Are auto trans shift points based on engine RPM or Vehicle speed?

John

 

I'm pretty sure that the stock shift points are based on MPH. Ed set mine on RPM's so the taller Hoosiers don't throw them off at the track.

 

I'm not saying it doesn't vary by year model but my '00 went by MPH.

 

Really, the changing of the tire height shouldn't make the shift point lower even though the shifts are MPH based. The speedometer and the actual speed of the car will be off by the same amount.

For example, let's say that the 2-3 shift normally occurs at an indicated 70 MPH with 26" tires. We'll say this was 6300 RPM. Now, change the tires to 29". Now the shift will still occur at an indicated 70 MPH, correct? Yes, correct. But what actual MPH will that be? 78 MPH. What RPM does that give us? Same as before. 6300.

If anything the shifts should be slightly higher due to the taller tires placing greater load on the converter causing more slippage.

Hmmm...maybe with the RPMS rising slower due to the taller tires, the shifting process can take place sooner in relation to the RPMS rising. I still wouldn't think that no more tire height change than we're talking about that it would make a noticable difference.

I'd like to see some Autotap data with nothing showing but the RPMs between shorter and taller tires. A few passes with each and with the selector in the same spot on all runs (i.e. D or OD.)

 

Let's move this to drivetrain... <img src="gr_stretch.gif" border="0">

 

The taller the tire with the same gear ratio the lower the engine RPM will be to run a given MPH.

As an example:

26” tire 3.23 gears 4000 rpm = about 95mph
Now if we increased the tire to say 28” the overall effective gear ratio would be numerically lower. (Less rotation to go the same distance).
At 4000rpm your speed would now be about 102mph. So the RPM stayed the same but your speed increased.

When I run through the traps my speed is about 112mph but AutoTap and my speedo show 108mph however the engine RPM will be what is required to turn a 26.1” tire through 3.23 gears at 112mph regardless what the PCM thinks.

So in my case I had to raise the shift points because with my 25.6” Goodyears the 1-2 shift occurs at ~6000rpm/42mph. The1-2 shift occurs at ~5800rpm/42mph with the 26.1” ET Streets (lower RPM). If I want the shift points to stay at the same RPM with the ET Streets I will need to raise the shift point MPH to make that happen.


John

 

Thanks guys. I noticed that with the taller than stock ET Streets my speedometer reads low and the shift points are low. I guess that the 98 also uses MPH. It seems to me that it would be better / more consistent to use RPM.

John