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New tires: speedometer is reading 2mph above actual speed

Discussion in 'Gen 4 Prius Main Forum' started by Torrential, Apr 16, 2024.

  1. Torrential

    Torrential New Member

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    Ideas appreciated.

    Stock size Michelin X-Tour put on earlier this month. Three tires were from the same batch, one is from a different batch. Oddball is on right rear. Speedo reading is 2 mph higher than GPS readings.

    Where is the speed sensor? Could one too-small tire cause this?
     
  2. fuzzy1

    fuzzy1 Senior Member

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    Speedometers traditionally read slightly high, this has been standard industry practice since almost forever, for both practical and legal reasons. And a topic of very many threads here. Be glad you are in the U.S., as European offsets are (or used to be, back in the days of mechanical speedometers) significantly higher.

    Did you make a similar speedometer calibration check before this tire swap? If so, how much did the offset change?

    What is the RPM (revolutions per mile) spec on this tire model? What was the RPM spec on the tires you just took off? Do their RPM specs match, or are they a bit different? (An additional difference between new and worn tread depth will also factor in, but not as straightforward as simple circle equations would suggest.)

    Beware that speedometer and odometer offsets and errors are not the same. And deliberately so.
     
  3. jdenenberg

    jdenenberg EE Professor

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    In addition, a worn tire has a reduced radius by about 1/4 inches. that is a reduction in circumference and therefore an increase in revolutions/mile. New tires (same model/size) will therefore rotate slower at the same speed and the speedo will read lower with new tires. Note that this also means that the new tires will cause an apparent reduction in MPG.

    JeffD