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315s and actual speed vs. speedometer?

Discussion in '80-Series Tech' started by CodyRx7, Aug 18, 2006.

  1. CodyRx7

    CodyRx7

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    I tried a search for this, but didn't see what I was looking for.

    I know there is a speedo correction when you go to larger tires - but how far is the speed off going from stock LTX's, to 315 Terra Grapplers?

    If I had to guess, at 55 on my speedometer, I'm about 7 mph faster than the other cars.
     
  2. cruiserman

    cruiserman

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    Easiest way is to use a GPS to get your speed. Divide by what your speedometer indicates, and that is your correction. Don't forget your odometer is off, too.
     
  3. SOCALFJ

    SOCALFJ

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    or drive next to a new civic or a hybrid lol
     
  4. pulse98

    pulse98

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    Yeah, GPS is easiest.

    Another ways it to count the mileage markers on the highway and see how far off your odometer is compared with those markers. The percentage error of the distance will be the same percentage error of your speed.

    Yeah, GPS is easiest, haha.
     
  5. landtank

    landtank SILVER Star

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    doing the math there is a 12% change in tire diameter when going to 35s

    so if you are going 55 mph x 1.12 = 61.6 mph

    Just rough estimates as I rounded off the tire sizes to 31 and 35
     
  6. NorCalDoug

    NorCalDoug problems solved daily...

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    with my 315/75 R16 MTRs, my speedo is 10% off true speed when checked against my GPS.

    actual tire sizes vary from advertised sizes, so the actual speed doesn't always match the mathematical calculations
     
  7. hoser

    hoser SILVER Star

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    Yeah, ditto on the GPS. The OEM speedometer is often inaccurate to begin with!

    Speedometer Scandal
     
  8. landtank

    landtank SILVER Star

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    you also need to remember wear on the tires. When I calibrated my truespeed I roughed it in and then did a final when the tires were about 1/2 way worn. There is quite a bit of error between new and old tires at that size.
     
  9. Trapper

    Trapper

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    Using GPS, mine is off 10%. Makes for easy calculation in your head when you don't have GPS/speedo correction. Probably was a little more (+1%) when the tires were new. I've got 33,000 miles on my MTRs now.
     
  10. Martin White

    Martin White

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    Another vote for GPS. Use it for a while and do an odometer calibration while you're at it. It should be more or less the same % adjustment as your speed, but that doesn't always seem to be the exact case, though it will probably be within about 1-2% between the odometer and the speedometer.
     
  11. Walking Eagle

    Walking Eagle

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    Add one more data point for 10% off with 315's. I've checked it several times with my GPS, at different speeds, and always essentially 10% off.
     
  12. CodyRx7

    CodyRx7

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    Ok...

    Everyone was saying its around 10%-12% difference in actual speed vs. the speedometer.

    I had another vehicle next to me communicating via cellphone, and at 50mph, she was doing 11-12mph faster than I was.

    Thats more than a 20% difference.

    Is that even in the realm of possibility - or are 1 or both of our speedometers incorrect?
     
  13. cruiserman

    cruiserman

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    Too many variables to say. Use a GPS :).
     
  14. CreeperSleeper

    CreeperSleeper SILVER Star

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    Just do what I do... guess.
     
  15. CodyRx7

    CodyRx7

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    "Son, do you know how fast you were going?"

    "Nope."
     
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