differential gear ratio opinions

[andrew]

I'm contemplating changing the differential ratios on a '66 109, S2a from the standard 4.71 to either a 4.11 or a 4.43.

The engine will be rebuilt so it will be strong, and I will have 235/85 tires (31.7") and an overdrive (Fairey; .78 ratio). This should take the RPMs (with o/d) at 70mph, for example, down from 3132 to 2946 (with the 4.43) or to 2733 (with the 4.11). I do commute in the truck so the highway RPMs matter to me; I also drive around town a lot. And, I don't want to lose the off-road performance; the low range 1st gear overall ratios would change from 40.81 (with the 4.71) to 38.38 (with the 4.43) or 35.61 (with the 4.11).

At this point I'm seeking others' opinions, particularly if you have experience with a similar setup. I can imagine the modest but nice difference on the highway but I have little sense for whether the changes would matter that much off-road, or even around town.

Thanks for any thoughts on this.

[jp-]

Ashcroft has a few more ratios to consider. I went with the 4.37's with limited slip. Unfortunately, it will be some time before I get to drive it to give a report. My experience is therefore limited to the Rover 4.7's and 3.54's. IMO, the 4.7 is too low (makes everything scream to highway speed), and the 3.54 is just a little too tall (my 110 struggles to pull them, even with the Turbo). But many people report that they are happy with the 3.54's in a Series rig with a strong motor. That said, I think you'd notice more a difference (in the positive) with the 4.11's.

https://www.ashcroft-transmissions.co.uk/differentials/stock-rover-type-diffs.html

[TeriAnn]

I'm contemplating changing the differential ratios on a '66 109, S2a from the standard 4.71 to either a 4.11 or a 4.43.

The engine will be rebuilt so it will be strong, and I will have 235/85 tires (31.7") and an overdrive (Fairey; .78 ratio). This should take the RPMs (with o/d) at 70mph, for example, down from 3132 to 2946 (with the 4.43) or to 2733 (with the 4.11). I do commute in the truck so the highway RPMs matter to me; I also drive around town a lot. And, I don't want to lose the off-road performance; the low range 1st gear overall ratios would change from 40.81 (with the 4.71) to 38.38 (with the 4.43) or 35.61 (with the 4.11).

At this point I'm seeking others' opinions, particularly if you have experience with a similar setup. I can imagine the modest but nice difference on the highway but I have little sense for whether the changes would matter that much off-road, or even around town.

Thanks for any thoughts on this.

The sweet spot of a 2.25 petrol is in the 3000-3200 RPM range. Good power, reasonable torque & fuel efficiency. That engine is designed to happily run all day in that RPM range. Drop the RPM too much and your engine has to work harder to turn the tyres and your fuel mileage suffers. Get into the hills and you will really notice the difference as you gear down sooner. Personally I wouldn't touch it. You will be happier at 40:1 in low first than the taller ratios.

[andrew]

Thanks for the feedback, jp and Teriann, I appreciate it.

Another perspective from a well-informed source was that the 4.11 is about 12% or so higher than a 4.7 -- whereas 12% at 65 mph is a lot, 12% at very slow speeds (e.g., when negotiating an obstacle in low range) is not much at all. So, their view was that the change would buy me more at high speeds than I would lose at low (off-road) speeds.

I am really torn as to what to do (4.11 v. 4.7). I like the idea of an easier time on the highway and reduced RPMs, but I'm reluctant to diminish the off-road capabilities too much. Also, it's unclear just how much difference this would make in around-town driving, which I do a lot, but I think the 4.11 might actually make things a bit more civilized in that instance (smoother starts off the line in 1st gear, etc.). The problem is of course that I only know my own, current truck well and have had no chance for a direct comparison to really feel the difference.

Once this is all settled out I'll report back on my driving experiences for others, but in the meantime I'd appreciate any additional info/opinions/thoughts that might help me make the right call...I need to decide now.

[TeriAnn]

Thanks for the feedback, jp and Teriann, I appreciate it.

I am really torn as to what to do (4.11 v. 4.7). I like the idea of an easier time on the highway and reduced RPMs

The problem is that with the 4.11:1 is that your engine will be working harder to push your tyres. Unless you are going downhill the carb throttle will be open more to get to and maintain those speeds. Your acceleration will suffer, and you may find yourself in third over on slight inclines into a mild headwind. Its your money, & a pair of 4.11:1 R&P gears are not all that cheap, esp if you have someone set the diffs up for you. Personally, if I still had my 8:1 2.25L petrol engine I would not do it. Choice is yours but you will be cruising below optimum RPM for that engine.