TSR2 · Jan 17, 2020

Assuming level ground etc. what should the expected engine rpm be for 70mph in 5th gear?

I ask because previously my engine would run at just under 3000rpm, now it sits at just below 2750rpm at the same speed in same conditions. I have had an injector replaced last month and a new clutch a few weeks ago, but it's only today the RPM has done this.

I know this may be an impossible question to answer, but 250rpm seems a big jump.

EDIT: I've a 2004 TD4.

freelance · Jan 17, 2020

Could be new injector making it more efficient, perhaps consider changing the other three !

TSR2 · Jan 17, 2020

freelance said:
Could be new injector making it more efficient, perhaps consider changing the other three !

I thought that, but then I changed the injector at the start of December and it's just today I've seen a change. Does the ECU (or whatever it's called) control things like this?

freelance · Jan 17, 2020

Could even be air temps up a bit, brakes not binding, doesn’t need much to make a difference

Nodge68 · Jan 17, 2020

The speed the engine spins for a given road speed is purely dependent on gear ratio and nothing else. If so then the clutch has likely been slipping which would account for the higher engine RPM. For reference, my TD4 SE auto engine spins at about 2700 RPM, when the speedometer indicates 70 MPH, when the box is in 5th, with the TC clutch locked. If the TC clutch is open, then the engine spins at about 3100 for the same 70 MPH.

J_D · Jan 17, 2020

as above. Ratio of rpm to speed is flywheel speed through the gearbox to wheels. Doesn't matter on state of the engine.
Could have been the clutch slipping, that would have meant you pressing on the accelerator though.

TSR2 · Jan 17, 2020

Thanks all, you learn something new every day.

Avocet1 · Jan 17, 2020

TSR2 said:
Thanks all, you learn something new every day.

Have you altered your tyre pressures recently?

TSR2 · Jan 17, 2020

Avocet1 said:
Have you altered your tyre pressures recently?

Nope, they’re religiously maintained at 30psi all round!

Avocet1 · Jan 17, 2020

TSR2 said:
Nope, they’re religiously maintained at 30psi all round!

Can't think of anything else then. I agree that slight clutch slip seems the most likely explanation.

Jayridium · Jan 19, 2020

You've just fitted a new clutch - it might be that the old clutch was slipping allowing the enigne to spin faster at any given roadspeed whereas thenew one is proplery connecting the two. But Nodge is right:

Nodge68 said:
The speed the engine spins for a given road speed is purely dependent on gear ratio and nothing else. If so then the clutch has likely been slipping which would account for the higher engine RPM. For reference, my TD4 SE auto engine spins at about 2700 RPM, when the speedometer indicates 70 MPH, when the box is in 5th, with the TC clutch locked. If the TC clutch is open, then the engine spins at about 3100 for the same 70 MPH.

GrumpyGel · Jan 19, 2020

Have you recently put 33" muds on?

Nodge68 · Jan 19, 2020

GrumpyGel said:
Have you recently put 33" muds on?

It makes no difference to the relationship between the rev counter and speedometer, what size of wheels are fitted. Only the actual speed would change at a given RPM, with different size tyres, not the indicated speed.

Jayridium · Jan 20, 2020

I've got a spreadsheet I put together a while back for calculating tyre sizes * gear ratios * Diff * Transfer Box * Portal Box and comparing road speeds versus dyno outputs I'd be happy to share? It's still a bit work in progress, as in I haven't got the pull down menus fully working, but the basic calculations are there and the pretty graphs work.