Menu
What's new
By:
Advanced search...

What speed would hypothetically yield the highest MPG ?

J

Jamie91

Members
Messages
153
Reaction score
1
Location
Slough
Car
2005 iCDTI Saloon
Without wanting to get side tracked into a debate about going too slow on a motorway.

On a flat road at what constant speed would the iCDTI engine yield the highest fuel economy ?

My educated guess is that 50 mph (1500 rpm in 5th) would be the answer as any slower and the engine struggles in 5th.


(I get that pumping tyres, anticipating road conditions ahead, not carrying heavy objects in the boot, not using air con etc etc will help with fuel economy, but I'm just interested in speed)
 
56mph is the accepted conventional rule within the motoring industry. I found from my own experiments (spending a lot of time on motorways lol) that on the 5 speed ICTDI, the best speed/mpg compromise was around 1900RPM which gave you a decent enough usable speed for the slow lane at around 65mph and decent economy.
 
i do Ten miles to work on rural A road gives me UPTO 60mpg doing 45mph in 6th gear, the stretch is mainly flat, on the other hand I get 45mpg doing 70 80, when im running late
 
F6HAD said:
56mph is the accepted conventional rule within the motoring industry. I found from my own experiments (spending a lot of time on motorways lol) that on the 5 speed ICTDI, the best speed/mpg compromise was around 1900RPM which gave you a decent enough usable speed for the slow lane at around 65mph and decent economy.
Click to expand...
Surely a blanket speed of 56 mph won't apply to all cars though? 56 mph in my brothers 800cc Daewoo Matiz isn't all that economical for example as it's still working moderately hard at that speed, compared with say 40 mph where it sits happily in top gear. Whereas a Veyron wouldn't be able to use top gear until doing 100+ I imagine.

Interesting experiment. I usually sit at 60 in the slow lane if I'm not running late and have plenty of time, will try 65 mph and see if that boosts my economy.
 
diesels and petrols will have completely different economy figures which is probably obvious, apart from the engines the rpm figures etc are totally different.

60mph in my car is mundane to say the least but i would expect well over 400miles on 60/65litres for that, i would say a steady cruise control 65mph is still pretty economical for a petrol.
 
toffee_pie said:
diesels and petrols will have completely different economy figures which is probably obvious, apart from the engines the rpm figures etc are totally different.

60mph in my car is mundane to say the least but i would expect well over 400miles on 60/65litres for that, i would say a steady cruise control 65mph is still pretty economical for a petrol.
Click to expand...
I drove from London to S/W Scotland the other week covering almost all the motorway miles at 80 with Cruise Control on. I still managed 425Miles before bottling it and getting fuel (didn't know how far away the next garage was) and even then the warning light wasn't on.

At 60-65, I'd disappointed not to get 450+ from a 2.4 petrol and would probably say that was the best speed. If you stay south of 3000rpm it surely must be sipping fuel?
 
i would find it hard to keep at 65mph, its normally 70 (or more :p) like yourself its over 400miles when the light is on, 450miles is a good return.
 
I am currently regularly travelling both on the A1 and across to Norwich which involves lots of single carriageway at usualy 40-60mph depending on tractors/lorries etc. When sitting doing 40-60mph I can regularly get 60+mpg indicated on the trip computer but this drops once I get on the motorway. An overall average of 600-700 miles out of a tank is quite easy though.
 
With a diesel it's bad news to let it slog, your "educated" right foot is the answer, feel it run easy then thats best for it, As you say 50mph in 5 it is not a happy bunny.
 
edgeoftime said:
With a diesel it's bad news to let it slog, your "educated" right foot is the answer, feel it run easy then thats best for it, As you say 50mph in 5 it is not a happy bunny.
Click to expand...
Maybe it's because mine is remapped but on flat ground mine doesn't make any grumbling noises if I put the cruise control on at 50 in 5th. It will make grumbling noises if I cruise at 45 mph in 5th.
 
You reckon? I've gotten 450 from my 2.4 using lots of cruise control. I guess it depends on how you use it.
 
I've been told by a lot of people it does.

I would tend to agree as the CC really does seem to 'floor it' when it comes to hills or when you resume it

Ive never been bothered and use it all the time lol
 
Jamie91 said:
Maybe it's because mine is remapped but on flat ground mine doesn't make any grumbling noises if I put the cruise control on at 50 in 5th. It will make grumbling noises if I cruise at 45 mph in 5th.
Click to expand...
drive it as you will, but keep us informed when the bottom drops out of it!!
 
Cruise control accelerated up hill and lifts off down hill, oposite to eco driving.

56mph is the fastest you can go before you get hit with a expodentially increasing curve of drag. (Basically wind becomes a issue from that point onwards)
 
You must log in or register to reply here.
Top