HOV

When I put on my 285/75's, I reprogrammed the tire size with Smarty Jr. The truck still shifts at higher speeds.

However, this may be due to the DTT Shift Enhanser I installed the next day...

I like the higher shift points. It makes OD lockout a no-brainer in city traffic, whereas that setup was a little frenetic before. It makes 2nd taller - comfortable up to the mid-30 MPH range - and 2nd is important when towing heavy with a trans with only 4 gears.

MTC

So, let me see if I got this right?

I just put 285/75's on my truck, and the rpms are very low compared to where I was with the stockers. So low on the highway that it feels like a turd now. Will the truck shift and drive better if I change the tire size with the Jr. back down to a stock size tire?

sloride

MTC, you wanna change it to the new tire size so that the speed you are seeing on the speedo is the correct speed. Our trans shift points are determined by speed. I assume the 07's are the same as the 04's and up.

MTC

Immediately after mounting the tires, I corrected the size with the Jr....The rpms just seem to be too low now

I mean, I knew the taller tires would affect the rpm's, but not this bad

sloride

Those tires are only 33" tall(almost 34) so it really shouldn't make that much difference going from stock. Did you check your speed with a GPS after you adjusted for the tire size and make sure your speedo was correct? Sounds like it may be off.

MTC

No, I havent checked with the GPS...Ill do that tomorrow and see whats up.

Before the taller tires, my rpm's @ 70mph were right around 2K. Now @ 70 mph my rpms are 1650-1700...It just seems like a turd there.

sloride

I have 4.10's so I have no idea where your RPM's should be.....sorry.

WhiteMule

I calculated that you should be at 1788 rpms using 33.86" tires @ 70 MPH.
Check your speedo.

MTC

Thanks Mule, Im going to check with my GPS and see whats up.

WhiteMule

Try this http://www.csgnetwork.com/multirpmcalc.html
Plug in .69 for OD, 1.00 for Drive.