speedo calibration #2

[Hammerhead]

Just did a little research and his is what I found out



265/70/R17 = 638 revs per mile

315/70/R17 = 587 revs per mile



If that tech was right ,and I hope he isn't, then the stock 265's are about at the maximum size range that the dealership can program for



Hope that maybe somone can benefit from this little bit of info

 

[Hammerhead]

whoops

Sorry about this being a new thread!! I was meaning to put it as a reply on the bfg tire deal thread. Again sorry, my mistake

 

[crebelein]

that doesn't make any sense... if that is a variable in the program than why can't it be set to virtually any value?

 

[David Muench]

Just Dodge being a pain in our asses. It's a range in the scan tool you can select from. 636 has to be completely arbitrary.

 

[crebelein]

Originally posted by David Muench

Just Dodge being a pain in our asses. It's a range in the scan tool you can select from. 636 has to be completely arbitrary.

so you are saying that if we had some other interface tool we could put in pretty much any value??? hmm... the corvette/mustang guys write their own interface sotware... maybe we could to...

 

[David Muench]

Tough to say. I mean, is it the scan tool that is limiting the values, or the PCM in the truck? Could be either I suppose.

 

[Rockcrawler]

gps calibration

got the tires on last night (315/70/17 BFG All

Terrain)- sure did help the 4. 10's. Anyone calibrate the speedo error with GPS yet- will do soon but curious what other results are.



FWIW- I brought a 37" BFG Krawler to but on for grins and a front clearance check. After mounting the 315/70 I could see the krawler would not fit- 1" more radius. it did look like a 35. 5" tire would fit at stock height. At this time I know of none, maybe the next set of tires. It did appear a 2" leveling kit would be just enough to run the 37's though. I may do that if the 4. 10's are still driving me nuts when I need the second set of tires.



Leaving Friday for Denver- 24hr RT towing my Jeep to a buyer from UT- should help break in some and give me a feel for the new RPM's at 80 mph

 

[PCline]

Anybody know what the upper limit is on the speedo calibration?



I have the 4. 10 gears and plan to go with the BFG's. My thinking is that if I change the rear end ratio AND the tire size in the PCM I'll have an accurate speedo.



In other words:



4. 10 gears X 601 revs/mile = 3. 73 gears X 660 revs/mile



Now I just have to figure how to convince a dealer to program the PCM with 3. 73 gears and 660 tires.



I know this doesn't help you guys with the 3. 73 gears.

 

[sag2]

This is from a post I made in December.



sag2





Just played a bit with the scan tool after reading this post. I went to Anti-lock brakes, Set Pinion factor, and there is something new this year. There is a selection for 10. 5-11. 5 diff with UNKNOWN tires. It then lets you set the tire size in rev's/mile from 636-690. Don't have my book at work with me that has the conversion table. I will post it when I get it

 

[PCline]

Sag2



Does that mean that the diff ratio cannot be reprogrammed? So my slick idea goes down in flames?

 

[David Muench]

I don't know this for a fact, but it's the theory I've been working from. . I think in these new trucks, the speedometer is picked up from the rear ABS tone rings (or whatever they have that is the equivilant). . So unlike earlier trucks that picked it up on the transmission or t-case output shaft, axle gear ratio would be irrelevant.



Anyone want to throw water on my theory? I don't have my FSM with me to look it up.

 

[Rockcrawler]

The tire size computes to about 9% difference- I have not been at highway speeds yet but at 40, 50 and 60 GPS confirms the 9% error-



no matter where you measure before you get to tire size you will have a difference if you change diameters. I'd love to get the speedo recalibrated- In the mean time i have to fudge. As luck would have it- I usually drive about 10% over the posted limit.

 

[PCline]

I think in these new trucks, the speedometer is picked up from the rear ABS tone rings (or whatever they have that is the equivalent). . So unlike earlier trucks that picked it up on the transmission or t-case output shaft, axle gear ratio would be irrelevant.

Now that you mention it, I think you're right. The speedo pickup is definately in the axle, and judging from the exterior location I don't think it's picking up pinion speed. Therefore gear ratio would be irrelevant.



Nuts!

 

[fritzimax]

Got about 9% too with the GPS