Optimum grey cruise RPM

Includes clutch, transmission, propeller shaft,
universal joints, differential and rear axle.

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Rooooboy
Posts: 5
Joined: Tue Aug 23, 2016 5:29 am
State: VIC
Location: Melbourne

Optimum grey cruise RPM

Post by 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?
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Harv
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Joined: Mon Oct 08, 2007 2:00 pm
State: NSW
Location: Sydney, Australia

Re: Optimum grey cruise RPM

Post by 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
327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.
Rooooboy
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Joined: Tue Aug 23, 2016 5:29 am
State: VIC
Location: Melbourne

Re: Optimum grey cruise RPM

Post by 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.
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Errol62
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Location: Adelaide

Re: Optimum grey cruise RPM

Post by 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

getting my FB ute on the road
EK van on rotisserie
Blacky
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Joined: Tue Jul 20, 2004 8:58 am
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Location: up in the Perth hills

Re: Optimum grey cruise RPM

Post by 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
When you're faced with an unpleasant task that you really don't want to do, sometimes you just have to dig deep down inside and somehow find the patience to wait for someone else to do it for you.


Foundation member #61 of FB/EK Holden club of W.A.
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Harv
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Joined: Mon Oct 08, 2007 2:00 pm
State: NSW
Location: Sydney, Australia

Re: Optimum grey cruise RPM

Post by Harv »

Rooooboy wrote: Mon Sep 27, 2021 7:45 am 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
327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.
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