Thread: Reducing engine revs on the highway without sacrificing low gearing

Don't know if it helps, but my preference would be: change diff ratio > ashcroft HR kit > roamerdrive.

I have gone with the ashcroft kit in my SIII, but I have a feeling I will be putting the standard kit back in the car and fitting a roamerdrive in the future.

Cheers,

Hi Tim,

Thanks for your prompt reply. Yes I do like the benefit of the overdrive in that it can be selected or deselected when and where driving conditions change apposed to a more permanent modification.

pulling 2+ tonne, i'd keep it standard

i can pull 1.5 tonne and sit on 90 all day long

pulling 2.5 i would be sitting on 80-85 and that would be about it but mine is an 88"

adding an over drive etc pulling that sort of weight will cost you more fuel not less in my experience

Hi Incisor,

Many thanks for your advice.

I would only use the overdrive when traveling long distances on the bitumen without a trailer to bring the revs down.

This is why I feel sceptical about putting in a high ratio kit as I cannot deselect it when I need to tow a trailer and would then have the engine labouring unnecessarily.

First I have to get it roadworthy and then I'll run it for a while to see.

How hard do they rev at 80-90kph? I haven't ever driven mine on the road since I bought it 9 years ago!

The Rover engines, both the 2.25 and the 2.6 are quite happy to run indefinitely at the rpm needed for cruising at 100kph, and using higher gearing from either a high ratio box or overdrive will have little impact on fuel consumption - which won't look too good travelling at that speed anyway. A Holden engine is a different situation - they are not happy.

The 2.6 engine in particular is usually considered to be less happy running at full throttle at a lower engine speed than producing the same power at higher rpm - the higher rpm improves coolant circulation, minimising the possibility of exhaust valve issues.

As Dave says, if you are planning on consistently towing a heavy trailer, the only advantage you would get from an overdrive is the ability to split between gears, as the only time you would be able to use the overdrive to reduce rpm while cruising would be downhill.

Thanks John for that helpful info. Obviously I need to try it out first before modifying it.

As John said, the 2.6 is very happy to rev where it needs to with standard gearing to give you 100KPH easily - I've driven an Ex Army series 3 at over 110 no dramas and that same vehicle satin a convoy of much newer vehicles for hours on the freeway at 110 - all with standard gearing although fuel usage does go up obviously.

These engines also pull well down low when breathing properly - a set of extractors and a 2" exhaust makes them able to pull from 20KPH in top gear.

If you really feel a need to knock some RPM off consider if you're ever likely to need a PTO at any stage for anything as fitting an overdrive will preclude this.

The Ashcroft kit allows you to still use a PTO if ever required and doesn't heat up like an OD does.

I'd try it in standard trim and think about what you may do long term before making a descition. There's pros and cons to all options.

Thanks Homestar. It's interesting that a long stroke motor is happy to rev. I also find the Vanguard 6 cylinder engine in my Triumph 2500 is much happier sitting on 110kph at 3,500rpm in direct drive.


I do plan to alter the exhaust. That manifold is so inefficient.


Did you put on Cookey's extractors? If he's still making them by the time I get round to putting the FFR back on the road I hope to be able to buy a set from him.