Is speedo recalibration manditory? And explain shifting points

[notthejeepnomo]

Hello,
Question: changing stock 96 fzj 80 r/p to 5.29 not changing tires, yet(yes i know it will run 4000 rpm at 75mph, don't care) what i do care about is is this going to screw with the transmission shifting? I understand that rpm will increase, inversely actual speed will decrease. However im not sure how the transmission (ie stupid computer) will be effected by the new descrepency between higher rpm and display speed on speedo and actual slower mph.
will it still shift at the correct rpm or will it hold out until i get up to a certain speed. Or exactly what will happen by not recalibrating the speedo. Will there be any codes.
I realize the speedo will be incorrect vs actual speed, but will the car go from Point A (zero MPH) to point B (65mph) and run through all the gears like it is supposed to. Also say i am not accelerating , but traveling at a constant speed say uncorrected speedo speed 40mph/ actual speed 20mph (bare with me this is a rough approximation) will the tranny want to shift into 4th (overdrive) since the uncorrected speedo is 40 or would it be in 2nd since the actual speed is 20mph
Thankyou for your responces.
P

 

[d0ubledown]

somone will correct me if im wrong...but i dont think the ecu looks at the actual speed figure to control its shift points. most likely it depends on things like throttle position, rpm, load...etc iirc, calibrating the speedo requires a speedo gear thats available from slee. i dont think the ecu will know if youve changed the speedo gear or not. only you will know once you see the needle at the corrected speed.

 

[landtank]

the ECU does look at the speed sensor for calculating the shift points. I used a truespeed calibrator on my truck and a yellow box on a build for someone else. I also understand that Slee offers something as well.

The truespeed allows two tire sizes and uses a switch to select which one. The yellow box doesn't but can be selected by setting switches to a different calibration. I'm not sure about Slee's.

I'd go with the cheapest one unless one was simpler to install and that made a difference to you.

 

[notthejeepnomo]

so what would happen if left uncorrected

 

[landtank]

it would shift at the wrong times. If you are asking if something will blow up, I don't think so. But the yellow box is something like 100.00.

 

[Tools R Us]

Is it mandatory, IMHO no, but you may want it. It's not going to damage anything running that way.

I don't have any experience going that way, but run 37" tires on a stock drive train and it's drivable. The rig determines speed from the sensor on the transfer output, so doesn't know what tires, diff gears are in it. The relationship between tach and speedo will remain the same as viewed when driving, but you will be going slower than indicated.

Shift points are determined from indicated speed and various motor sensors. With your setup load will be lower for the same speed, so shift points will be slightly off from stock. How much that is noticeable, affects the drivability, etc, is up to you. I would install the stuff then decide if you want/need more changes.

 

[landtank]

when I had the truespeend installed one switch position was uncalibrated (stock) and the other was calibrated. While driving down the highway I could just switch from one to the other and the tranny would down shift. This was when I had stock gearing and 35" tires. So in that case it made driving the truck much nicer.

In your case I'm not sure what would happen.

 

[UWdave]

when I had the truespeend installed one switch position was uncalibrated (stock) and the other was calibrated. While driving down the highway I could just switch from one to the other and the tranny would down shift. This was when I had stock gearing and 35" tires. So in that case it made driving the truck much nicer.

In your case I'm not sure what would happen.

I just lifted at put 35s on stock gearing and noticed that sometimes it does nasty down shifts to high RPM if you jump on the throttle. I wasn't sure if this was because of load or speed.