I upgraded my stock tires/rims (15's) to 235x40x18's from TireRack. After the change, I noticed I got better mpg then my 15's which is odd to me, and now brings me to this question. By putting on these 18's, does it effect my speedo/odometer? I searched previous threads about this but nothing specific to this tire size.

 

The short answer is yes. This site will help you calculate the difference
http://www.miata.net/garage/tirecalcold.html

 

TY! Looks like my speedo is .04% to fast with these tires on. Great tool! Thanks for your help!

 

Assuming that stockers were 205/65-15s, the overall diameter of the tires is very close - differs less than 1% so the speedo will be off by less than 1mph when driving at 100mph. So the short answer answer is no.

 

So what did you get?

 

is there a way to adjust this, because if you put bigger tires on isnt that effecting the milage on the car, assuming if it does wouldnt that count as odometer fraud if say you sold it? correct me if im wrong

 

it does affect mileage, but it's such a small amount who cares?

 

it's such a small amount, I wouldn't worry about it. over hundreds of thousands of miles, it might start to add up, but even then. What the next buyer doesn't know, can't hurt em!

 

Not fraud. Fraud is when you do something maliciously. They won't put me in jail because I put bigger tires on. Haha. I'm sure if it were a big difference in the speed I could get the computer adjusted for the size I got, but not worth it. With my old 205/65/15's I was getting around 250-270 miles a tank with mixed city/highway driving, and with these 235/40/18's, I get about 300-330 a tank. My old tires might have been low on pressure, not really sure.