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?
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?