FAQ about the entry on
automobile tires

New Tires and the Speedometer

The original equipment tires on my vehicle were P195/75R14, and I bought 4 tires from a tire dealer which are size P205/70R14, which are supposed to be Optional for my vehicle. My question is, how will this difference in size effect the reading on my speedometer? Will my speed on the speedometer slow down or speed up, and will I have to change anything in my speedometer? I have a 1994 Plymouth Voyager Minivan, with a 2.5 liter engine and automatic transmission. <name and email address withheld>

Reply

It will make a difference, but such a small difference you may not want to bother with it. Let's begin by estimating the distance from the rim to the road with the two different tires. We can get this by multiplying the width of the tire (the first number in the tire designation) by the aspect ratio (the second number).
Distance from rim to road, original tires (P195/75R14): 75% of 195 mm = 146.25 mm
Distance from rim to road, new tires (P205/70R14): 70% of 205 mm = 143.5 mm

The difference is only 2.75 millimeters, about a tenth of an inch.

The diameter of your rim is 14 inches (which is 355.6 mm). We need to add two rim-to-road distances to get the diameter of the tire on the rim. For the original tire, 355.6 mm + (2 × 146.25 mm) = 648.1 mm. Multiply this diameter by pi (about 3.14159) to get the circumference, = 2036 mm.
Doing the same thing for the new tire,
(355.6 + (2 × 143.5mm)) times pi = 2018.8 mm.

With each revolution of the wheel you are moving about 17.2 mm (two-thirds of an inch) less far, or, percentage-wise, 0.8 percent (less than 1 percent). For this difference to be detectable on your speedometer, at 60 miles per hour you would have to be able to read the speedometer to the nearest half a mile per hour (60 × 0.008), plus the speedometer itself would have to be that accurate.

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