Getting New Wheels

different tires are needed. The current ones are the improper height.

 

Changing wheels does not change the speedometer at all. It is the difference in height of the tire (plus or minus) that can cause the speedometer (and odometer) to be off. There is "plus 1 sizing", meaning that if you go from a 16 inch diameter wheel to a 17 inch diameter wheel, you get a shorter tire (less sidewall height, to keep the overall wheel and tire diameter as close to stock as possible.

I went from a 16 inch wheel with a 225/50r16 tire to a 17 inch wheel with a 255/40r17 tire. The tire diameter is nearly the same as the original, though the wheel itself is taller. The aspect went from 50 to 40, and created a shorter sidewall, but same overall diameter.

Wheel width does not play a factor.

If your overall diameter is greater or smaller, then your speedometer will be incorrect. It will also affect your odometer, ABS, and your shift points on an automatic.

5% difference greater or smaller in tire diameter is when you need to start looking at correcting the speedomter.

 

F cars speedometers are notoriously inaccurate. It is accepted fact amongst many 360 owners that the darn thing is almost 10% off.

 

Really....the 360 speedometer is usually like 10% off?

I have a 360 and it doesnt seem like anything is wrong with it. or is it that i just do not know?

 

If you know that your speedometer is inaccurate by a certain percentage, you could correct it by getting larger or smaller diameter tires. On my truck, the speedometer was reading a little low. When I bought tires one size bigger, it was closer to true.

Though not scientific, you could play with tire sizes to "re-calibrate" your speedometer. Your speedometer is based on tire diameter and rear end gear ratio. Since you really can't change the rear end ratio, tire size is the available variable. But if you increase or decrease it too much, you will put the engine out of it's optimum operating RPM at any given speed.

A car or truck with very large tires, will usually downshift a lot to maintain speed since the RPM is too low. I had to go from 3:73 to 4:56 gear ratio in my truck when I went to much larger tires, since the truck would "lug" at too low an RPM. With the new gears, the RPM is back in the "sweet spot" which means more power getting to the groud and better fuel milage, since the engine is operating at the RPM it was designed to.

Sorry to get long winded, but I have had a lot of experience and lessons learned with tire sizes on my truck and Ferrari.

 

Bart: I used that argument to win a ticket (well, I don't know if I won on the merits of my argument, the cop never responded. Gotta love CA's written declaraction).

--Dan

 

Try this site. http://toy4two.home.mindspring.com/tire.htmlvalue It showes changes in speedo readings with different tire sizes.

 

oops! Try this: http://toy4two.home.mindspring.com/tire.html

Sorry I'm still rying to get used to this new software.

 

Tire rack has charts for each brand and size of tires. There is one parameter that tells you everything you need to know; Rolling radius. If the new wheel + tire package has the same rolling radius as the originals, then the spedo will be as accurate/inaccurate as before.