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Question for those who have recalibrated their Speedometers with a Tazer and/or the like.

How do you measure tire size?


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Gren71

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i swear my GPS apps must be off cause I had to set my tires to 31.25(ish) and I am within 1mph...I'm running measure 33in
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KurtP

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i swear my GPS apps must be off cause I had to set my tires to 31.25(ish) and I am within 1mph...I'm running measure 33in
gps accuracy can fluctuate and different units have different sensitivities to location distance and vector. Gps over open terrain is best, and theres always radar stands etc.

if you measure 31.25 and set for 33, your revs per mile is off by a touch over 5%, which would have your speedo off by about 3.5mph at 70.

my guess is your tires that say 33” actually measure about 31.75 if you use the circumference of the center band method to measure. That would have you off by 1mph based on revs per mile calculation

this is why i advocate for a circumference measure method.
 
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XraytecH

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i swear my GPS apps must be off cause I had to set my tires to 31.25(ish) and I am within 1mph...I'm running measure 33in
Yeah the number on the side wall almost never match the real running size. My 35" DuraTracs are actually 33" at 35psi. If I inflated them to the max, 65psi, they would measure 34.8 according to Goodyear.
 

KurtP

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Yeah the number on the side wall almost never match the real running size. My 35" DuraTracs are actually 33" at 35psi. If I inflated them to the max, 65psi, they would measure 34.8 according to Goodyear.
that would be such a comfortable ride. Hahahaha
 

SquirrelNuts

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I'm having a hard time getting MPH properly set with my Tazer. Got a JT with Max Tow package. Went up to a 315/70r17, and measured hight of tire at proper PSI using the 3ft level method off top of tire (measured 33.25). I set the Tazer accordingly, still off by 1.5mph according to two different GPS apps I have... adjusted now a few times and I'm all the way up to a 35" measurement and I'm still off by about 1.5mph. 🤦‍♂️ I guess I'll keep adjusting, but didn't think I'd need to be that far off actual measurement to get it calibrated properly. At this rate I'll be at a 37" before getting things set... hmmm.
 

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CrazyCooter

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I posted the most accurate may to manually measure with tools anyone has above.......Mark the tire and ground, roll vehicle/tire one revolution, and measure distance traveled......Divide that measurement by 3.14....This is your loaded tire diameter.

Also plan on your MPH to be high by 1 or 2 to get your odometer perfect.....Everything I've worked on has that little bit of error to the high side to CYA.
 

ElDirector

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I used the size from the manufacturer, and then adjusted slightly to match my GPS-indicated speed. Was only off by 0.2 MPH at 60.

Actually feels... strange... to have an accurate speedo in a Jeep.
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