Is there a DIY something i can adjust to change the tranny shift points as i have installed bigger tires? I went from stock to 33 inch on 18 inch wheels. I am seeking better fuel mileage... 2002 F_250...

 

Do you want it to shift earlier or later? I don't understand why changing tire size would cause a change in when it shifts.

Either way the only way to change it is with software.

 

The irony of your statement aside, you'll need to have the speedometer recalibrated. That requires a compatible tool to reprogram. With the speedometer recalibrated, the shift points will be back to a more stock-like feel.

Shift points are determined in the tuning by pedal position and speed in MPH. If you throw off the speedo by adding larger tires, the shift points are then affected. Slightly.

With that said however....... Going to a 33 inch tire over the factory 31.5" tires is not a huge difference, so recalibrating it won't make much of a difference on your shift points to begin with. And you won't be getting better fuel economy either.

 

No, the shift speeds are not affected AT ALL by changing tires. The trans shifts by the rotational speed of the rear axle. It does NOT matter what speed the truck is actually moving over the road. Changing the tire size does not affect the relationship between engine speed and rear axle rotational speed. It affect the relationship of the rear axle rotational speed to actual vehicle speed, but the computer doesn't care what the actual vehicle speed is. It just doesn't matter.

If you reprogram to make the speedometer read accurately it also doesn't matter. The trans software simply doesn't care. It looks at the rotational speed of the rear axle, and you can't change that.

 

Exactly.

The PCM cares about how many teeth on the ring gear reluctor wheel pass the vehicle speed sensor pickup in a given period of time. This is what the PCM uses for vehicle speed. While the shift points are in fact based on accelerator pedal position and vehicle speed, changing the tire size has zero effect on the relationship between the accelerator pedal position and the speed that the ring gear in the rear axle is spinning. If the PCM commands a shift at 35 MPH (indicated) with 31" tires at a given load, it will STILL command the same shift at 35 MPH (indicated) with 44" tires if the load is the same.


The distance from the centerline of the axle shaft to the outer circumference of the tire is what changes the ACTUAL vehicle speed in relation to the calculated (indicated) speed based on the ring gear's RPM. When the ring gear is spinning at 300 RPM with 28 inch tires, it's going to spin 300 RPM with 44 inch tires and the indicated speed on the speedometer will read exactly the same but the vehicle itself will be traveling MUCH faster.

The ONLY reason that the shift speed would change slightly based on tire size would be due to load and the accelerator pedal position required to attain the same speed and rate of acceleration based on the increase in drag/power required to get to that speed. By going to a larger tire and increasing the load on the engine and decreasing the amount of actual power to the pavement at a given axle torque, the shift speed would actually increase a tiny bit.....but don't confuse that characteristic with any tuning changes or PCM calculations that change simply because of tire size changes.

 

Well, this is one of those subjects that is hard to understand even when clearly explained. I am just going to accept the explanations as true, even though it is way over my head. Keep up the good work Mark and Cleatus!

 

Mark, I think you missed what I was getting at. Cody kinda explains it but in a very long and round-about way.

Yes ring gear doesn't change, hence when you change gear ratio that has zero effect on the speedometer.

However, shift points are determined by vehicle speed and pedal position.

If you change tire size, you've now changed the ACCURACY of the speedometer, hence vehicle speed input has been altered. If you alter one of the inputs, you alter the outcome.

So while the truck might still shift at the speedometer indicated 45 mph mark for "X" pedal position, the truck itself is no longer actually traveling at 45mph.

What that means is the shift points to the driver feel different, because they are different. Shifts are now occurring at a different RPM. For example. You go from stock to a much larger tire. The speedometer indicates you are doing 45 mph but are actually traveling at 55 mph. However, the transmission tuning at that pedal position wants to shift to OD at the speedometer indicated 45 mph. Now re-calibrate the speedo so that it reads actual speed. What are your RPM's now that you shift at 45 mph instead of 55 mph actual speed? See where I'm going with this? Hopefully that explains it more.

But as I stated before, the effect can be slight, especially when there's only a small jump in tire size. Recalibrating the speedometer will correct what the PCM sees in terms of actual speed, which will slightly alter when the truck shifts.

I've had first-hand experience with this several times. Having the lift on my truck, re-calibrating the speedo, then re-gearing, then taking the lift off and going back to stock, then re-calibrating the speedo again. It's really fun to go back to much smaller tires while the speedo is still calibrated for very large tires. Yes, speedo calibration does have an effect.

 

No, I think you didn't understand what was said.

The shifts ARE NOT based on vehicle speed. Yes, the parameter says vehicle speed, but it is really rear axle rotational speed. The PCM does not read the speedometer. It reads the VSS, and all it cares about is how many rising and falling edges it sees from the tone wheel on the ring gear.

It is true that you've changed the accuracy of the speedometer. The point that you're missing is that the speedometer is not an input for shifting. The speedo can read 0, or it can read 100. The PCM doesn't look at the speedo, so it doesn't know.

Exactly. So what?

If I were to paint the truck purple with pink polka dots it would also change the truck, and it would have exactly the same effect on shifting as changing tire size.

ABSOLUTELY NOT!!!!!!!!!!!!!!!!!!!!
For any given accelerator position the shift will occur at EXACTLY the same RPM. It will occur at the same indicated speed, too, unless the speedometer is corrected. Then it will occur at the same INDICATED speed that it occurred at before the tire and speedometer change.

This is a flawed example because it is NOT true. It will shift at the indicated 45 MPH which will be the same RPM it was at before the tire change.

However, the transmission tuning at that pedal position wants to shift to OD at the speedometer indicated 45 mph.Now re-calibrate the speedo so that it reads actual speed. What are your RPM's now that you shift at 45 mph instead of 55 mph actual speed? See where I'm going with this? Hopefully that explains it more.[/quote]
I see where you are going with this. You are explaining that you don't understand how the PCM software works. IT DOES NOT CARE WHAT THE SPEEDOMETER IS READING BECAUSE IT DOESN'T KNOW WHAT THE SPEEDOMETER IS READING.

However, the transmission tuning at that pedal position wants to shift to OD at the speedometer indicated 45 mph.But as I stated before, the effect can be slight, especially when there's only a small jump in tire size. Recalibrating the speedometer will correct what the PCM sees in terms of actual speed, which will slightly alter when the truck shifts.[/QUOTE]
No it will not. The speedometer doesn't see what the speedometer is reading.

It was my full time job at Ford to calibrate this for 19 years. Yes, I know how the software works, and I understand it is complicated. Your understanding is wrong.

 

Obviously we are on two different pages. So let's make things clear and see where the misunderstanding is.


Yes the VSS reads rear axle speed. Think about that as revolutions per mile, because that's exactly what it does. Your rear axle speed is also the same as your tire speed in revolutions per mile. So for one tire rotation, the axle also rotates one time. A 31.5" tire is going to rotate 661 times in a mile. A 36" tire (which is what I used to run on my truck for instance) rotates 576 times in a mile. So now you've reduced the number of times the rear axle has rotated in one mile. This affects what the VSS see's as rear axle speed, because now the rear axle is spinning slower due to the larger tires.


The VSS gives the input. But if the input changes, whether that is the signal going to the speedometer, to the PCM, whatever.... then it's going to change the outcome of whatever is programmed to read it.


Same page so far?

 

Mark- I'm going to throw in a slight curve ball here and ask possibly a silly question. Considering the OP's situation, would torque converter lock up speed be inadvertently effected with an uncalibrated speedo? I understand TC lock up happens at around 42 MPH with minimal throttle input. Say lock up happens at 42mph with stock tires. Then switch to 37" tires & an uncalibrated speedo TC lock up still occurs at an indicated 42 MPH (51 MPH actual speed). Then recalibrate the speedo for larger tires, would TC lock up occur at the new recalibrated indicated 42 MPH? Or would it now occur at 51 MPH on the recalibrated speedo?

 

THE PCM DOESN'T KNOW IF THE SPEEDOMETER IS READING 0, 50, OR 5000. It does NOT matter if the speedo is calibrated, not calibrated, or even working. The PCM doesn't see what the speedo reads.

I know that the truck moves further one tire rotation with larger tires. The relationship between engine RPM and rear axle rotational speed is unaffected by changing tire size, and this is why tire size and speedo calibration don't effect shift or lockup speeds.

Curtis, no, we are not on the same page. Changing tire size does not affect the relationship betweend RPM and axle rotational speed. It does affect true speed over the ground, but that doesn't matter.

 

Thank you for clarifying for tc lock up mark.

 

OK, now I see where the disconnect in our conversation is. Maybe there's a piece of info I'm missing on my side.


So to get on track and put me on the same page, please answer these two questions for me...


1) Where does the speedometer get it's input from?


2) Where does the transmission get it's rear axle rotation speed?


I have more questions, but I want to take it a step at a time and not skip anything or overlook something important.

 

They both come from the VSS.

There is a computer (I don't know which one) that converts this to MPH for the speedometer to display. Separately the PCM reads the VSS to determine axle speed. Changing what the speedo sees DOES NOT change what the PCM sees.

 

Guys, Mark is totally correct about this. I've been around 100's of these trucks with tire sizes all the way to 44" and shift timing is exactly the same for a truck with stock tires and wheels as it is for a truck with 44"'s. This goes the same all of the way back to the E4OD days.