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.
'97 Black F250HD SC 3.55s, Punisher trans, PI Converter, Dana 60 RSK w/ V & C SD springs,
PIS 175/80s, HB Jr, Comp910s, Smith Bros PRs, ARPs, Fluidampr, IC, E-Fuel, CJC Traction blocks, HFHV IDM, T4/BASB, 262K
-1987 Buick Grand National (Staged)
-2004 Mercury Marauder
-1993 Geo Prizm (beater)
-2016 F350 DRW Utlimate