doesntplaywell

if anyone is running that setup, can you tell me the rpm's @80 mph..... I'm thinking of doing a gear swap & have access to a set of 4.10's but I take alot of long trips (1000 miles one way) & am a bit worried about high rpm's for that length of time.... I know the taller tire will help but it's only 1" taller..... I've read many threads & it seems that most people say go with the 4.10's

also is there any formula or anything that I can use to get the rpm raise from 3.31 to 3.73 to 4.10?

jobeonnet

i believe it is as easy as dividing the gear you want by the gear you have and multiplying your present rpm, so multiply your rpm now times 1.24 for 4.10 and 1.13 for 3.73. i think thats it. there are some spreadsheets around but i couldn't find one.

danzcool

I don't have the setup, but mathematically it should be at 2635 RPMs, given a tire height of about 28.45"

Formula:
You first have to calculate the RPhour of the rear wheel, which is engine RPM * 60 * the 5th gear ratio (0.68), and then divide that by the rear end ratio (4.1 in this case).
take that measurement and multiply it by the wheel tread length (tire height time Pi) then divide that by 5280.
That will give you the speed in 5th at the given RPM and gear ratio.

ChiDiddy

your fuel economy will suffer slightly by about 2-3 mpg so thats the only downside. Higher rpms really doesnt hurt anything except your mileage.

jtstakelin

Give this a try:
http://www.miata.net/garage/tirecalc.html

Put in your stock wheel info and your new info and it will tell you how much your speedo will be off

Shotokan1509

Pm me your email, I have an excel file that a Francis Chang created, I tweaked a little bit that gives great info by adjusting tire size and gear ratio in comparison to hp & tq peaks and redline. (you would need to find the automatic's gear ratio's)

This is a screenshot of my combo.