pretty much what the title says will bigger tires than stock make the truck shift later? i will be going from the stockers to pro comp xat in a 305 60/17 thanks

 

depends if you tell the PCM the new tire size or not.

 

Yes it will change your shift points and our final drive ratio. From small tires to bigger like you want it will raise the speed that your auto trans shifts. Example if your truck used to shift into overdirve at 50 mph it will then shift into OD at 55 with the larger tires.

All of this will occure reguardless of if you reprogram the PCM. It will however change the odometer/ speedo if you don't reprogram.

 

ok so i have a smarty so all i have to do is change the tire size in the settings and it should remain the same correct. really what im asking is that if i have a 33 inch tire and want it to shift earlier i would put it down as a 31 inch tire can you do this or will it mess up the trans? thanks guys your great help

 

Bump...i wanna know too

 

My 06 auto 410 gears actually shifted and drove better when I put the 315/70/17 tires on it with no corrections to computer. Occasional 1-2 hunt was gone and a pleasure to drive. Don't know why.

 

I`m no expert , but I don`t think it will do anything other than make your speedometer read fast...I`m pretty sure it doesn`t control the trans shift points.

Of course I`ve been wrong many times before...

Why don`t you try it and see what happens , then let us know for sure

 

Yes it will change them. Your speedo will be off and our trans shift according to speed so if it shifts at 49mph now, 49mph after the tires will be in the 50's for actual speed. You're not going much bigger so it may not be noticeable at all. Smarty will fix.

 

I did not realize that , Ill have to give a try.
I hate that early shift into overdrive.
10 miles an hour higher would be nice.

 

when does yours shift into od doug?

 

I would say about 50 mph. At that point the rpms are a little low .
So theres more heat than I like to see, when accelerating slowly onto a busy highway.
Delaying the OD shift would keep the Egt's about 200 / 300 degrees cooler.
I'm going to try entering a 28 or 30 inch tire size into Smarty.
32.5 is currently entered.

 

You need to regear when you add larger tires. Regearing will put your engine back into its peak powerband and return your truck to stock performance.

 

Well setting the ABS tire size to 28.00 did delay the shift into over drive .
That was very annoying, seems alot better now.

 

My o6 megacab 4:10 rear drove so much better after I installed my 35's I have a smarty and all I did was tell it the correct tire size and set for 60 HP bost and thats it. Never had a shifting problem or do I need to change the gears. Now when I am on the HW at 70 MPH I am below 2000 RPM's used to be 2200 RPM's

 

You really dont need to regear with a cummins, it has plenty of power/torque to overcome pretty much any tire you put on it. If you tow heavy its a good idea so you arent lugging as bad(but even if you have a manual trans you can drop a gear to get the rpms back up when climbing a grade)

I had a 4" lift w/35s on my '05(48re/3.73s), and got 16-17mpg, stepped up to 37s, got the same fuel mileage, even a hair better on the highway with the lower revs, the added weight of the 37s really never made a difference either.

You dont have to regear a truck with larger tires to still get great performance, especially since almost no one has a bone stock truck, any programmer/box/CAI/exhaust is adding more than enough power to overcome the larger tires.