Minimax tire height / speedometer sync - Ford Powerstroke Diesel Forum
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Old 04-21-2013, 05:34 PM
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Minimax tire height / speedometer sync

I was wondering what tire height you guys are using with either 35's or 37's to get GPS speed to exactly match speedometer?
Seems I am always off a few - then I forget to write down height lots of times, then go to change but cant recall previous setting
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Old 04-21-2013, 06:29 PM
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Depends on the tire, the same "size" tire from a few different manufacturers can have different heights. Check the manufacturers site for an exact measurement. I run 325/60-20 toyo's and I put in 34.75 IIRC and its dead on.

Good idea to check with GPS, I downloaded an app for my phone that goes down to the tenth of MPH to make sure I got it right.

if your still running the Toyo MT 315/70-18 in your sig, they spec'd out new at 35.7
with 21/32 of tread depth.
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Old 04-22-2013, 06:00 PM
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I still have the 315's - over half gone...
I run a bit lower air pressure, so is tire height really the distance from the ground to the center of the tire x 2 - so as to really be rolling radius / diameter?

I am not off by much, but damn, GPS and speedo are nice when exact match!
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Old 04-23-2013, 10:45 AM
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I always run into this problem myself and have yet to find the perfect calibration of the two, and of course when I find one a measurement that's close i forget what it was when i reload another tune. The closest number that works for me is 34.75 for a 325/50/22...Now if I remember correctly though, H&S suggests to measure the actual tire height as it sits on the truck not just the manufacturer's spec's, but I can't remember where I read that. The question I've always wondered with this is does the tire measurement change the speedo of the truck or just the speedo on the tuner?
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Old 04-24-2013, 02:04 PM
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I believe it changes the speedo on the truck as well, not that mine ever agrees with the readout on my blackmaxx. I forgot to mention that, I don't bother with the speedo on my truck as it seems to change slightly everytime I get in my truck, some times it reads slow sometimes it reads fast sometimes it dead on, where as the display on my black maxx always reads correctly.

I thought speedometers run off the rolling circumference of the tire, they ask for the height as its easier to determine and because the circumference can be calculated from the height.

Air pressure can effect things as well.
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Old 04-24-2013, 03:40 PM
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34.5 matches me exactly to my GPS.
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