The math above is not correct. You have to work in percentages. 30÷40=0.75 or 75%. 29÷0.75=38.7 . You can also go the other way, 40÷30=1.333 or 133%. 29x1.333=38.7 .

Three people go to share a hotel room. They pay $30. After they get settled in, the manager realizes that he overcharged them by $5. So he sends the bellboy up with $5. He decides to give them a dollar each because he didn't have change. That means they paid $9 each for the room. $3 X $9 = $27. $27 + the $2 the bellboy kept = $29. Where's the other dollar?

Go back to post #19. You said you have 225/60-14 tires, which by the book are 24.6". Chart says 24.6" diameter & 3.23 gears = 34 teeth.

So if you don't have an existing gear, like me, go by the chart, then once installed why would the math/calc be different? 255/60/15 with 3.91s = 38 tooth gear, which apparently isn't a valid gear in those calcs.

These guys are basing their math on some variables that are just that, variable. So, I'm glad to see he is basing this on a chart/on-line calc. I think that's the best bet.

I have changed gears & tire diameters several times with great results. For the ones I have done for myself & others in the last ten years, I have been able to check the speedometer at 55 MPH & my GPS has agreed, every time.

Tire diameter changes with wear. Diameter is also affected by rim width. Wider wheel = smaller diameter. Plus the fact that the op indicated that he was not extremely accurate with his gps or even his gauge readout. I figured a 36 but I'm sure he'll be happy with the 35.

Did you use the chart None of that really matters about wear and width that your mentioning. The op said they measure 24 inches. Plug that into the chart, 35. But I agree, 34-36 probably will keep him close enough to right.

How did you measure the tire diameter? Only way that I know of that will give you a semi-accurate answer is to measure the circumference and do the math: D = C ÷ 3.142

Because a lot of old speedometers are not accurate. So going by the chart might not always be 100% accurate. By the way, a 28" tire with 3.91 gears = 37 tooth per Brewer's chart.

But the radius of a 27" tire would be about 12" from the center of the wheel to the ground (if it's a radial).

Your kidding.....right? Take a tape measure and see how tall it is....... Uhh..., I dont think so. How does a steel belted radial shrink? The sidewall varies to accommodate the rim width... "... Note: Because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width." https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=194 Now remember that these tables from Mopar were using bias ply tires that sat higher than a radial with its flatter and larger contact patch. So a "24 inch" bias ply tire may be in fact a 23 inch radial as the car sits. Maybe that explains the 1 tooth difference in Brewers and Mopars. Run your 35 and tell us how it worked.

Anybody who has ever stretched out a 195X75 tire to an 8" rim after having removed it from 4 1/2" rim knows more about tires than "TireRack.com". Experience is the best teacher.

That does not matter. It is the circumference of the tire that determines how far it travels down the road, and thus determines the exact gear ratio, and so affects the speedometer. If a circle of 85" is round (your 27" tire), or if one side is squished down and it is not exactly round, the distance around it is still 85".

Then why do underinflated tires cause your engine to run at a higher rpm? And overinflated tires cause your engine to run at a lower rpm.

Impossible to measure something like that with a tape measure. Well, maybe to ±1". Maybe get a huge set of calipers? Or just measure around it... that's the easiest way. Or just look up the tire size on any number of charts. The tire companies know the diameter of their tires when mounted on the rim they recommend, at standard inflation. Which I'm guessing most of us adhere to? If not, you'll need to start measuring, somehow, more accurately.