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OK, I admit to being numericly dislexic (sp), I recently replaced my 235/70x16 Goodyears with 225/75x16 Goodyear MT/R's. Given that my speedo' was registering 4% fast, 60 mph on the dial, actual speed about 57 mph, does the larger tire diameter make the difference worse or better? I tried to do the math but gave up after getting 3 different answers. HELPPPP !!!!
 

· These pretzels are making me thirsty
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I found this thru google. On a Miata website, but math is math. I plugged in your tire size and according to it, with the new tires when your speedo says 60 you are going 60.7. So, if it's right, not a big change. Anyway, here's the link
http://www.miata.net/garage/tirecalc.html check it out.
 

· MG
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llesra and PI are correct.

Basically, in inches, you've just gone from a 28.95" diameter tire to a 29.29" diameter tire.

Let's suppose that your speedo was perfectly calibrated for the first set of tires: when it showed 100 mph, your wheels should have been turning at 581 rpm. At that same rpm (same indication on the speedo) the second set of tires will be travelling at 101.17 mph.

Your upgrade has, in fact, reduced the indication error.

Working it backwards, given that your speedo was NOT calibrated for the original set of tires (you mention the 57 mph real vs. 60 mph indicated), your speedometer will indicate true with a 30.47" tire (245/75).

If you like, I can break it down even further (formulae, radians and other unit conversions), but that'll have to be over a beer or three...

Cheers,
 

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Do what I do when I change tire sizes. Drive by one of those portable radar speed signs at a steady speed and see how far off it is. You can check the odometer with mile marker signs. or set it on 60 and see how far it goes in 60 seconds (1 mile per minute).

My D2 with the stock tires had the speedometer reading fast. I put 245 75's on it and the speedometer was right on the money.

My D1 had 255 65 16's. Speedo read about 4 MPH faster than I was actually going. I put 245 75's on and now the speedo is about 2 mph fast instead of 4 mph. I guess I need 265 75's then it will be right on the money.
 
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