Revs/mile and speedo correction

[6speedsd]

Been having some confusion about all this. I'm running stock sized tires (245/75/17). Noticed the other day my speedo was off from what the gps stated my actual mph was. Before you say GPS isn't accurate, it has always ran within 1-2mph of my actual speed (checked with AE). My stock sized tires are 31.47" tall, which translates to 641 revs/mile. I went in and manually set my tire size with AE yesterday to 31.5" (again, stock size). Went up the road and my speedo was registering 7-8mph higher than my actual speed. Can't figure out why this is happening.

I hooked up the SCT after that, and chose the option to correct speedo for oversized tires. The default setting that it showed was 611 revs/mile! I manually changed it in SCT to 640 revs (yet again, according to my stock sized tires), and it got the speedo within 2-3mph of actual speed.

Seems to me my ABS system is somehow calibrated to a taller tire, if that is even possible? I would think changing it manually with AE would correct this, but it doesn't. I hope all this makes sense, as some of my terminology might be off. Anyone have a clue what's going on here?

 

[6speedsd]

Bump....

 

[6speedsd]

All you smart people on here and no one has an opinion?

 

[Cknox121]

Mine seems way off as well. I'm tuned via h&s. Set tire size to 35 and seems like my odometer just tacks up the miles so quick!

I stopped by a mile marker sign, reset it and it was .2 miles off.

Same with speed, anywhere from 3-5mph off from gps and cop radar.

Reentered it a hundred times and it still won't do right. Even tried smaller and larger tire size.


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[tbsimmons]

If I remember right my 6.0 truck the tire size just did shift points not the speedo. Been about 6 years since I messed with one though.

 

[6speedsd]

With the X3, you have the option to adjust for both tire size and shift pts. I'm pretty sure that's correct anyways. I know you can adjust for oversized tires, and in the adjustable options menu you can adjust there again, which if I understood it correctly from Eric, that one adjusts for shifting.


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[tbsimmons]

I only had the X2 from Eric.

 

[Derkperk]

I have not had any problem calibrating my speedo with my x3. I switch between stock 265's and 35's and my speedo is pretty accurate. It kinda sounds like your x3 and ae are double calibrating.

 

[6speedsd]

I have not had any problem calibrating my speedo with my x3. I switch between stock 265's and 35's and my speedo is pretty accurate. It kinda sounds like your x3 and ae are double calibrating.

But why? LOL This whole situation makes no sense to me. My front wheel speed sensors read 7mph higher than actual speed, but I've been told by a couple different people that those are estimated speeds, and that those sensors don't really matter and to not worry about what they are showing. But, when I enter my actual tire size, my speedo is exactly 7MPH off! That's what I can't wrap my head around. Why do both of those numbers match if those sensors are irrelevant?

Which sensor sends the speed signal that controls the speedo? VSS?

 

[Derkperk]

Don't get so stuck on your actual tire size. If your speedo is reading lower than your actual speed, decrease your revs per mile or vise versa until you get it right on according to the gps.

 

[6speedsd]

So basically it's no big deal, and I shouldn't even be worried about it? Just get the speedo accurate, and forget about it?

When I change tunes, the X3 defaults to like 663 revs, which gets me within about 2mph. Maybe I am just thinking too hard about this.

 

[Derkperk]

Mine defaults as well. I have my revs written on the back of it.
I don't see what would matter as long as you speedo is correct which is then keeping your odometer correct. Then the truck will know how fast it is going to make the proper shifts.
I can't tell you how it works bit I think that's your best solution.