Tire Sizes Explained

by Bob DeP.


Dear Bob;
When buying replacement tires: How much larger can a tire you purchase for your auto be, before the "actual" speed & mileage disagrees with the "gauged" speedometer & odometer? I have a 1990 Camry. I replaced the 14" original 195/70 Dunlop tires with 14" 205/70 Michelin tires for better handling( I drive 20,000 miles annually). My friends tell me that the actual speedometer and odometer readings are off because of the new tires. The tire jockey was clueless with his explanation. He would have sold me anything.
What really happens?
Thanks.
AL

Al,
Any time you change tire sizes, you are affecting the rotational aspect ratios. But let me first tell you what the tire sizes mean.

205/70r14 means you have a tread width of 205 mm (millimeters) and the 70 means that the height of the tire from the end of the rim to the surface (or the sidewall) is 70% of 205mm or 143.5 mm with all of this is sitting on a 14 inch rim and it is a 'R'adial tire.

The originals were 195/70r14. This means that the tread width is 195 mm and the aspect ratio is 70. So the sidewall height is 136.5 mm. So the ride height has changed from 14 inches plus 273 mm (136.5 x 2) to 14 inches plus 287 mm. The is a difference of 14 mm. That's about 1/2 inch. What probably would have been better was 205/65r14. Same width as the new tire but a slightly lower profile. Only 133.25 mm. Slightly lower than the originals (but the difference is less by about half as much, a quick calculation shows only a 1/4 inch difference). Now, how does all of this pertain to your problem? What will a difference of a 1/2 inch or a 1/4 inch do to your speedo? Probably not much, but it WILL be different because now your final drive ration has changed! Most speedos are off by around 3-4 mile per hour slower at 60 anyway. I hope this helps.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man


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