[Jebb]

I understand that the speedometer and odometer take their input signal from the axle ring gear sensor - changing tire size affects the speedo calibration but changing the ring and pinion ratio does not.

So then, what does changing the axle ratio affect as far as the ECU is concerned? Are transmission shift points determined by engine rpm or vehicle speed or, I suspect, some combination of both?

In older, all mechanical powertrains the tranny shifted based on throttle (vacuum) and engine rpm. The speedometer took its feed from the prop shaft. If you changed tire size you could change axle ratios to match and everything was fine; you could achieve the same final ratio; the tranny shifted where it should and the speedo was correct.

I don't see how you can do that in these ECU-controlled powertrains. If the speedo takes its signal from the axle (vs. prop shaft) it seems that the ONLY tire/axle combination that works right is STOCK. Even if you change gears to match a new tire size, the ECU calibration is still wrong because it's reading axle speed, not prop shaft speed.

Do I have this correct or am I misunderstanding something here?

 

[AIR_RAM]

I understand that the speedometer and odometer take their input signal from the axle ring gear sensor - changing tire size affects the speedo calibration but changing the ring and pinion ratio does not.

So then, what does changing the axle ratio affect as far as the ECU is concerned? Are transmission shift points determined by engine rpm or vehicle speed or, I suspect, some combination of both?

In older, all mechanical powertrains the tranny shifted based on throttle (vacuum) and engine rpm. The speedometer took its feed from the prop shaft. If you changed tire size you could change axle ratios to match and everything was fine; you could achieve the same final ratio; the tranny shifted where it should and the speedo was correct.

I don't see how you can do that in these ECU-controlled powertrains. If the speedo takes its signal from the axle (vs. prop shaft) it seems that the ONLY tire/axle combination that works right is STOCK. Even if you change gears to match a new tire size, the ECU calibration is still wrong because it's reading axle speed, not prop shaft speed.

Do I have this correct or am I misunderstanding something here?

The dealership can adjust for the "PINION FACTOR" which is when they just tell the PCM your gear ratio. I had the 4.56's installed for over 2 years before I had them do mine and honestly there was absolutely NO change in shifting... It was like NOTHING at all had changed. So I would say, don't worry about it, if you can get it done for free then you have nothing to lose... but if they want to charge you... don't waste your money on it.

SPEED SAFE, AIR RAM

 

[Jebb]

I had the 4.56's installed for over 2 years before I had them do mine and honestly there was absolutely NO change in shifting... It was like NOTHING at all had changed. SPEED SAFE, AIR RAM

Like you said above, I don't see what the PCM would do with a "pinion factor" if it already has axle rpm and engine rpm. You only need two to calculate the third one (unless there is a lot of slip in the torque converter).

So what does the PCM use to determine shift points? Engine rpm, road speed (axle rpm) or both?

 

[HemiDak]

Like you said above, I don't see what the PCM would do with a "pinion factor" if it already has axle rpm and engine rpm. You only need two to calculate the third one (unless there is a lot of slip in the torque converter).

So what does the PCM use to determine shift points? Engine rpm, road speed (axle rpm) or both?

It's actually a little more complicated than that. With a 45RFE/545RFE it uses the following:

Shift lever position
Throttle position
Engine load
Fluid temperature
Software level