Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/50-17 4.4in 12.9in 25.9in 81.2in 780 0.0%
235/45-17 4.2in 12.7in 25.3in 79.6in 796 -2.1%



So with it being -2.1% what does that mean??


How do I calculate say I am going 60 mph with the OEM what speed would I be going with the new tires?
Thanks!

 

http://www.miata.net/garage/tirecalc.html

your new tires are smaller than the old ones. so your speedometer will be reading 2.1% too fast. if your speedometer says you are going 60 MPH you are actually going 58.8 MPH.

 

ok.. so does this affect the mileage also?

 

yes, but not all that much. The actual error % is
2.055420216. Basically that means when you are goin an indicated (speedo reading) 100 mph, the actual speed is 97.94457978 mph. The same error occurs in your odometer as well. The slower you drive, the smaller the error as well.

 

Not sure about the mileage, but I know it will affect your speed governor...

 

I thought the odo. readings are taken from some kind of sensor detecting the wheel itself's rotation, not the wheel + tire? Are you sure the % affects the odo. too?

 

It used to be that at 80mph my speedo is at 3k.

Now, with the lower profile tires. I'm roughly at 2700rpm at 80mph.

The nice thing, although barely noticeable, about lower profile tires - is it seems my turning radius improved slightly.

 

I hope your wheels aren't rotating at a different speed from your tires!!!! Both Odometer and speedo read axle rotation.

 

stupid me..
So, this would mess w/the odo. is that what you're saying? If so, a -2.2% speedo error would affect odo. readings how (i.e. more/less miles logged than actually traveled)?

 

yes, you can figure out the true mileage travelled by takin out the avg error % from the total mileage. Go drive a car with stock tires on a certain loop of roads. Record the distance, then do the same with your max at the same exact indicated (speedo reading ) speeds. Note the difference.

The faster you go, the more the error in speed and the greater error in distance.