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i went to tiresize.com to compare the stock 175 to a 185/60 i will be installing. it says the speedo will be running faster. i don't see how this is possible when the newer tire has a greater circumference therefore in one revolution it goes a greater distance. if you were to measure say a mile the 175 would read 60 while the 185 would read less because it does not have to turn the same revolutions to go the same distance. the speedo does not measure distance but the revolutions of the tire. the bigger it is the slower the wheel turns.

what am i missing
 

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i went to tiresize.com to compare the stock 175 to a 185/60 i will be installing. it says the speedo will be running faster. i don't see how this is possible when the newer tire has a greater circumference therefore in one revolution it goes a greater distance. if you were to measure say a mile the 175 would read 60 while the 185 would read less because it does not have to turn the same revolutions to go the same distance. the speedo does not measure distance but the revolutions of the tire. the bigger it is the slower the wheel turns.

what am i missing
Because the larger tire is spinning a little faster at its edge so this faster spinning tire spins the axle faster too, so maybe that is where the speedometer pickup is? (on or attached to the axle shaft)
 

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The first number on the tire size is the width of the tire. the second number is the height percentage of the tire


Tire Width is the width of the tire measured in millimeters from sidewall to sidewall. The first three-digit number in the tire size refers to the tire width. For instance, in a size P215/65 R15 tire, the width is 215 millimeters. Aspect Ratio is the ratio of the height of the tire's cross-section to its width.

If you use tire rack's website, it tells you under specs how many revolutions per mile.If the stock tire turns say 877 and you replace it with a tire that turns 930 revs per mile the speedometer will read faster than you are going.
Most tire comparison websites will have a table that shows what the speed difference is or... use an app on your phone to give you speed.
 

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still don't "get it"

The wheel turns the same amount. What you are missing is that that same revolution takes you a little further than the speedo "thinks"! It is correct, your speedometer will read x revs per amount of time and convert that to y miles per hr. The larger tire is actually taking you further than that so the speedometer reads slower than you are actually going.

Reading your more recent post. Which is turning what? The ground is turning the tire, or is the engine turning it?
 

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You have an approximate 3 mph error from smart, as in at 70 you are going 67. Going up one tire size brings you even. Not guesswork, by GPS.
 
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