I'll add the ones I use on a regular basis.

http://gs.tolan-hoechst.com/tirecalc.htm

http://powerdog.com/tiresize.cgi

http://www.mindspring.com/~toy4two/tire.html

 

Here is another

http://www.mr2.com/FORMS/tire.html

 

Wich tire does the speedo take its reading from, front or rear?

 

It is actually the transmission which in effect makes it the rear tires which is why your speedo gets screwed up if you change sizes, the transmission is still saying the same speed as before but the car is physically going a different speed if the tires are different diameters than before.

 

I am not sure if it would work on the 350Z (need to check my service manual) but there are some electronic solutions for fixing speedo error out there.

Check out this website:
http://www.blackrobotics.com/index.htm

 

Tire/Wheel Size Speedo Error Calculator
http://www.miata.net/garage/tirecalc.html

 

Are you sure about the transmission reading? I was pretty sure it read from the wheel sensors located at the front wheels and the outsides of the differential in the rear. These are what ABS/VDC/TC use, so I can't see why they'd use the transmission for the speedo.

 

I haven't checked the manual, and you are right the sensors are used for those functions so they might also be used for the speed, but either way they do not read the outside diameter of the tire so it will read the same speed as before changing tires since it will be reading at the same spot but the car will actually be going faster or slower depending on which way you go with tire size.

 

Right, it still measures the number of turns of the wheel, then computes speed based on a set formula.

 

Looks familar, huh?

 

Very oddly familiar.

 

That's my work. I also wrote the one on the MR2 site mentioned above. I suggest going to http://toy4two.home.mindspring.com/tirewheel.html

There is one which links you directly to tires on tirerack.

Enjoy,

Mark

 

So after you changed the tire/rim size, the speedo will be wrong??!!!! Any way to fix/reset it?

 

No one still knows were the speedometer sensor is located at. I've been trying to figure it out, I disconnected the 2 sensors located on the transmission and the speed was still being taken. They must be on the wheels? Someone has to have a manual by now

 

I know my base, stock is reading 1.5 mph faster than its really going, versus what my streetpilot III GPS is saying. GPS is pretty accurate (handheld reads the same in tenths as my aircraft INS/GPS combo) so I'm going with that versus my speedo. Was it because the car was designed for the 18s?

 

The "speed" is calculated in the ECU. There is a setting to switch
between 17" and 18" rim styles. The sensor is the same on all models.

If you are that confused, simply measure the circumference of your
new rim/tire and do the math for foot-distance traveled in a mile.

Geez…

 

This should be a sticky Ares!!