Optimum grey cruise RPM

[Rooooboy]

Hi all,

What is the recommend cruise rpm for a grey motor.
I need to rebuild the diff and have opportunity to change ratios.
Going off the factory spec 6.50 x 13 tyre size, revs were 3,254 with factory 3.89 ratio.

Should I be aiming for around 2800rpm?

[Harv]

Depends a bit on how you like to cruise. Your diff ratio can be changed a bit (2.78 - 3.89 for the banjo) as can tyre diameter. With some work, you can adapt a box with overdrive (the crashie is 1:1).

If you are chasing fuel economy, a lower cruise speed helps (towards the 2.78 end of the scale). You need to be prepared to row it through the gears though on bigger hills, and it will be sluggish in the city. Try to chirp the tyres and the poor thing will shake it's head and sulk.

If you like to keep up with city traffic, the big end of the scale (3.89) helps. I run 3.89 in the FB, and it keeps up with Sydney traffic pretty well. It does a lot of highway miles (often with a trailer), and have never had an issue with overly high RPM. Pulls well up hills, and will readily break the speed limit if you were so inclined.

All up its a balance, and often depends on what mixture of city/cruise driving you do, and how you like to drive.

Cheers,
Harv

[Rooooboy]

Thanks Harv,

What size tyres are you running? Or do you know your revs at 100kmhr?

Mine currently has a trimatic so it will be doing the work if I drop down to. A 3.08. The grey motor is freshly rebuilt, and is a little bit of a warm motor.

[Errol62]

Used to run 3.08 behind 202 trimatic. 205/60R13. 3.36 ratio would be the go behind a warm grey trimatic.

FB ute driver, EK van project

[Blacky]

I have tried all ratios from 3.08:1 up to 3:55:1 on runs across the Nullarbor coming home from the Nats and have settled on a 3:36 as the best compromise. I had the 3.08:1 in it when we went to Mudgee and before the nats we went down to Victor Harbor and were down to 40km/h in places on a 100km/h road - pissed me off and no doubt the people behind me …….
Mild grey with 3 speed , 14” rims 215/65/14 tyres

[Harv]

What size tyres are you running? Or do you know your revs at 100kmhr?

It's got 175/70/13s on it currently. Have had 185s on it but the local tyre mob had none when it was last due for new shoes.

No idea on RPM, but a happy motor. Still pulls well when overtaking at highway speed.

Cheers,
Harv