Tire size vs transmission

[copeland]

If I move up from 265 x 75 to 285 x 75, how does this affect the automatic transmission shift points? It would have to affect the speedometer as well.

[H ED S]

Copeland from what I understand you need to have it reprogramed for the new tire size. Your PCM uses this to calculate your speed and also this caluculation is used by it to control the injection timing. It will not be far off with that small of a change, but might affect effeciency. You will be actually turning less rpm. This is in addition to changing the shift points a little to.

Maybe somone with more knowledge can jump in.

Take care [img]images/icons/smile.gif[/img]

[scotpart]

I have 305/70R16's and my Ford mechanic told me that that the shift points don't change until you go to real big tires, 37" and up. My tires do change the speedo, but not much.

scott

[POLR BR]

I went to 295's which makes approximately 5% difference in the revolutions per mile from the 265's. This difference might seem initially to be insignificant but in fact does make a real difference. The data for my tire change is as follows:

265's 285's 295's

Circumference 92.87 96.70 97.70

Speedometer error
20 mph 20.0 19.33 19.01
40 40.0 38.66 38.03
60 60.0 57.99 57.04
80 80.0 77.32 76.05
100 100.0 96.65 95.0

Odometer error
1000 966 950.63


Differential Ratio 3.73 3.61 3.55

RPM change 2000 1933 1901


If the PCM calculated shift points based solely on speed, the tire change would not make a large difference. However, RPMs are also considered, and here the difference is significant, amounting to 100 rpm at 2000 rpm.

I recalibrated my speedometer for the 295's and would probably do so even for the 285's although that is a closer call as the change is only a little over 3%.

The gear ratio change is also significant and noticeable. An effective 3.55 ratio is a real change however it does not seem to translate into fuel economy improvement. For me it seems to have been a wash; the efficiency of the gear ratio was lost to the increased rolling resistance of the tire. I do run my tires a little soft, 50 psi front,55 psi rear. If I ran them at 80 psi perhaps I would get better economy but the ride would be awful.

The next time I change tires I will definately consider the 255/85R 16's which are narrower, probably better in snow, and more fuel efficient. Only draw back is that BFG does not make them in the TA KO model line.