Contour Enthusiasts Group Archives Forums Modifications Wheels & Tires This might seem like a dumb question

I have the stock 15" rims on my mystique, I wanted to get 17" rims to enhance the looks of my car. My parents say that my speedometer will be off because of the difference in the sizes. Is this true and if so how can I fix that problem?

Yes it will be off and you can go to a shop to have it recalibrated.

Check this link to see how far you will be off.

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

It is not a dumb question but your parents are right ! It's a simple physics problem. For a given rpm, a 17" wheel will roll in one hour over a larger distance than a 15" wheel. Your speedometer would then give you a false reading. It must be recalibrated.;)

Go to www.blackrobotics.com they make a yellowbox that will allow you to correct the problem if you want.There under $100. bucks and you can buy a cheap GPS receiver to calibrate it yourself.

It is not the rim size but the overall rolling diameter of the tires. Plus sizing is when you go from a 205/60/15 to a 205/50/16. The rolling diameter is similar due to the shorter sidewall of the tire. I have 225/45/17's on mine and the speedo is only 5% off as the rolling diameter is only SLIGHTLY taller than the stock combo.

Originally posted by andy v:

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It is not the rim size but the overall rolling diameter of the tires. Plus sizing is when you go from a 205/60/15 to a 205/50/16. The rolling diameter is similar due to the shorter sidewall of the tire. I have 225/45/17's on mine and the speedo is only 5% off as the rolling diameter is only SLIGHTLY taller than the stock combo.

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That's right. The 5% increase (because of the bigger diameter of the rims/tires) means that if your speedometer indicates 100mph, you are actually rolling at 105mph and that is why the speedometer has to be recalibrated. Same applies to the odometer.

Ok so if I get it recalibrated how much would that run me and can any car place help me out with it?

Frankly, I would not do this (costly) mod because, besides the recalibration of the speedometer, rubbing of the tires would be the big issue. You are going from 15" to 17" wheels so be sure before deciding to go ahead. Also, the 17" wheels will require low profile tires and these are less comfy than the standard ones and are usually associated with a specially tuned suspension. Just my opinion ...

As long as you stay close enough to the stock rolling diameter, you will be fine. Use the Miata wheel diameter link that someone posted earlier to find out what size you need. But I can tell you off hand that for 17's , Contours should ride on 225/45R17 tires.

Recalibrating your speedo is unecessary IMHO. Who really gives a flying fugg if the tacho says 50mph and your doing 52.5mph? Save the money and spend it elsewhere!

As for the comment about rubbing and comfiness - that's also BS!
If you get the correct offset wheels (42mm for a 7" wide wheel, for the Contour)rubbing will be minimal, and nothing a rubber mallet and 30 minutes of your time can't fix.

I have 18's and the ride is still good. You'll just have to learn to be more alert while driving, so as to avoid potholes. And being more alert when driving can only be good!

The forums are refreshed and BS is already flying - ahh you gotta love the internet!

Thanks for your very stylish response!

I am not the kind of guy who would fix a problem with a rubber mallet... And even a "minimal" rubbing is unacceptable to me! As for the necessity of the speedometer recalibration, I agree with you but I was merely answering to a question.

Good luck with your slalom driving.