Tire size - speedometer off
Thread Starter
5th Gear
Joined: May 2024
Posts: 5
Likes: 0
Tire size - speedometer off
New 250C owner here and love the car! 2010 with 78k on it.
Tire size question: Previous owner(s) put 225/45-17's all around. When driving on the highway, with cruise at 75mph, Waze reported actual speed at ~71mph.
I realized probable cause is lack of 245/45-17's at the back.
So, with the speedometer being slow, I'd guess miles are being slightly underreported as well. Wondering if that would affect MPG as shown on the board display?
Tire size question: Previous owner(s) put 225/45-17's all around. When driving on the highway, with cruise at 75mph, Waze reported actual speed at ~71mph.
I realized probable cause is lack of 245/45-17's at the back.
So, with the speedometer being slow, I'd guess miles are being slightly underreported as well. Wondering if that would affect MPG as shown on the board display?
Racer
Joined: Jan 2021
Posts: 1,913
Likes: 150
From: CO
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I don't know offhand the percentages, but for sure your odometer and speedometer don't really measure your relative speed to the surroundings, but measure the rpm's of the wheel turns. Then it figures what the equivalent speed should be based on the stock tire at stock pressure. One problem with absolute correctness is that there are also variables such as tread wear, tire air pressure when heated, and difference in exact manufacturers' tire specs. Even two tires from the same mfgr can be slightly different.
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I don't know offhand the percentages, but for sure your odometer and speedometer don't really measure your relative speed to the surroundings, but measure the rpm's of the wheel turns. Then it figures what the equivalent speed should be based on the stock tire at stock pressure. One problem with absolute correctness is that there are also variables such as tread wear, tire air pressure when heated, and difference in exact manufacturers' tire specs. Even two tires from the same mfgr can be slightly different.
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