4th vs 5th gear mpg'ing with revs

[Phil_beeza]

anyone have any opinions or facts (!?) about this...

say i am doing 50 mph, up a slight hill.

4th gear needs little foot pressure, but slightly higher revs to keep going
5th gear will be lower revs, but i'll be pressing the accelerator more to keep going

anyone know which is meant to be the most efficient?

hard to tell from the display...4th gear may be 45 mpg, 5th gear may be 50mpg

 

[Danny_FR]

Ideally you want the engine rpm to be as close to the peak torque rpm as possible for the best fuel efficiency... That sounded better in my head than it does seeing it black on white lol!

 

[ChrisUK]

Whack it in 3rd & boot it - should get you up the hill quicker

 

[beefy]

i just always thought the lower the revs, the less the engine was working and therefore the less diesel it had to burn to keep the engine going. If it is turning 1500 times in a minute would you not need to burn less as opposed to 2000 times in a minute....

I feel as if when im driving at 2000ish rpm my mpg always seems to drop as opposed to 1500ish and my peak torque level is 1900, i hope/think.

 

[muddyboots]

i just always thought the lower the revs, the less the engine was working and therefore the less diesel it had to burn to keep the engine going. If it is turning 1500 times in a minute would you not need to burn less as opposed to 2000 times in a minute....

That would be true if it only ever injected the same amount of fuel per revolution.

But it doesn't

 

[Phil_beeza]

i just always thought the lower the revs, the less the engine was working and therefore the less diesel it had to burn to keep the engine going. If it is turning 1500 times in a minute would you not need to burn less as opposed to 2000 times in a minute....

aye..but if your foot is closer to the floor in 5th gear to keep going up the hill, you're trying to inject more fuel per squirt (? ), even at lower revs..?

 

[mgrays]

Full throttle best fuel efficency is roughly peak torque... but part throttle fuel efficency is tricky
... and then there are higher frictional losses at higher rpm too.

Really have to trial and error it but 1700-2000 rpm seems a sweet spot.

I have never seen the 3D load map of fuel volume/torque/rpm for any engine, let alone an electronically injected turbo diesel.. they are normally full throttle 2D ones in kw/cc or something with a nice peak at peak torque.

 

[Phil_beeza]

reason i ask is because my journey to work is Junc 6/7 of the m3, which basically consists of a long shallow u-shape, could result in a bit of real difference, perhaps

 

[rice-c87]

i only have a little leon tdi 90 and i cant get above 43 mpg i dont have a trip thingy (base modle booo hooo) i mainly drive about on A roads and some short dule carrageways less then 5 miles i am not bothered beacuse my last car did about 30 every where crappy petrol

 

[Triple D]

Whack it in 3rd & boot it - should get you up the hill quicker

Failing that, brick on the accelerator and jump out to push