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What MPG are you getting at a steady 80 MPH?

ship69

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'08 i-DTEC EX man 5D
Hi

What MPG are you getting at 80 MPH on the dashboard? (i.e. probably a real, what 75MPH? according to the Sat-nav)

To be specific:
- If you're on a long journey - doing a journey of say 40+ minutes and you're on the motorway, and
- after say 5 or 10 minutes and you engine has warmed up nicely,
- there is no significant wind,
- the road is broadly flat,
- it's this time of year (March) in the UK (5-10 DegC)
- you're using premium quality fuel (e.g. BP Ultimate diesel).
- your driving a manual 2.2L i-DTEC
- you have no extra weight in the car
- you're car has recently been serviced
- you're using normal tyres (I ***ume the pressures where checked)

What MPG would you expect to get on the Honda's built in computer if you set the cruise control to exactly 80MPH? I'm getting about c.41-43 MPG in my 2.2L i-DTEC. Whereas I swear my i-CDTI got 50-53MPG! I've only had the car a few days so I havent managed to see how far a tank get.s
Do I have a problem or is this normal?

J
 
That's about right, anything over 70mph and you get a big drop off in mpg. Optimum for most cars is about 65mph for best mpg. I tend to stick to 70mph max these days and notice a lot more people tending to do this too in the last few years. For me it's the saving in fuel and not wanting to waste money on tickets and points, loads of unmarked police cars on the roads these days too.
 
Yep your better off sticking to the speed limits at 70 mph to get the best performance,
 
On any vehicle driving on a flat road in no wind, 80mph requires 49.27% more power than at 70mph.
 
my old 202K CDTI used to average 45-47 at that sport of speed, week in week out on normal esso diesel
 
On any vehicle driving on a flat road in no wind, 80mph requires 49.27% more power than at 70mph.

That's a very precise number ;)
Out of interest where did it come from?

J
 
Okay so it looks like I'm going to have to re-run my testing at 70MPH...
 
That's a very precise number ;)
Out of interest where did it come from?

J
the power required to overcome drag is a function of the cube of the velocity

so 140mph requires 8 times the power to overcome drag than it did at 70mph

and the difference between 80 and 70 = (80/70)[sup]3[/sup]

this doesn't mean you would use 49% more fuel, just that the car needs 49% more power to push its way through the air
 
Interesting, freddofrog. For whatever reason I certainly dont think my previous car used an extra 49% of fuel going from 70 to 80MPH. Over about a year of regular experimentation I can tell you that at 80MPH on a long journey it would use about 50MPG (+/-2MPG) and at 70MPH it would use close to 60MPG (+/-2MPG) - but that's accord to the car's computer. i.e. Only and extra 20% if true.

Update.
At an exact, steady 70MPH over 30+ miles this evening I got only 51MPG on the car's computer on a night without very much wind. Like I say my previous car the 2.2L iCTDI used to do the same journey in 58-60+ MPG on the computer. The computer may of course be wrong but I notice that the official Extra Urban was 58MPG for the iCTDI and 61.4MPG for my current car. i.e. 10MPG more than I'm currently getting. :^(

J
 
unless i want to end up in a maximum security prison i got no choice but to stick to 70

any thought of me getting anything bigger than a 2.4 now are just a distant memory, like the nebula that's 2.6 billion billion light years to your right above the sky now...

rosette_lula_1700.jpg
 
Real life experience has taught me that going over 69mph (on my phone's gps) cripples fuel economy.

On the Accord - 6th Gear cruising should be done sub 1900 revs.

In my mapped Tranny - 2400 revs.

However much funnage can be had on private roads if you know what your steed can do. Redlining in 4th is certainly not big or clever.
 
Interesting, freddofrog. For whatever reason I certainly dont think my previous car used an extra 49% of fuel going from 70 to 80MPH. Over about a year of regular experimentation I can tell you that at 80MPH on a long journey it would use about 50MPG (+/-2MPG) and at 70MPH it would use close to 60MPG (+/-2MPG) - but that's accord to the car's computer. i.e. Only and extra 20% if true.

Update.
At an exact, steady 70MPH over 30+ miles this evening I got only 51MPG on the car's computer on a night without very much wind. Like I say my previous car the 2.2L iCTDI used to do the same journey in 58-60+ MPG on the computer. The computer may of course be wrong but I notice that the official Extra Urban was 58MPG for the iCTDI and 61.4MPG for my current car. i.e. 10MPG more than I'm currently getting. :^(

J
The reason why the increase in going from 70mph to 80mph is an increase in 49% of power but not fuel, is due to both BSFC and Pumping Losses.

If you had a car that had a maximum speed of 80mph, then you would find that the difference between driving the car at 70mph and driving at 80mph would be close to a difference of 49% in fuel consumption. The reason is that the BSFC would be relatively flat because you would be at maximum throttle at 80mph, and close fo maximum throttle at 70mph. Also, if it was a modern car, then the engine would be small, so the Pumping Losses would be small.

Pumping Losses give you a fixed offset, like a constant fuel leak, which is why you do 0 mpg with the engine running and you're not moving.
So, due to Pumping Losses, your fuel consumption relative to power is not y = mx, but y = mx + c (it's more complex than that when you take throttle opening and BSFC into consideration, but hopefully you get the point).

So if x is the power required, then when no power is required, the fuel consumption y = c.

If you need say 25 HP at 70 mph, then your fuel consumption y = 25m + c

If you then need 49% more power (80nph), your fuel consumption is now y' = 37m + c

So it depends on m and c as to the percent difference between y and y'.

If c is small and m is close to 1, then the difference between y and y' would be close to 49%.

That's a very basic description of the relationship between power and fuel consumption.
EDIT: and a more complex description would mean that m and c both vary as well
 
Basically you need to drive more slowly to get better mpg.

Dont get too obsessed with mpg figures, its not healthy.
 
I have had 2 Accords which were 7th gen diesels and they both overstated the actual mpg by about 5-6%. However my current 8th Gen Tourer consistently understates the actual mpg by around 8-10% based on my initial few fill ups. If this is the case with yours it is possible that the fuel consumption may not actually be that much different between the two.
 
i agree,. i just fill up and dont worry on what mpg i get.

all i know is i get around 250miles ish on a full tank with city driving and 400 plus on long runs
 
Toffee pie, what engine is that your quoting? my old 2.4 8th gen did 350 mixed driving to a tank easy and that was using the odd bit of heavy foot. I think 250 would be concerning unless it was around a race track!
 
my 2.4.

well its probably more than that with the reserve tank and all. this is city driving, short runs each day im talking 5/10 minute trips.

the engine is hardly warmed up each time so hardly surprising.
 
my 2.4.

well its probably more than that with the reserve tank and all. this is city driving, short runs each day im talking 5/10 minute trips.

the engine is hardly warmed up each time so hardly surprising.
seems reasonable to me ;)
 
looks ok to me too, i am surprised it is not losing oil as the engine is hardly warmed up each time.
 
The reserve tank is good for 20/30 miles too remember, but i try not to let it run for any long period of time with the light on, not good practice.

I usually just brim it when its almost out of petrol.
 
Remember that the engine is most efficient at peak torque. Or a peak of torque, not necessarily the biggest, as then tends to be higher up the RPM, which has a richer fueling map.

Look at my dyno graph for my Accord, notice the torque curve,

This is measured at the wheels so includes transmission losses. Firstly ignore the first bit on the left side graph thats the dyno latching on. Notice the lump from 2400rpm-3000rpm That is manufactured in by Honda, and most car marques will try and create the same. The speed that rpm range is in is 50-65mph in 5th gear. So for me to maximize mpg, the speed i need to be doing is 60mph really.

dynograph.jpg
 
Not quite so, an engine is at its most efficient when its Specific Fuel Consumption (SFC aka BSFC) is at its minimum.

This article is excellent at explaining SFC.

SFC generally includes two separate factors to engine efficiency: the thermal efficiency (the amount of heat energy that ends up pushing the piston down); and the pumping losses (the amount of energy lost in breathing, the bigger the engine, the bigger this is).

An electric motor does not have pumping losses, which is why an electric vehicle travelling slowly will always go further on its charge than travelling more quickly. However, for a car with an IC engine, this is not true. There is an optimum speed at which an IC powered car can travel, and this is entirely due to the pumping losses.

Here is a spreadsheet that I have created that gives the mpg for a car. If you download it, you can adjust the constants to see how they affect the mpg. The constants are:

A the fuel kWh per litre (11 for diesel, 10 for petrol)
B the thernal efficiency (this is the efficiency of converting stored fuel to movement of the piston, diesel are higher than petrol)
C the transmission losses in kW (basically gearbox and driveshafts, you'll have to guess this)
D the pumping losses in kW (depends also on whether the engine has a turbo)
E the coefficienct of rolling resistance (see coeffs sheet, depends on tyre type, road surface, and tyre pressure)
F the car weight (kg)
G the acceleration due to gravity (converts kg weight to Newton weight, you can't change this unless you go to the moon)
H the air density (affects the drag, depends on altitude)
K the car width (you can't change this once you've bought the car)
L the car body height (do not include the wheels in the height, you can't change this once you've bought the car)
M the car's coefficient of drag (you can't change this once you've bought the car)

Here is the equation (you lucky people will not find this anywhere else on the internet, trust me, it's correct)

MPG = 10215 x A x B x V ÷ { ( [C + D] x 1000) + (E x F x G x V) + (½ x H x K x L x M x V[sup]3[/sup] ) }

Note that V is the velocity in m/s ....to convert mph to m/s, divide mph by 9 and then multiply by 4 e.g. 9 mph = 4 m/s

If you download the spreadsheet you will find that adjusting the losses has the biggest impact on shifting the optimum speed point, the bigger the losses, the higher the optimum speed.

changing the thermal efficiency and the coeff of rolling resistance has the biggest impct on the peak mpg achievable.

Notes:
1. the equation is mpg versus a steady speed in m/s
2. the equation in effect ***umes an optimum rpm at each car speed
3. this optimum rpm is ***umed to give the chosen thermal efficiency
4. the pumping losses are ***umed to be consistent in that rpm range
5. transmission losses should really vary with speed, just keep them low in the spreadsheet (would be very high at 250 mph)
6. the spreadsheet as it stands has constants set for a 7th gen Honda Accord diesel turbo running on efficient tyres on a good road surface

From this, it becomes obvious why driving around town returns poor mpg, it's not just the stop-start and idling, it's the very fact that you're driving slowly. Hence a hybrid car is better around town, because if it's running on electric with the IC engine off, then the pumping losses are zero (and in fact the thermal efficiency as the electric motor converts stored energy to motive power is high too).


Here is what the spreadsheet looks like

0018_zps033a0259.jpg
 
Never saw thisnpost, but I like it.

I'd take 6mpg if I could have 250mph from the old tractor.
 
I've still got the spreadsheet somewhere. I think I modified it later, to make transmission losses also proportional to speed , which reduced the MPG at higher speeds.
 
Never seen anything above 41mpg, on my cw3 auto estate but who cares?? The 7th icdt manual estate used to get well over 50mpg, but no computer doing the sums.
 
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