Changing tire size with EFI

[DetailsUnlimited]

Need to change the tire size and I know how. But the question is that when changing size in the tune to 285/70/17 its 32.71 inches and 830.83 mm. Drive ratio is 3.73. But in the trans tune it shows 4.10 final ratio and the tire size is a few(830.80) mm off. Do I manualy correct or whats the correct fix.

 

[TrentNell]

Need to change the tire size and I know how. But the question is that when changing size in the tune to 285/70/17 its 32.71 inches and 830.83 mm. Drive ratio is 3.73. But in the trans tune it shows 4.10 final ratio and the tire size is a few(830.80) mm off. Do I manualy correct or whats the correct fix.

Leave the trans set to 4.10 , make all tire size changes in the ecm just as you have already done , I usually use a GPS to test and adjust from there .

 

[DAVe3283]

x2 don't worry about the trans. I never do.

A good way to get the speedometer dialed in exactly is to start a trip meter on your truck and your GPS, and drive as far as you can. Straight is better (highway). Then, write down the truck's distance vs. the GPS distance. Use a little math and you'll have a speedometer that is dead on.

 

[SgtKilroy]

Instead of just putting in the numbers from the sidewall you can also call up whoever made your tires and they can tell you what the exact measurement of revolutions per mile your tire has. Then input that number directly into your speedometer adjustment tool in efi live. If you want to adjust the transmission shift points do the same thing to your trans tune. If I remember correctly the final drive ratio is a combination of gear ratios in your drive line. Your drive ratio is the ratio from your actual differential. 4.10 is correct for final drive ratio assuming you haven't changed your ring and pinion gears.

 

[S Phinney]

I always do a run out on the tire. Mark it a do one complete revolution and put that distance in the circumference spot. Has always worked out well for me. Don't adjust in the TCM.

 

[LBZ]

Leave the trans set to 4.10 , make all tire size changes in the ecm just as you have already done , I usually use a GPS to test and adjust from there .

I do it the same way. I take it up to 100km/h or 60mp/h and just match the speedo to the gps. Easiest and fastest way to do it. I've never touched the TCM tune just changed the ECM.

 

[DetailsUnlimited]

I guess the mod I dont have for my truck is the gps! I just use my I phone. Thanks for the schooling guys. Good to know how to more accurately dail the size tires in.

 

[Redbowties88]

i corrected the tire size in my tcm tune, seemed to help my buddy's truck (with larger tires) quite a bit. i also dropped the final drive down to 3.73 becuase when i saw it since im running a different os then came in my tcm i thought it was just wrong didn't seem to change much though

 

[05smoker]

Leave the trans set to 4.10 , make all tire size changes in the ecm just as you have already done , I usually use a GPS to test and adjust from there .

X2 - My new 285s were shorter and I was off 2 mph from my GPS.