Speedometer calibrations

[PDF]

What ways can the speedo be re-calibrated after changing the tire size.

 

[DennyDay]

Just get an Abbott conversion box. The ratio chart will get you real close for the settings, and then you can fine tune it if you wish. I bought mine from Jannetty when I went from 245's to 285's. About a 12 % change. I had him do the install, about 1/2 hour, and make the initial setting. Using a stop watch and a calculator, I stopped 3 times on the way home and made small adjustments, and now it is on the money at 70 mph. Worth the bucks!

Denny

 

[klenger]

If you can borrow a GPS, that is one of the most accurate references I know of for checking spedo calibration, and it's easy to do.

 

[Blakers]

My speedometer indicates about 10% faster than I am actually going. So I use my GPS as my speedometer. Eventually I will just get some bigger tires and that will make my speedometer more accurate.



Blake

 

[RADdodge]

C. D. Day - You don't say what year you truck is. If it's 98 or later, the speedometer is electronically adjusted. Then it depends on how much you change your tire size. If the new tires are within the range of sizes that Dodge supplies, the dealer can change it with the DBR II Scan tool. You will need to know the revolutions per mile which is published by the tire manufacture. Otherwise you need some other method. There is a thread in the Products/Accessories forum by fest3er that describes modifying an Abbott box that will allow your speedometer to be corrected. This splices into your system after the CAB so it won't affect the ABS performance, especially if you have 4 wheel ABS. You still need to know the old/new tire revolutions per mile for the correction factor. There is a product that Rickson sells that will do the same, but it's more expensive than the Abbott box. The GPS method mentioned above is a good way too.