My question is in regard to speedometer andcalibration of that speedo. I have a 2004.5 Dodge Ram 3500, auto, 4X4, with the5.9L Cummins. Now I have changed tire size on a number of vehicles in the pastso I have an idea of how it should work. For example, if you go with a largersize tire than what the speedo is set for you will now be reading slower thanyou are actually going - speedo says 65 you might be doing 70.

I bought this truck used and the tires currently on it are 285X70X17. Theoriginals were 265s so I have larger tires than the original. I have noticedthat when driving on freeways at 70 I get passed rather regularly - I know, Iknow, everyone drives faster than 70 but this is much more pronounced than whenI do 70 in my Grand Cherokee. So I did a GPS check on the truck yesterday andwhen my speedo says 70, I'm doing 65. That is the opposite of what it should beso I'm confused. How did this happen?

Something that may be a contributor. I have a Smarty S06 tuner with this truckand I had recently programmed in the tire size. It might be my imagination butthis slower speedo seems to have become more noticeable around that time. Couldthat have an effect on the actual speedo since the Smarty impacts the ECM or ismy speedo more a mechanical one?

Kinda stumped here.

 

Use your Smarty programmer to correct the tire size.

 

This isn't rocket science. The speedo is controlled by the chassis computer and it clocks the rear wheel abs teeth. It is displayed with the DBR3? in revolutions per mile and can be easily changed with the same tool. It is best to do a long trip and compare your GPS log with the trip readout on the speedo. Determine the difference as a percentage. Then adjust the number read by the DBR3 up or down accordingly by the same percentage.