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Hello,
I have been trying to get my speedo corrected on my 2006 Cummins with 35" tires. I have changed the Pulses per mile as well as the Tire revolutions per mile using the formulas I have found on the forum. So far I haven't seen any change when I go out to compare the speedo to GPS. Am I missing something or doing something wrong?
Thank you,
Josh
Have you figured this out? I am trying to get this figured out as well...
what did you end up using for tire rev? I just switched to 35s and calculated 593 tire revs per mile which is over double of the factory 250?
found this in another thread, is this what you tried?
Adjusting the speedo is easy. You take the stock size tire. Change it to inches to make it easier..
In my case 05 Dodge ram diesel came with 31.5 inch tires
I got 35 inch tires.
You take 31.5/35 = .9
Take .9 and multiply it by the VSS / Mile setting (10% bigger overall)
8000 *.9 = 7200
Calculation is always (Original Tire Height/New Tire Height)*VSS/mile.
The theory is your gonna see 8000 pulses per mile because your tires are 31.5. You should see less per mile if you have bigger tires, and more if you have smaller.
Hope this helps! Worked great for me.
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.
thank you I just got the tuner and this is the main purpose that I bought I for
Thanks Jim! this worked great
Thank you so much for this tip. I have been chasing my tail since forever. This worked 100%.
@Jim P 2.0
I know this is an old thread but I have not been able to get this to work. I made the adjustments as outlined by Jim and my odometer is still off by a little more than 10%. The speedo is spot on with my numbers. Anybody got further recommendations to try? I have wrote the tune file with both the latest Beta and the latest good software. Neither have worked beyond correcting the speedo. Any recommendations?
2005 Ram 3500 MT
What I had wrote way way back is the way, if it’s not working could be something with the software or something elseQuote:
Originally Posted by John05Cummins
Don’t feel bad , my trucks the same way . Mph is spot on and odo is off by 1.14Quote:
Originally Posted by John05Cummins
Seems odd the odometer would still be off. Maybe try alfaOBD, that’s what I use on my 2012 for speedo correction and odometer has no mismatch issue
Thanks Jim. I am going to go back to stock settings and then try again and see if that makes a difference. I couldn't get my AlphaOBD to connect to the truck. It did not like the fact it was a 2005. I ended up selling it.
Spooled05 - maybe it has something to do with our very weird year. If I figure out how to get it to work I will let you know. I was planning on getting an Edge CTS3 Monitor and it is supposed to have the ability to adjust for tire size too...maybe that will work. Have you had issues with tuning other functions of the ECM?
On a side note, I do appreciate the wisdom you have tried sharing with others on this forum Jim. Trying to soke up what I can before I start making actual engine adjustments.
For a side note, the Edge CTS3 won’t change anything on your vehicle, it’ll just adjust the speed readout on the monitor itself.
John , I have not had any other problems with tuning at all , just the odo being off .