Tyre size / revs

I don`t know if this will help or not.

M.P.H = rpm x wheel dia. in inches divided by gear ratio x 336.
Regards G.T.

This " target="EOF" class="lbg">www.miata.net/garage/tirecalcold.html will give you a percentage difference in speed for different tyre sizes. How that translates to RPM is another matter.

Mike

Sorry, problem with the link.

Try this " target="EOF" class="lbg">www.miata.net/garage/tirecalcold.html

Hi nissnut,

All a matter of ratios:

31/33 x 2500 = 2350 rpm for a 33" tyre @ 100 km/hr

31/35 x 2500 = 2210 rpm for a 35" tyre @ 100 km/hr

Cheers

Captain

Geez, my method was no where near as elegant as Captain's, but same result. To get 2000RPM, need around a 38.5 inch diameter.

My 80 series does 2500 to give 100kph in 5th gear.

Circumference is 2 pi R or Pi D

It will therefore give the following for each revolution

31" = 97.38937" travelled
33" = 103.6726" travelled
35" = 109.9557" travelled

Engine running 2500 revs for 31" wheels

33" reduces revs by 6.4516 % or 161.29revs = same speed at 2338.71 revs
35" reduces revs by 12.9032 % or 322.58 revs = same speed at 2177.42 revs

hi nissnut
i don't know how much a differance to your speedo it will make by running higher profile tyres worth looking at. When i changed my from 650/16 to 750/16 it made about 10k's differance on the speedo and we don't want to contribute to the police funds
cheers steve

change your diff ratios....

Captains reply is an elegant solution

31/33 x 2500 = 2350 rpm for a 33" tyre @ 100 km/hr

31/35 x 2500 = 2210 rpm for a 35" tyre @ 100 km/hr

so 2000 rpm requires 2500/2000 x 31 = 38.75" dia

But the question in my mind is 31/38.75 X 100 = 80% reduction or 20% reduction in revs and also power.

So I assume you have thought the following through?
Can you afford to loose 20% in power in top gear and still have enough power to push the car along the road?

20% is about the difference in gears between 5th and 4th.
So 4th will then be like 5th now, so you can use 4th for overtaking.

But if 1st gear is 20% higher, will you get off the line on a hill?

A further cost may be a rock crawler set of low ratio gears so low range isn't 20% higher.

I went up 10% in gearing with bigger tyres (new 32") to go from 2870 to 2580 rpm at 100k, but I can not go any higher without a lower 1st gear. After a bit of research I think I may have found a set from a newer model, and 10% higher diffs to go with it. (The higher diffs saves arguing about illegal larger tyres, and I can only go to 33" max under std guards). Will give me 2320 revs at 100k which is right on the good power/ max torque point for economy. 4th will be needed around town as 5th will be too high, and for overtaking on the highway, and for hills. May use 3rd more too for steep hills. But with the lower 1st, 2nd should still be OK since the 1st to 2nd gap is pretty small. I can start off downhill in 2nd now. I can get rock crawler gears too. With 10% higher gearing fuel usage has dropped about 5% at a steady 100k. (Stuck to same width tyre, just taller so rolling resistance doesn't go up too much, went from 235/75/15-29" to 235/85/16-32")

So if I spend a few thousand dollars on diffs, a new 1st gear and rock crawler gears I can get another 10% to make 20% in total. How many years does it take to pay me back in lower fuel usage and less wear and tear? Reason I have not gone down that road :o)

Look at the calculators on this web page , it will take care of all your questions