A better tire size

[asirvr4]

my current tire sizes on stock OEM wheels are as following:
front: 225/45-R18
rear: 255/40-R18
which is stock i guess

i want to run a different size as following:
front: 245/40-R18
rear: 285/35-R18 or 275/35-R18

away from affecting speedo meter.. will i face any other problems????

advice pro ppl.. need help..

thanks

[esqu1re]

Speedo will reflect a larger margin of error than stock as the 275/35/18 sized tires are smaller diameter than stock---speedometer will tell you you're going faster than you actually are. Odometer will also begin reflecting more miles if you change to 275/35/18.

[ANILE8]

245/40-R18 will only be around 1% faster than shown speed.

275/35-R18 will only be around 1.7% faster than shown speed.

Pretty close that I would not worry about it and within acceptable tolerance.

The above work fine on a stock Z4-M and is very close to what I previously used to run.

285's are too large IMHO for a 9" rim and would rub anyway if driven hard.

[SmartBastard]

I'm running 245/40/18 in front and 265/35/18 in back at the moment with Yokohama Advan Neova AD08 and it fits, but I would recommend going bigger rims (going with 8.5" front and 9.5" rear) to fit the size of the tires. You should also know that with the 245 the front (also because mine tires are so sticky), the tires will drag you a lot more when driving on street waves/rails (not sure if this is the correct word, I mean streets are being used by heavy duty trucks and therefor have tire "rails" or w/e it's called). Nothing really to worry about, just a lil bit inconvenient if you use the car for DD and have shitty streets ..

PS: I wouldn't worry about having smaller diameter if it's under 3% of the original size, but you should NEVER go bigger diameter as this will exploit your ABS

[esqu1re]

Originally Posted by ANILE8

245/40-R18 will only be around 1% faster than shown speed.

275/35-R18 will only be around 1.7% faster than shown speed.

Pretty close that I would not worry about it and within acceptable tolerance.

Are you sure it's not 1.7% (the sensor is in the rear wheels, and assuming 1.7% is the circumference difference from stock, not the diameter difference; otherwise multiply 1.7% by pi) faster than stock? Stock is already some percentage faster than shown speed. I haven't tried anything other than stock, but I know on stock tires, the speedometer is already like 5% fast (shows 80mph when actual is like 76).

[ANILE8]

See for yourself Miata.net Tire Size Calculator

[esqu1re]

Originally Posted by ANILE8

See for yourself Miata.net Tire Size Calculator

Awesome. I'll refer to that page in the future as well.
So, 1.7% in addition to the stock speedo difference if OP switches to the 275 tire.

[beta]

I'm running PSS 245/275 and love it. The difference in grip from stock is very noticeable.

[RichardTS]

Originally Posted by esqu1re

Awesome. I'll refer to that page in the future as well.
So, 1.7% in addition to the stock speedo difference if OP switches to the 275 tire.

Calculators are nice but several years ago I found out you need to get the actual values for the particular brand of tire you are going to run. They don't necessarily have the same diameter for the same size. And you need to take tire inflation into account. This all results in as much as 2-3 % error.

If you look hard enough each manufacturer has the actual rolling diameter at a given inflation.