With all the interest in the 4.77 final drive-- my track set of wheels and tires are almost identical to that 4.77 final ratio. they are 24.4 inches tall-- do the math. I can tell you this---you do NOT want this final drive ratio for everyday driving. It will be much to buzzy. I can tell you something else--its the biggest thing that has helped accleration in all my mods.
Just some examples
4th gear at 8.5 k(my shift points) is 118 mph(before on stock tires/wheels is was approx 124
3nd gear is 10 mph per 1k rpm's.
rotor on
olddrager

 

Everyday driving in town, or on highways?

 

Town would be ok. But for anykind of driving that requireds interstate type speed--no way for me.
olddragger

 

whats your mpg change after you got the final drive?

 

I don't think he got the Final Drive modification. He is saying having smaller wheels is similar enough that its not worth doing so for normal highway driving.

-hS

 

Noob question here, but whats the formula?

-hS

 

Since I bought the same wheels as you...what's the clearance to the fender lip with the 235's? I'd like to cram the 255 RA-1's on my set, but I'm afraid the offset is a little too much for the 255.

 

(OE tire OD/new tire OD) x OE gear ratio

but you should really use loaded radius rather than OD

you're only talking about a 7% increase, this would only knock you up from 4000 to 4300 rpm to achieve the same speed, I don't see how this qualifies as "buzzy"

 

Hey Team-- No kidding man put a set on and try--you will see. 4th gear pulls almost as strong as 3nd does with oem wheels/tires. But it is just a little taller. The rotary with its large rpm range LOVES lower gears.
Mycalculations are with the 17's-- 4th gear is equel to approx 14 mph per 1k rpm while with the oem 18's it is 14.7.
at 8500(my shift point) the top speed in 4th with the 17's is 119mph with the oem 18's it is 125. BIG differance on the track.
5th gear at 8.5 k is equal to 140mph
5th gear at 70 mph is apprx 4.25 k--didnt figue 6th but est that to be about 4k for 70 in 6th? 4.5k for 80mph--est
The formula I used was
.003x rpm x tire diameter/diff ratio x trans ratio--this way you can figue what speed in whatever gear you are in. I got the formula from GRass Roots Mag- some of our NASA buddies. The article was about proper gearing for the particular track you are running.
Tomfree---plenty of clearance. I am running koni's with tein h springs and with the 17's on the clearance look like the stock wheel fender clearance with the oem 18's with oem suspension-- to me. May I ask why would you want the bigger 255's on an 8 inch rim? Wont you lose a little sidewall support? The extra aprrox 1 inch rubber to me is not worth that.
olddragger

 

OD - I guess I'm having a mental block with going down to a 235 when I know so much more tire will fit under the car. My street tires (which I usually run on Saturday of a 2 day HPDE) are 245/40/18 Azenis 615's on 18x9's, and it just seems odd to downsize tires. I know a 255 is less than optimal on an 8" rim, but I know that some folks are running the Dunlop SSR in the 255 on the 8" wide OEM rims with success. I'm sure you lose a little turn in response with the 255 on a skinnier rim, but it should be fine beyond that. Of course your description of the gearing difference may convince me to go w/ the 235.

Thanks for your input. Only 2 more weeks until I'm at VIR...

 

roger that tomfree--also look at the weight differance.235's are light. But I see your point also.
In the end the differances will be small (except for the acceleration factor.) The shorter tire will exit the turn faster and will definitly pull stronger. It will also heat up a little faster and I probaly will not have as strong of a brake as the 255's
only 5 more days before road atlanta--with about 12 rx8 's showing.
rotor on man
olddragger

 

ran 245/35-18 V710 Kumhos all last year, just a tad bit taller, approx. 4.66 effective gear, can't say that my butt dyno jives with your own, maybe we should compare graphs on 2nd thought maybe not

 

Hey Team

My butt has been chewed out so much it is probaly a little more sensitive than yours.
The V710's I looked at and really liked them, but the price of my 75% thread left Toyo Ra 1's(shaved and heated before they where ran) was only $310 delivered and I couldnt pass that up. The wgt of my wheel/tire and lugnuts is only 40lbs (i dont know what your's is) which may also make a little differance.
All I know is I really like this combo for the track I run --Road Atlanta and Roebling.
Olddragger