Quote:

Your calculation is cnot orrect because as I said before you are not going up an inch in diameter because the tyre sidewall also reduces, so your outside diameter remains basically the same.

Karlos wrote:

With due respect the reason I wanted to increase tyre size is for better ground clearance and off road performance. Having bigger wheels and lower profile tyres with not help this. As reference to the site

http://www.miata.net/garage/tirecalc.html (nice find)

indicates increaseing the overal tyre diametre by 1" will change the speedo by about 4% and close to my approx calculation.

With all due respect, you did not state increasing the tyre by 1", but stated instead about the wheel rim with lower profile tyres which is what I commented on, but now what you are saying is you want to increase only the tyre size by 1", which is probably not possible on the car without interference between the wheel arch or suspension. As stated before you do not increase the tyre by 1" on it's external diametre, unless generally you are looking to make serious mods to your vehicle.

The fact remains that if you stick to the guidelines of increasing rim size and tyres to match then your speedo will not rellay change as shown in the calculator!

A change of wheels and rims from the smallest available on the FL2 235/70-16 to 235/55-19 will give you only 0.8% difference in the speedo reading.

Increasing your rim size will probably not aid your off road capabilities in the FL2 as you need to also reduce the tyre profile.

Thanks Gambba, now clear. No reason to have increased wheel size to increase off road performance is the anwer to my original question.