...how high should the revs be when I am at 80 mph? Does anyone have a formula? Thanks!

 

it's not too hard to figure out if you know what the gearbox ratio is and what the TOTAL diameter of your wheel is (like, are the tyres 5 series on 16x3 's, or are they 125 series on 16x7's?? ...i know, i know, just an example )

 

One of my friends has that exact setup on his 87 GXL. GTUs rear(4.33), and S5 turbo wheels with 245 45 16's. The wheels are slightly taller than stock, but its enough to offset the 4.3 gears. His speedometer is dead on, without any adjustment.

 

Whats the point of putting in the 4.3 rear if youre just going to offset it with taller tires??

 

I agree, but I'm gonna do the samething. But I want the internal gears to be different also.

 

well, at least the taller tires didnt kill his final drive this way...

 

You need to know the actual height of the tires to calculate.

 

There used to be an awsome calculator that took all this stuff into account, and it was made for RX7's, it had all stock trannys, and some of the racing ones at the click of a button, plus you could enter your own #'s, does anyone remember that link?

 

I did a tech column on this for my club's newsletter last year.

Just use this formula:

V=(TCxRPM)/(1056xARxTR)

V is velocity in mph
TC is tire circumference
RPM is engine speed
AR is axle or rear end ratio
TR is trannsmission ratio

To find TC, you must know tire diameter, TD.

TD=(sidewall x 2)+Wheel diameter

Sidewall for a 205/60 tire would be 205mm x .6. Multiply by two, of course. Then convert to inches and add the wheel diameter.

TC is just pi x TD.

 

As mentioned, wheel size is irrelevant without the tire size. But assuming the overall tire size is the same as stock, replacing a 4.1:1 diff with a 4.3:1 diff will see revs 5% higher at the same speed.
You don't say what your car is, but for a T2 with a stock diff, 80mph is 3550rpm; with a 4.3:1 diff that would be 3730rpm.
The difference between the two ratios is quite small, so I don't see the point in spending money to change, especially if you go and buy a wheel/tyre combo that negates the difference...

 

wow are u serious here? at 80 mph my rpm's r at like 3000+/-

 

... or you could search this forum for the Evil tech column.
https://www.rx7club.com/forum/showth...hreadid=124399

 

As I said, they're for a Turbo, which has a shorter 5th gear than your NA.

 

Baaahhh!! My article in the Rotary Flyer was early 2002, so maybe you stole my formula!

It's a pretty well cicrulated forumla anyway, and not much harder than using those fancy online spreadsheets. I wrote up a simple program for my TI-86 for the few times a year I need to use the formula, it justs prompts me for the variables and spits out the unkown value I want.

 

Hehehe, the base formula has been around for decades, and is published in the Mazda Motorsports catalog, as well as in most high school auto shop textbooks. Much like you, I just added the tire sidewall information to estimate the total tire height for the equation. Also, much like you, I have my own Evil spreadsheet that calculates all kinds of goodies.

 

Thanks guys, appreciate the help!

BTW, the 4.3 just came with the torsen LSD that I bought off another guy, so it was practically free. As mazdaspeed7 said, at least my bigger wheels don't kill my final drive. My vert is slow enough off the line!