I am getting ready to go up in tire size with my 48re and 4.10s, I am assuming this will cause later shifts and lockup?

If I change the tire size in my smarty to the new larger size will the speedo only be corrected and shift/lockup still late or will the speedo and shift points both change?

TIA, Eric

 

I really do not notice a later shift, but my RPM's are definatly lower.

I am judging my shift points off the RPM's, not the speed. I lock up around 45 when I am going slow.

 

Is this after changing the Smarty tire size function?

 

Yes it is.

With changing the tire size, I normally get closer to the speed by measuring the tire from the top to bottom, and putting that size in the smarty. For example, my tires are 37's, but I am within a mph or two at 65 with 35.75 on the smarty.

 

The speedo and the trans control rely on the ABS signal, when you reset the tire size it adjusts both the speedo and trans control.

Like madhat said, measure the rear tire as it sits on the ground and you will come a lot closer to the actual size. On my truck with 33" static tire height I had to back the tire size in the Smarty down to 32.5 " to get the speedo within 2 mph of a GPS.

 

10-4 fellas, I am thinking about leaving it alone b/c my 1-2 shift is so early and 2 could be held a little longer. My 3-4 shift is great though. Theoretically it should delay them if I don't update the size, correct? I am just hoping for a couple of hundred RPMs on the delay nothing huge.

 

I might add with 4.10s its amazing how quick it wants to go through the gears compared to a 3.73 truck. When I first bought it I thought I would hate it, but every time I drive a 3.73 truck I miss my 4.10s.

 

Ya know... maybe I'm missing something, or it's the fact that I drive around with about 550 hp every day, and my truck acts different... I really did not notice the change in shift points. My truck shifts differently for my different driving habits.

I guess if it bothers you, then play with the speed adjustment until you find the range you like. You will just have to know what rpm is what speed, unless you have a gps.

 

You will get slightly later shift points with bigger tires. With 4.10 gears I would see that as a plus!

RJ

 

Yes I am thinking it will be a good thing

 

Not really, with the Smarty and removing some of the TQ management you want early shifts to use the TQ and keep the rpm's down. I always figured the shifts were too late, especially under hard throttle. The problem with that is the line pressures are low on a stock unit and you slip clutches. Smarty is not easy on things in the bottom end of the power band. Thats why a shift is really needed to make things live longer.

Its a challenge to get enough TV pressure so they don't shuttle shift under light throttle and not too much that it hangs the shift points to redline.

 

When you put larger diameter tires on, the "indicated" speed at the various shifts is the same, but the truck is going faster. Correcting with the Smarty or doing a "pinion factor" adjustment changes the shift points relative to the actual speed of the truck but not the indicated speed on the speedo.

This is one reason it can feel sluggish. Mine did when I went to 315s from 265s, but was still running stock power. After doing a pinion factor adjustment it drove a lot better. Of course, more power makes up for the higher gear ratio that larger tires bring.

 

I played around with tire sizes tonight on the smarty, regardless of what I input for tire size the truck seemed to lockup around 45-46 verified by GPS. This is telling me the trans. is not using wheel speed for lockup/shift strategy, rather output shaft speed.

If this is true then no matter what you input for tire size on the smarty lockup will still occur at the factory set output shaft speed, but different wheel speed depending on how much one changes their physical tire size. Whether or not you update the ABS module with the new tire size will determine if you see the actual lockup speed on the speedo.

Let me know if this sounds about right?

 

Thanks for checking that!

 

I added a shift enhanser from DTT to help with the shift point.With a bunch of power and tall tires it isn't necessary but it is much nicer to drive and easier on the equipment