Speedo correction 2007 Ram 3500

[cscott]

How do I modify the speedometer on my 2007 Ram 3500 5.9L? I've tried the typical settings under Speedo -> Calibration. None of them had any affect. The little bit of info I found in this forum didn't have an answer either. Is it even possible?

[Jim P 2.0]

There’s a thread somewhere in the dodge section where I explain exactly how to do the speedo correction

[Jim P 2.0]

Your other option would be to buy alphaOBD and use that if HP Tuners isn’t doing it.

[cscott]

Your other option would be to buy alphaOBD and use that if HP Tuners isn?t doing it.

Thank you. For posterity's sake, here is the [URL="https://forum.hptuners.com/showthread.php?75249-2006-Cummins-Speedo-Calibration&p=623672&viewfull=1#post623672"]post[/URL

What I do is go for a drive, set cruise at a speed, higher is better but whatever is safe to set to and within speed limits, say 70mph, check your speed with GPS. Record the two speeds, observed vehicle speed from cluster or scanner and actual GPS speed. Divide observed by actual. If you set cruise at 70 and GPS says 75 then, 70/75=0.9333333. Multiply VSS pulses per mile by the 0.933, 8000*0.93333333=7466.66664, copy that into VSS pulses per mile.

The tire revolutions per mile is scaled wrong, the number for this parameter is NOT 251, its 654. But anyways, multiple 251 by the 0.93333333, 251*0.93333333=234.2667, copy that to the tire revolutions per mile.

Finally your final drive ratio high and low, multiple those by the 0.93333333, 3.54*0.93333333=3.30399999, copy that into the final drive ratio high and low cells.

Change ALL these cells as they all play into things like your speedo reading, odometer reading, out-of-gear detection, transmission parameters, etc. I wouldn't use tire size. A 35" tire if you look at manufacture specs, the nominal size can vary between manufactures and could be something like 34.1", now factor in loaded tire dimensions, it's going to be less than that. Using vehicle speed reading versus GPS speed takes into account all factors that will be affecting tire size.