I hope iam not beating a dead horse. My 2001 dak with bigger tires than is stock. I had a transmission guy put a computer on the dash connector , it showed that the truck computer has been re-calibrated to the larger tires. I allso changed the rear end gear from 3.55 to 4.1. I gained 3-4 mpg. In town only, no hiway driving. When I look at the exciter ring in the service manual it doesnt say much about it. But it looks like it sends the speed signal to the computer. The way I see it , I think changing the rear gear , will not change what the computer sees. If so my speedometer is correct. yes?-no?

 

If I remember right, changing your gear ratio will not mess with your speedo

 

I work in a Garage working on cars all the time and we have had this question before from people that are like your self wanting mod there rigs in some way. The awnser to your question in no changing your rear end will not change that reading your speedo reads out, but having the larger tires will now having send that and having read your post thats not a problem for you because you had your speedo re-calibrated to the new tires you have installed n your truck. On a side note what kind of tires are you running i'm just curious?

RitualJman

 

the over size tires was on the truck when i bought it. i think the stock tires was 205 ? the tires on it now are 255. i will look at it tomorrow, to be sure. thanks. now i can put some new tires on for winter.

 

Quickest way to tell if your speed is reading right or not is to use a GPS and see what speed it says and compare it to your speedo as your driveing .

 

Or drive by a mobile police radar scanner.

 

How do you adjust your computer to accommodate the new tires? Mine is anywhere from 2-7 mph off when I lifted my truck and put on the rims/tires... Dealership?

 

i did not adjust the computer, it was like that when i got it, course i didnt know that when i bought it. i think that is done at the dealership. the door post calls out p215/75r15 15xx7, the tire is p255/65r16. now to look for 2 new tires, i want a tire that goes through the snow good. new subject, in 1968 my dad and 1 put a 1958 chrysler 392 hemi engine in a 1959 plymouth, a very tight fit, dad tryed to race GTOs but he was giving 1000 lb, so he couldn't beat them. we changed the rear end gear from a 2.92 to a 3.55. so i changed the trans speedo gear to get the speedo right.