VTJeep

this was a part of another reply of mine: if you put your aw4 into 3rd (out of OD) you will keep the rpm's up allowing you to turn your oversized tires a little easier. however. this doesnt lower your final drive ratio right? only by installing a diff set of gears, say 4.10's, do you actually change the ratio. my guess would be that you would be using more gas if you leave the stock 3.55 and run w/ OD off vs. 4.10's and leaving it in OD? any input would be cool. i dont wanna mislead...

5-90

Depends on how you're defining "final drive ratio."

Generally, it's taken as the differential ratio - the axle in FE/RWD & 4WD applications, and it's in the transaxle in FWD and RE/RWD and ME/RWD applications. In that case, the only way to change it is to change differential gearing.

Some people define "final drive ratio" as the total drive ratio - transmission x tcase x differential. To those people, you change your final drive ratio just by shifting gear ranges.

The Hell of it is, both are semantically correct.

If you want to compute your optimum gear ratio for best cruise, you need to know what your tyre's rolling radius is (it's slightly less than true radius - bear in mind the compression at the bottom of the tyre due to vehicle weight) and the peak torque crankshaft speed of your engine combination (because "best cruise RPM" = "peak torque RPM." Any piston-engine pilot can tell you that.) Crunch around with every other variable known (cruising speed, drive/overdrive ratio, ...) and plug in various axle ratios until you get the crankshaft speed you want.

I don't recall the formula offhand, but it should be available readily with a little digging.

We are greatly assisted by the AMC six having such a broad, flat torque curve - the larger target is easier to hit! Also, with an automatic, there is a slight "multiplication effect" in the torque converter in first and second gear - but this usually goes away in third and fourth (or higher, if available) due to the presence of a lockup clutch (which makes the torque converter a direct 1:1 coupling.) However, I have still noted that the AW4-equipped rigs will get "best economy" cruising speeds at a slightly lower crankshaft speed than a manual transmission - even with the lockup converter - I have yet to be able to explain this.

For RENIX, I have noted "best economy cruise" to be about 2600-2800rpm (at the engine) with a manual transmission - the automatic will get that around 2300-2700 (a bit lower, a bit broader. I am as yet unable to explain why this is so - so I merely accept it for the moment.) Those are your targets for calculation.

If you want to get a "rolling radius" for your tyres, it's best to measure between TDC on the hub hole and ground line - then at BDC on the hub hole and ground line. Average the two measurements. This will get you closer than merely "eyeballing" the centre of the hub hole. Any similar pair of extreme opposite points will serve - even TDC and BDC on the rim edge! Calculate your "rolling circumference" from your "rolling radius" for purposes of determining your tyre revolutions per mile.