I had my new rims installed yesterday (see below for specs) and it seems like my odometer is running faster now...

do i need to recalibrate???

my stock tires were 215/50/17 with a diameter of 25.46 inches
my new tires are 225/40/18 with a diameter of 25.08 inches...

I know a lot of you have the same size tires...what did you do?

Thanks

 

The math says there is a 3% difference between the two. Since your original odo was off app. 1 to 1 1/2% you're now reading 4 to 4 1/2% high (YMMV). I'm not aware of any calibration available to our speedo/odo.

 

The short answer is -- it cannot be adjusted.

The long answer is -- it's quite difficult to adjust and you need to tweak the ECU which may not be easy. Somehow I remember Jens commented on this once. Do a search for Jens & odometer.

I'm sure you'll be fine with your new set-up. Enjoy.

For a better comparison, or to figure out how much off your speedo will be, compare roll out of the tires rather then size. The stock tire rolls out at 819 rev/mile. The Dunlops in your size roll out at 827 rev/mile which makes a difference of about 1%.

You can also check things out by driving at a set speed on the freeway (I choose 60 MPH -- set the cruise) and timing with a stopwatch between mileage markers, then do the math to figure out how much off you are. I did this and my speedo is pretty accurate.

 

 

You can do all that math... or just go to this site and enter in the different tire sizes and it does it for you.

http://www.dakota-truck.net/TIRECALC/tirecalc.html

 

Good link... even better then the miata.net one!!

I just got the same size tires (225/40/18) after having 225/45/18 on my car for a year. I figure that the 225/45 were a tad bigger, so the fact that the new tires are a bit smaller should actually balance out my mileage

FYI, that size is 1.5% smaller then the stock size... so all you have to do is multiply the mileage by 1.015 and you'll have your difference. For example.. if you go 20,000 miles, your odometer would read about 20,300 miles... adding 300 miles to your odo.