Most efficient way to run my TDI 90 around town

A

andrewparkeruk

A2OC Donor
I have a bog standard ’54 plate TDI 90bhp (ATL) which I have owned for the past ten years. I have been a participant on this forum for slightly longer: I bought the car on here with 39,000 miles on the clock; it now has 162,000.

Over the years I have heard people discuss the optimum engine speeds, and the term “power band” has been used again recently, e.g.
depronman said:
Keeping the engine in the power band, 1800 to 3700 with a remap is key, if the revs drop below 1700 especially in higher gears then you are outside the power band and performance and economy will suffer greatly
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On a flat, straight, unbroken road, my car will do 30mph in 4th at 1400rpm, or in 3rd at roughly 2000rpm
Here in Manchester, many of the inner city roads are now 20mph. On a flat, straight, unbroken road, my car will do 20mph in 3rd at 1400rpm, or in 2nd at roughly 2000rpm

I have always erred towards the higher gear, believing it to be more fuel efficient.

My fifteen year old car has been driven by me for ten years and 124,000 miles, and the way I drive it hasn’t been too detrimental. @timmus has told me mine is the quietest 90 he knows.

I am interested to know your thoughts. Should I use the higher gear at 1400rpm, or the lower gear at 2000rpm?

Thank you, Andrew

PS please ignore the Fuelly thing in my signature; I don't use Fuelly any more
 
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My opinion is to use a lower gear. Reason is the DMF is happier with more rpm. As far as economy is concerned do not think we are talking a big difference here. You could always fill the tank to a known point use the low gear for a say 50 miles, refill and work out mpg then repeat with the higher gear to see for yourself. The diesels are not the quietest around and the lower gear should also reduce kamikaze pedestrians.
 
I think anything from 1700 rpm is ok
If it drops to 1600 I would not be rushing to changing gear if on the flat and not in a rush but not keen on going lower


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Even with a remap I'm not keen to go to 4th gear until around 40 mph - you feel the car struggling a little below this and this wont do the DMF any good...
 
andrewparkeruk said:
I have a bog standard ’54 plate TDI 90bhp (ATL) which I have owned for the past ten years. I have been a participant on this forum for slightly longer: I bought the car on here with 39,000 miles on the clock; it now has 162,000.

Over the years I have heard people discuss the optimum engine speeds, and the term “power band” has been used again recently, e.g.


On a flat, straight, unbroken road, my car will do 30mph in 4th at 1400rpm, or in 3rd at roughly 2000rpm
Here in Manchester, many of the inner city roads are now 20mph. On a flat, straight, unbroken road, my car will do 20mph in 3rd at 1400rpm, or in 2nd at roughly 2000rpm

I have always erred towards the higher gear, believing it to be more fuel efficient.

My fifteen year old car has been driven by me for ten years and 124,000 miles, and the way I drive it hasn’t been too detrimental. @timmus has told me mine is the quietest 90 he knows.

I am interested to know your thoughts. Should I use the higher gear at 1400rpm, or the lower gear at 2000rpm?

Thank you, Andrew

PS please ignore the Fuelly thing in my signature; I don't use Fuelly any more
Click to expand...
Contrary to the flow of opinion, I'd go for the higher gear/lower revs. At those speeds, you're on a very light throttle (I know, a diesel doesn't have a throttle, but you know what I mean), so the firing pulses are very light, so the torsional vibrations going into the DMF are very light.
My 6-speed TDi 90 is happy at 40mpg in 6th (~1200 rpm), but only if its drifting along. If the road rises at all, or to accelerate, you have to drop down two gears.
It's a lot quieter that way.

I suspect it's down to personal preference in the end.

If you're at really low engine speeds and on high throttle opening (yes, yes), then there is the risk of working the DMF hard, but it's not pleasant to drive that way anyway, so who would?
 
I'll pootle around town down to around 1500rpm (75 TDI) though any incline will have me dropping a gear. 90 will be fine at low revs too if you don't labour the engine.
 
A 1.2Tdi will change up to 4th with minimal accelerator at 30mph @ about 1200rpm.

RAB
 
I drive as @Merlin3046 describes. I accelerate to road speed then switch into the higher gear. In a 30mph zone (long, straight, open road, good visibility, unbroken road surface) I will often miss out 3rd gear: 1st to whatever speed; 2nd to 30mph; 4th. When my speed drops I will change down in order to get back to road speed.

Cheers, Andrew
 
andrewparkeruk said:
I will often miss out 3rd gear: 1st to whatever speed; 2nd to 30mph; 4th. When my speed drops I will change down in order to get back to road speed.
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Longer gears offer no guarantee of better mpg, just higher stress on the drivetrain. Routinely bypassing 3rd gear is definitely the start of a slippery slope unless you're someone who only drives downhill.
 
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