Speedometer programing.....just some info - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 3 (permalink) Old 05-09-2011, 09:40 PM Thread Starter
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Speedometer programing.....just some info

Went from 265/75-16 Coopers to Nitto Terra Grappler 285/75-16 tires. So I wanted to reprogram my speedo to display the correct MPH.

I go onto Nitto's website and it states for my tires that the diameter is 32.76".
I do the calculation (20186/32.76)=616.17 then do (616.17/.6666)= 924.35. So I use the 924 as my calibration code. I go for a drive, the GPS unit states that the speedo is 5 mph fast. I don't believe the GPS and use a roadside radar signboard and it also states that the speedo is 5 mph. The milemarkers do not coincide with distance vs odometer. When I'm cruising down the road, with the cruise set on 73, I'm really doing 68

Yesterday, I do the rolling measure method. I stick one of those plastic headed tacks into tread of the rear driver's tire. I roll the truck by hand until the tack touched the pavement. Placed a piece of duct tape on the pavement and marked the location of the tack. The tire makes three revolutions and I mark the pavement in the same fashion.

I measure between the two marks and get a total of 24' 8 1/4" or 296.25"
296.25" / 3 = 98.75
98.75 / 3.14 = 31.44" diameter

I do the calculation for the speedo programming and get a code of 963. I did not program the speedometer yet.

What I've learned from this, don't trust the manufacturer's tire spec.

Billy T.
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post #2 of 3 (permalink) Old 05-10-2011, 06:01 AM
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Re: Speedometer programing.....just some info

According to the tire size calculator at miata.net Tire size calculator , that measured diameter is more like your old Coopers. Plug your new size in, and it comes out about the same as Nitto's web sited. Did you double-check the size marked on the sidewall?
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post #3 of 3 (permalink) Old 07-12-2011, 02:18 AM
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Re: Speedometer programing.....just some info

Tire specs vary from each manufacturer. Do you have the tire on the specified wheel width? How much air do you have in the tires? How much of a load do you have on them? All this has to be taken into consideration

I've seen identical tires, bought at the same time, that were not the same height. I personally think that there should be some kind of standard for tires so that every tire marked 285/75R16 from every manufacturer is the same size.
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