Best mpg lower gear at peak torque or change up?

[DanCat]

Ok, not that interesting but what gives best mpg 3rd gear at peak torque (ibiza with 110bhp 1.9tdi) 1900 rpm or change to 4th and possibly strain engine at about 1550-1600 rpm.

Assuming road is flat.

thanks

[morgrp]

A copper once told me lower gear and less engine load is better for economy (no it wasn't on a speed awareness course!)
As to the truth in this I don't know and I don't much care as my motor is an automatic

[willmagrath]

I've got a fabia vrs (1.9 tdi 130hp) and I always go for 4th gear at 30mph, don't know if yours is a six speeder but mine seems to sound too revvy in third, again at around 1900rpm. And it doesn't laboured in 3rd either, although obviously if you wanna accelerate at any more than a snales pace then change down a cog....or two

Edited by willmagrath on Thursday 21st November 22:04

[davepoth]

Best fuel economy in a turbo diesel is to change up before the boost threshold; the engine simply can't take much fuel in without the turbo, and besides that BMEP is highest at low revs.

[anonymous-user]

morgrp said:

A copper once told me lower gear and less engine load is better for economy (no it wasn't on a speed awareness course!)

Just another good reason not to believe everything Coppers say! ;-)

[ClassicMercs]

MrsMercs currently has a Jazz iShift - auto with flappy padels. She uses it in auto but I use the flappy padels to change up earlier. I always get better mpg doing these earlier manual changes.
The gearbox is set up to provide torque on more occasions for those solely allowing the car to do the thinking.

However - totally the other way on - old Mercedes auto's with switchable economy/sport mode always seem to produce better mpg in the sport mode, holding the gears and changing later.

[lightthefuse]

davepoth said:

Best fuel economy in a turbo diesel is to change up before the boost threshold; the engine simply can't take much fuel in without the turbo, and besides that BMEP is highest at low revs.

Agree 100% and borne out of experience with my 9-5 TiD.


The newer style of L-driving encourages you to change up as early as possible, I find in the 208 that taking it up to around 1700-1900 RPM then changing up is the most fuel efficient if not the most friendly to the components. I'd never let the clutch 'chug-chug-chug' though for more than a second or two before changing down mind and curse myself if I ever change up too early. Most modern manuals though are EXTREMELY tolerant through the gears though compared to cars of older vintages ('90s and earlier).

[Ozzie Osmond]

Petrol car is similarly most efficient if you keep the turbo at idle. But the mystery to me is why buy a powerful car and then drive it for economy? Might as well buy an economy car and save a serious wedge!

[veevee]

Difference is negligible IMO. Peak torque might not be ideal, but somewhere approaching the bottom of that range seems like the best compromise between driveability and 'economy'.

[chrisw666]

Ozzie Osmond said:

Petrol car is similarly most efficient if you keep the turbo at idle. But the mystery to me is why buy a powerful car and then drive it for economy? Might as well buy an economy car and save a serious wedge!

Because often the nicer cars are fitted with the less Eco engines and if you only have one car and want a big one why not save a few £ when you're not in a hurry.

[moreflaps]

Diesels should not be allowed to lug. That black stuff from an overloaded diesel (-DPF) is unburnt fuel -not efficient.

HTH