Hi, this is just a general question on the speedo, this is so I dont get too confused, so here goes.

I intend to put either 17" or 18" wheels on my 87gt
the question is will this affect the speedo? in terms of the speedo say its going 30mph when actually youre doing 40mph
and if so by how much and is there a device or modification to retify this?

any input is always welcome

jon

Well it's not the size of the rims that matters, but the outside size of the tire that does.

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

Use this caluclator to see what the difference is in outside tire size. Then divide the new tire size by the stock tire size. So if you get a number like 1.06 or whatever. Now take that number and multiply the speed the speedo displays by it and you will get your actual speed.

Assuming you are going to bigger outside diameter tires the first number you get should be slightly over one.
The resulting speed you get should be that your speedo is showing a speed that is less than your actual speed.

[This message has been edited by phonedawgz (edited 11-25-2010).]

thank you for the quick response and the tyre guide, the good thing from it it is telling me that the actual speed is slightly less than the speedo reading.


thanks again
jon

A gps (gps phone?) is a great way to actually tell how accurate your speedo is.

Your welcome. Have a great non-Thanksgiving day.

[This message has been edited by phonedawgz (edited 11-25-2010).]