Term 4 ratio of distance traveled to driveshaft rotations?

[jaywish]

I guess I am wondering if there is a term or preferred way to describe this ratio?

E.G. 3.08 rear with 26.6" diameter tires.

26.6 X 3.14 = 83.5" circumference = 6.96'

1/3.08 = .325 axle rotation/driveshaft rotation

.325 X 6.96 = 2.26'/ axle revolution or "2.26DR" 2.26' per Driveshaft Rotation

short method 1/3.08 X 26.6 X 3.14/12 = 2.26DR

This accounts for tire size as well as differential gear ratio.

[gadget73]

Final drive ratio? That usually also factors in the gear ratio in the transmission though.

[His Royal Ghostliness]

Revs per mile. Usually used for tires, that number is published for every tire size and model, Tire Rack puts them in their tables for every tire they sell. Multiply by your gear ratio to adjust for driveshaft.

[jaywish]

Ok So in the above example which is my old wagon

782 tire Revs/Mile (TRPM) X 3.08 = 2,409 Driveshaft Revs/mile or DRM?

If I compare a Base 2012 mustang with 2.73 Axel Ratio gears and 215/65-17 tires= 742 TRPM it shows
742 TRPM X 2.73 AR = 2,026 DRM

On this alone going from 2,026 to 2,409 this is a 18.9% increase in DRM.
Equivalent to going from 2.73 AR to a 3.25 AR (aprox a 3.27) or from a 3.08 to a 3.66 (midway between 3.55 and 3.72)

[His Royal Ghostliness]

Yes. You worried about having to re-gear the car to stay in the engine's powerband? Seems to me you're right where you need to be already.

[jaywish]

Yes, Particularly related to fuel economy. That is quite a change in the ratio. The above does not take into account the 11% increase in weight.

[His Royal Ghostliness]

Increase in weight is very close to increase in driveshaft speed tho. I'd say it's a good match, or at least a good place to start.

[jaywish]

Perhaps. I would think the only other option to add economy would be 2.73's maybe with smaller tires.

It should step out pretty quick with the existing ratio.

[gadget73]

Run the 3.08 and see what that gives you. Don't over-think it. The least expensive and least complicated thing you can do is nothing at all.

[His Royal Ghostliness]

^^^ What he said! If you actually needed some axle work I would understand worrying about it since ideally you want things to be done just once, however your axle is good right now so just roll with it initially and see where that gets you. Remember the new engines like to run faster, whereas the 302s do not - if you gear her too low numerically you may end up going too fast for your typical driving conditions. It'd be why I got two sets of gears for my small truck, if for some reason I decide the 3.55s are too steep for my needs I can always just drop the 4.10s back in. Going to grab a set of 4.56s too just in case, price is great and one can never have too many options to choose from. In your case, if you do find yourself needing 2.73s, worry not as sourcing those will not be a problem at all

[turbo2256b]

tHIS IS ONE calculator I USE to figure rpm / speed using different gears and or tires.

http://users.erols.com/srweiss/calcrpm.htm

There are also tranny sites that help to calculate driven speedo gears to correct speedo readings

[jaywish]

You guys make too much sense for me.

Thank You