Using GPS to calibrate tire size?
02-20-2008, 07:56 AM
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Using GPS to calibrate tire size?
On sunday coming home from a day's fishing one of the guys pulls out his "crackberry' and starts playing with it. He asks me how fast we're going on my speedo, I tells him 118 KPH, he says according to the GPS in his berry we're going 112 KPH.
So I'm wondering if anyone uses a GPS to calibrate their tire size? (assuming they have a BDTD like me, Smarty or whatever so they can adjust the tire size that is)
In my case I'd adjust my tire size a little bigger than it is now and try it again I spose...?
So I'm wondering if anyone uses a GPS to calibrate their tire size? (assuming they have a BDTD like me, Smarty or whatever so they can adjust the tire size that is)
In my case I'd adjust my tire size a little bigger than it is now and try it again I spose...?
02-20-2008, 08:01 AM
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I use mine to tweak the tire size setting on my Smarty. I have BFG 285/70R17s, the spec sheet says they are 32.7". When they are mounted at 60psi air pressure they actually are at 32.1" rolling diameter, measured with level on top of tire to ground. I used that number to set my tire size on the Smarty. Now my GPS and Odometer match exactly on mph. When set to the manufacturer spec the Odometer is around 2-3mph high compared to GPS.
02-20-2008, 08:32 AM
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Some of the towns around here like to set up radar trailers, which flash your speed at you if you are over the limit, and display it steady if not. If nothing else, they are handy for checking your speedo calibration. 
02-20-2008, 08:41 AM
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Used the GPS to correct my speedo.
You would be surprised that even through your tire is a 35x12.50, and you input that in the Smarty, it requires tweaking to get that matched up with the GPS. Sometimes I wonder if the fuel mileage reports that we consistently get are off because people have not calibrated their speedo?
You would be surprised that even through your tire is a 35x12.50, and you input that in the Smarty, it requires tweaking to get that matched up with the GPS. Sometimes I wonder if the fuel mileage reports that we consistently get are off because people have not calibrated their speedo?
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02-20-2008, 09:16 AM
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Used the GPS to correct my speedo.
You would be surprised that even through your tire is a 35x12.50, and you input that in the Smarty, it requires tweaking to get that matched up with the GPS. Sometimes I wonder if the fuel mileage reports that we consistently get are off because people have not calibrated their speedo?
You would be surprised that even through your tire is a 35x12.50, and you input that in the Smarty, it requires tweaking to get that matched up with the GPS. Sometimes I wonder if the fuel mileage reports that we consistently get are off because people have not calibrated their speedo?
Actually, I check the calibration on my vehicles' speedos and odometers separately. I don't trust them to have the same error. To check the odometer, I just watch the mile markers on the interstate and do a little math. I use a ten mile test to minimize errors.
02-20-2008, 09:16 AM
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Tire manufacturer specs do not take any rolling, weighted install into their Circumference spec. They just use Circumference = Pi (3.14) times Diameter (C=πD) as far as I can tell.
02-20-2008, 09:31 AM
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I use it all the time. I had my size calibrated for my BFG ATs. Then when I put the BFG MTs on, I had to recalibrate. Even though the sizes are identical, the extra tread depth adds to the overall diameter. IIRC, I'm using 32.5" for a 285/75R16.
02-20-2008, 01:44 PM
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