I have calculated that my speedo is out %20 percent.

I took my rx7 for a 5.2km drive and another car for the same distance. The rx7 read 6.3km and the other car read 5.2km on the trip meter.

The speedo is reading %20 out because of the diff change.

The car has the original 13 inch rims.

I would like to know what size rims to get that would correct this problem within a %3 tolerance of the speedo.

Any help would be excellent

 

I'm no math wizard, so could you let us know your current diff ratio as well as your previous ratio? Also, could you give us your tire circumference?

 

You do realise that you'd need to increase the overall tyre diameter by 20% to compensate. So unless you want to drive around with tractor tyres it's not really an option. Plus it would totally negate the reason for the diff ratio change in the first place. ie better acceleration. So, if you want the speedo to read properly either put the standard diff back in or install a ratio box between the speedo cable and speedo drive.

 

Or just use your brains and compensate for the 20% difference yourself.

 

:

I'll bet he's thankful for your help.

 

That is not going to help the fact that his odometer mileage is increasing greater than it should.

I have a similar problem. Here is a link someone gave me about the gears - Maybe you can find one that will work.
http://members.ij.net/mrmazda/speedo.html


-billy

 

Well I'm starting to be a nice guy since I took the time to fully calculate it to answer you question.

To compensate for the 20% you will need 18" rims.

With 245/50 tyres. Going higher then 18" is possible, but that would really be ridicules.

So the fix for your problem is to save up money to buy costum made 18" rims with 4 245/50 tyres.

Good luck