Speedometer re-calibration for bigger tires

[MikeGyver]

I replaced my 29.875 +- tires with 32.5 inch tall tires. I put 32.5 into the gear/tire calculator and committed to the changes. The speedo reads 42 when I am doing 40 by GPS. I changed the tire size in the calculator again to try and compensate and I noticed that in the general table, it changes the General VSS pulses per mile, but does not change the VSS output pulses per mile. Is this the reason that my torque converter locks up at an indicated 55 mph, when previously it locked at 50 indicated mph? I checked "Yes" in autoscale transmission tables.
If the VSS output number needs to be changed, why doesn't the calculator automatically change it?
If I do need to change it, should I just subtract the same percentage from 4000 (pulses per mile output) as is the difference between 29-7/8" and 32-1/2" tires?
Thanks.

[LEDbetter]

I'd like to know this too. I just put 32 11.50 18s on and my speedo is always showing about 2 mph faster than gps speed. I also drove by a radar detector on the the side of the road to verify if it matched my gps or speedometer.

[MikeGyver]

You just need to make your tire size about an inch smaller in the calculator.

I guess I should rephrase my question.

Why, when I change the tire size in the calculator and now have the speedo accurate by GPS, does converter lockup speed not stay at 50 mph?

[mecanicman]

When you change your tire size you are effectively changing your overall gearing, just as if you had actually changed your diff gears. The auto scale function changes your shift/lockup points to maintain the same strategy/keep engine rpms in specific areas.
General VSS pulses per
VSS Pules per mile= tells pcm how many pulses to expect per mile
Rev-trans=number of teeth on trans reluctor wheel, how many pulses from one complete revolution of output shaft.
Vss output pulse per
mile=pcm converts analog vss signal to digital signal, 4000 ppm is what items like the speedo expect to see(does job of drac from older vehicles if you know what that was). This value is normally not changed.
Mile-abs=not used for anything I have seen, if you look at wiring diagrams the same pcm speed signal out that feeds speedo feeds abs.
Final Drive ratio
Ratio=pretty obvious
Trans revolutions/mile=used to calculater transmission shift speeds. If you are doing manual changes to VSS pulses per Mile to fix speedo, this value needs to be changed by same percentage on automatic vehicles. Failure to do so can result in shifting quirks and failed transmissions.

[MikeGyver]

... I checked "Yes" in autoscale transmission tables. ...

... The auto scale function changes your shift/lockup points to maintain the same strategy/keep engine rpms in specific areas. ...

[MikeGyver]

It now locks the converter at an indicated 52 mph, which is actual mph. It used to lock up at an indicated 50 mph, which was an actual 48 mph.

[Mongorat427]

I just changed my ring and pinion in my F35 from 3.82 to 4.45 Is this where I change it. Or do I have to make some other change.

[MikeGyver]

Don't do that. Change it in the Gear/Tire Calc and click "Commit". That will rescale everything.

[Mongorat427]

Don't do that. Change it in the Gear/Tire Calc and click "Commit". That will rescale everything.

OK . When I click on Gear/tire Calc theres a blank box.

[MikeGyver]

That's different than mine.

[crazyeyez]

You just need to make your tire size about an inch smaller in the calculator.

question, when doing tire size, should i go off actuall size(tape measure and check) or go off the rated size? Went from the stock 265's on my truck, to slightlly larger 31's. Speedo is showing about 2higher then gps at 35mph and slower (havent drove faster yet with gps running)

[MikeGyver]

I tried both and ended up changing the numbers a few times anyway, until the speedo matched my GPS.

[mecanicman]

http://www.hptuners.com/forum/showth...ghlight=speedo

[crazyeyez]

thanx guys. anyone know a goos droid app for speedo? first one i had started acting up and see, each one i tryed was getting jumpy and not holding steady speed, while i had the truck on cruise