simply attribute this solely to the worsening weather conditions I've been driving in. High winds, loads of rain, colder mornings, it all adds up mate. I'd hold fire on making any decisions just yet ;) .
 
Jon, I can't think what else explains it. I know an additional 20kg on a 1500kg car shouldn't, in theory, make any difference. But if I went and strapped a 5kg lead weight to each of Sebastian Vettels wheels, I don't think he'd be too happy. I'm not an expert on physics, but the engine has to turn the wheels to propel the car, and an additional 5kg on each wheel just has to make more of a difference than 20kg lump of lead in the boot would.

I'm pretty happy to conclude that it's the extra weight of the wheels that cause the drop in mpg. That seems to be the experience of those who have changed to 18".

I'm beginning to think that 17" wheels would make a good compromise.

You asked where Freddofrog is, he's reading all this and either writing a 20 page dissertation, or more likely, having a good laugh at us (aren't you Brian :D )
 
Looks like our posts crossed Steve. Same statements mate, different writing style. I actually pefer your writing style, I've often been accused of being a lecturer or a barrister .... I wish in either case :lol:
 
Nala Nesnej said:
Jon, I can't think what else explains it. I know an additional 20kg on a 1500kg car shouldn't, in theory, make any difference. But if I went and strapped a 5kg lead weight to each of Sebastian Vettels wheels, I don't think he'd be too happy. I'm not an expert on physics, but the engine has to turn the wheels to propel the car, and an additional 5kg on each wheel just has to make more of a difference than 20kg lump of lead in the boot would.

I'm pretty happy to conclude that it's the extra weight of the wheels that cause the drop in mpg. That seems to be the experience of those who have changed to 18".

I'm beginning to think that 17" wheels would make a good compromise.

You asked where Freddofrog is, he's reading all this and either writing a 20 page dissertation, or more likely, having a good laugh at us (aren't you Brian :D )
Click to expand...
No mate, I've written the dissertation already, see 10:03 :lol:

As for Vettel's car ... I think his car weighs 500kg , and , the suspension and slip angles are radically different. You might as well talk about the difference between the speed your car reaches and the speed Voyager 1 is now travelling at .... no comparison :lol:
 
:unsure: Brian's already posted above Dude. Quite a lengthy one at that (he posted while I was typing - I think we're kind of getting at the same thing)

But, as I mentioned in the other thread. Formula 1, the Accord is not... Yes rotational mass will amount to more than static weight, but I find it hard to believe it will knock out the MPG to that amount. I'm sticking with my above points ;) .

Edit: :lol: Brian, not only are we thinking alike, but cross posting all the time... Textbook!
 
Stevearcade said:
:unsure: Brian's already posted above Dude. Quite a lengthy one at that (he posted while I was typing - I think we're kind of getting at the same thing)

But, as I mentioned in the other thread. Formula 1, the Accord is not... Yes rotational mass will amount to more than static weight, but I find it hard to believe it will knock out the MPG to that amount. I'm sticking with my above points ;) .

Edit: :lol: Brian, not only are we thinking alike, but cross posting all the time... Textbook!
Click to expand...
:lol:
I'm bailing out of this now, before I end up feeling like this again .......
banghead.gif


Goodnight folks :D
 
I'm staying outta this one. :lol:
 
If I fit roofbars do you think that might affect my MPG?
 
Most definitely. My dad had roof rails on his old Volvo for a while and it significantly impacted on MPG. Probably in a similar vein to running too low tyre pressure.

I mean the Accord rails are fairly low, sleek and inline with the body, so might not be so bad. But it definitely will make a difference as it increases wind resistance.
 
freddofrog said:
No mate, I've written the dissertation already, see 10:03 :lol:

As for Vettel's car ... I think his car weighs 500kg , and , the suspension and slip angles are radically different. You might as well talk about the difference between the speed your car reaches and the speed Voyager 1 is now travelling at .... no comparison :lol:
Click to expand...

You dont say? Well I never!! I really thought the Accord was like a F1 car?! God I hope everyone else who read it takes it as a tongue in cheek jokey type remark, or else I'll look really silly :rolleyes:


So if there's anyone else considering upgrading to 18" and is bothered about mpg, or just interested in things like this - read through the thread and form your own opinions, but in my experience, expect a little drop in mpg, but a big improvement in Looks.

Love and Peace everyone B)



Probably time to close this thread now Mods.
 
Nala Nesnej said:
You dont say? Well I never!! I really thought the Accord was like a F1 car?! God I hope everyone else who read it takes it as a tongue in cheek jokey type remark, or else I'll look really silly :rolleyes:


So if there's anyone else considering upgrading to 18" and is bothered about mpg, or just interested in things like this - read through the thread and form your own opinions, but in my experience, expect a little drop in mpg, but a big improvement in Looks.
Click to expand...
Dude, it's odd how you say the above, yet your opening senetence was
"Please don't close this topic Mod's - and please everyone no arguing I'm not trying to prove anything, start an argument, slag people off etc etc.........just a simple discussion on why a change to 18" should cause a drop in mpg"

If you want to introduce an engineering comparison to an F1 car, why turn around now and make the above sarcasm. It's classic verbal gamesmanship dude, not part of a "simple discussion".

The main point about an F1 car, is that the tyres on your/our cars would literally not even last half a lap. The point that you made about adding weight to the wheels of an F1 car is a valid point, but they are truly different animals. It would be like comparing a Cheetah to a Buffalo, the latter has heavier bones. Likewise the suspension on an Accord is much more heavily constructed, so adding weight to wheels on an Accord is same as adding weight in the car's cabin. On an F1 car, adding weight to the wheels is not the same as carrying lead weights in the ****pit. As previously discussed, the extra weight onto the Accord's wheels is like adding a bit of weight in the cabin instead. In a straight line, it makes no difference. For fast cornering it will make a difference, but the change of the tyres profile has the bigger effect in cornering.

To end the discussion more cleanly, let's look at the following scenario.

Two identical Honda Accord Tourers, both with 16" wheels. Let's call them car A and car B. Car B is fitted with a system that will keep it exactly 2 seconds behind car A. The owner of car A is told to go about his usual business, and the driver of car B is told to follow car A for 4 weeks, with the system controlling the engine of car B. We would expect an identical mpg for both cars. Now car B has 20 kg added to the rear seats, and car B continues to follow car A in the same way. We would expect a microscopic reduction in mpg of car B, virtually unmeasurable. Now car B has the 20kg weight removed, and the 18" wheels put on car B, and car B continues to follow car A in the same way. Any change in mpg will be down to any or a combination of
1. difference in the tyre pressures (which could be "tuned" out)
2. difference in the tyre compounds (which cannot be "tuned" out)
3. difference in the tyre profile (but unless the driver of car A was ragging his car round corners, I would not expect this to make a difference)
 
Just to add my 2 cents, I put 18's on mine (previously 17's 2.0 petrol) and saw a relatively small drop in mpg, btw I thought the display wasn't able to show actual mpg (though mine petrol)!!

In relation to the 16's v 18's, I'd also factor in ride hight as adding to the wind resistance if this wasn't already covered, lower cars have lower wind resistance underneath, therefore better drag co-efficiency, I'd be interested to know if your car was lower would it improve mpg on the 18's :unsure: And also 18's and lowered improves the looks even more :D
 
Guys stick to the topic or I will close this thread. Not here to argue amongst each other. If you have something constructive to say then do so otherwise we will take action. We are adults, do try to act like this.

I like this thread but its going off topic now.
 
His ride height has hardly changed, because there is hardly a change in radius to outer tyre edge.

Originally in the other thread, I thought that change in overall wheel radius was a contributing factor , because specs of the pre and post tyres was not posted.

As far as I know, I think that a 2.5% change of radius due to change of wheel and/or tyre spec is allowed. Originally I suspected an increase in radius, which results in increase in cirumference. If the radius/diameter/cirumference changes by N%, then this will result in a perceived change in mpg in the opposite direction i.e. increase of cicrumference by N% gives perceived reduction in mpg of N%.
 
Brian - If you honestly think I was making a genuine comparism between my car and a Fromula 1 car, then God help you mate.

I KNOW my car, and I KNOW that the mpg has dropped - I'm not wingeing or complaining, I'm stating a FACT. All you can do is spout off about the fact that it's impossible for the drop in mpg to be due to fitting bigger wheels. Well, I'm sorry mate you are wrong. I fitted 18" wheels and immediately I notice a drop in mpg. And all I get is people asking - has your car been serviced? Was the weather different? Were you driving differently? You refuse to believe that my mpg has dropped, you tell me my guage isn't accurate, tell me weather conditions are differeent, tell me that 20kg weight in a car will have absolutely no effect at all. Well you are wrong, sorry mate, my mpg has dropped - period!! A 20kg weight in the boot might not make any difference to the car. Add 20kg to the flywheel and you will notice a difference. Add 5kg od weight to each piston and you will notice a difference, add 10kg of weight to each drive shaft, and there will be less of an effect due to the way the forces are applied to the long narrow drive shaft - so 20kg of weight can make a difference..

I have fitted 18" wheels, they are 5kg heavier than the old ones, and suddenly the mpg drops. Search on the internet, for something like "fitted 18" wheels and my mpg has dropped" and you will find endless posts exactly the same as mine. The same view is echoed by owners on here, put bigger wheels on and the mpg dropped - so stop telling me I am wrong, stop telling me I am imagining it - I know what has happened.

My last mpg average over 1603 miles was 47.9 - thats exactly spot on the figure quoted in Notoring magazines etc. I've reset that to zero, in 1603 miles I'll put a post on to let you know what the new figure is. And you know what, you will still insist on making out I'm an idiot who doesn't know what he is talking about.
 
Alan, I wasn't going to reply (see . above) but I thought I'd give it another go.

Adding weight to pistons or flywheel or driveshafts would similarly not affect mpg.

Pistons: reason to keep piston weight down, is so as to be able to rev the engine higher, which gives more power (power is a function of rpm and torque). Limiting factors to revving an engine higher, is valve bounce and piston mass. That's why diesels don't rev as high as petrols, because diesel pistons are heavier. But in normal rev ranges of driving, piston mass (***uming all else it the same) will not affect mpg any more than adding weight in the cabin.

Flywheels: reason to keep flywheel weight down, is so as to make the engine more "responsive". This means that, when changing gear, you want the engine revs to change to the new rpm as easily as possible without stressing engine or gearbox. Hence you don't want heavy flywheels. In normal rev ranges of driving, flywheel mass (***uming all else it the same) will not affect mpg any more than adding weight in the cabin.

Driveshafts: as with everything else, why add weight where it's not required. Also easier to balance a lighter rotating mass than a heavy rotating mass. Driveshaft mass will not affect mpg any more than adding weight in the cabin.

As to other forums, yes what you say about other forums is true, seen it myself. But forums are not a reliable source of engineering fact.

I'm sorry if your new wheels are giving you poor mpg, but IMO, if you want to correct this, get some energy tyres for them, though that will cost more than the cost of the wheels. Not sure what I or anyone else can say here.
 
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