Big tires = Overdrive too early?

[410Fortune]

Okay people I have a question. I recent converted my highly modified bronco to an automatic.

Basically it's an Explorer with a BII frame and body.
I have a 93 OHV 4.0L with a few mods and a brand new cutom built 93 A4LD automatic. I have spent the last few weeks getting all the bugs out since my 5 speed to auto conversion, the kick down cable, shift linkage, etc....

Everything seems to be set up the way it should be.

I have 31" tires, 3.73 gears, and the proper speedo gear installed in the speed sensor (think it's black? 18 tooth?) cant remember, but it is the right gear according to the radar gun.

The problem is the truck shifts into 4th (OD) way to early, like at 1500 RPM's usually, I have to romp on the pedal to get it to kick down back into 2nd, then 3rd, the OD comes too early again......

I'm just wondering if anybody else has had this same problem and how do/did you fix it?


Okay I know the computer controls the torque convertor lock up and the 3-4 shift (overdrive) so a Superchip could be burned to help move the shift points up, but short of that is there another fix? I plan to run 33's and 4.10 gears very soon so I dont want to fork over the $$$$ for a chip until all that is done......



Any idea's?


THANKS!!!!!!!!!!!!!!

 

[GenerationX]

Some info that might help: I have 3.73 gears and I used to have 30x9.50 tires and the tranny did shift early. I can tell because now I have stock size on there now (235/75) and the motor winds up a lot more. at 65mph my tach is at 2100.