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So even though the ratio of the tire height vs gear ratio that I have now is very close to the stock ratio, my speedo can still be off. This is what you are saying, correct?


Yes. Think of it in percentages. For instance, if you change from the stock tire diameter by 10%, you'll need a 10% change of your final drive gearing to get back to where you started.

Example - we go from a 30" tire to a 33" tire, a 10% change.

Now if we had a 4.00 rear to start, we'd need to change to a rear that was 110% of the old rear ratio to get back to where we started.

4.00 X 1.10 = 4.40 and we're effectively back to stock. But we have bigger tires now...
<img src="/forums/images/graemlins/wink.gif" alt="" />

Just divide the new tire size by the old one, then use that number to determine the change needed in final drive ratio to get back to the same overall ratio...


Ok, went from 225's (28.4"-Firestone website) with 3.90's to

32's (31.8") with 4.30's

That's an increase of 11.97% in tire size and a 10.26% increase in gearing, so overall a 1.71% decrease, which means that 75 mph should be 73.7 mph. I'm overall 5 mph off, what gives?

Maybe I should measure the diameter of the tire on the ground and use that figure?

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