need help here

I don't know about the gear ratio question but when I changed the tire size on my old truck, I called Peterbilt with the tire revolution's per mile, trans model and gear ratio and they told me how to set the little dip switches on the back of the speedo. It was spot on accurate

 

All the newer trucks (can't remember what year they started doing this) the speedo will have to reset through the ECM. You can take it to almost any dealer and they will usually charge about an hours labor. If you are lucky enough and have a older truck then just call the dealer with all the specs and they can tell ya where the switches need to be on the speedo. As for the gear ratio...it all depends on where ya run, what rpms your motor likes, and how much weight ya pull. Also in the factor is how fast ya like to run...for me its only 65, but I like to walk up a hill with only dropping one gear at the most.

 

Both trucks are Peterbilt 379's the older truck is a 1990, the newer one is a 98. The 90 is the one I changed the tire size on and just set the switches on the back of the speedo. I have had the dash out on the 98 to replace the tach and was thinking it had the switches on its speedo too but I don't remember for sure.

 

I have 98 KW with no switches my friend has 99 with switches both have KW in the glass but we don't know if their original for sure.

 

If it's Paccar product, think that it depends on which plant built the truck, in addition to the year model. When I worked for major company, depending on how many replacement trucks we ordered, if they were built at both the plants, there were usually some differences in things like that. If all the trucks were built at one plant, they were usually identical.

 

Let's see. . . put on 22.5s go get calibrated. . . put 24.5s back on . . . yeah no more 62 mph truck.

 

depending on if is low pro 22.5 you may run 67, 68 if you put on 11r24.5