Tune and Speedo

[Deleted member 12951]

I've noticed that my speedometer is 3 miles fast. Meaning if I am showing 80mph, I am really going 77mph.

Now that I got a tuner (iTSX with 5 Star), I noticed that I can change the revolutions on the tires per mile. So instead of bringing it to the dealer before returning it to stock to fix and possible see that the PCM has changed, I thought I could try myself to fix the 3 mile difference.

I assume this is how the dealer would do it anyways?

I have stock beadlock SE wheels and stock BFG tires. What is the default revolutions per mile? And which way would I change it, up or down and by how much?

 

[squeak]

If you're running stock why is the speedo off? Although in the past I have heard they aren't even always accurate from the manufacturer. I didn't know that was still a problem.

 

[Deleted member 12951]

If you're running stock why is the speedo off? Although in the past I have heard they aren't even always accurate from the manufacturer. I didn't know that was still a problem.

Good question, I have no idea and I was going to take it into the dealer to have them fix it, just haven't got around to it as it is not critical. But if I can fix it myself now, I would rather do that.

 

[MTF]

How do you know it's off by 3 mph?

You not using a radar detector with gps that can display speed, because gps speed calculations is not accurate.

You could use a OBD11 scanner to see what the PCM is saying.

 

[squeak]

I had an edge programmer on my previous truck and when I put in the different tire size it was never right on the speedo. It didn't ask for a difference from stock just the tire circumference. I guess this one is different. Try calling the dealer maybe they can tell you what the stock input is.

 

[MTF]

That's gear ratio/tire revolutions that your seeing, it should be the same like my SCT 4X
If you press enter it will give you choices for gearing only, I believe.

If it does more please let me know, I have a Droid Maxx and I'm interested.

This is from BFG website for the Fords
(37614 LT315/70R17/D 121 S $383 F BSW 8" - 11" 12.7" on 9.5" 34.4 604 17)

The 604 is the rev per mile, the GM version is 601 per mile

And if the SCT iTSX does something different than the SCT 4X, like circumference.
All you need to do is make a mark on the tire and the ground,
roll forward till the line is straight at the ground again and measure the distance between the lines that's you circumference.

 

[Deleted member 12951]

How do you know it's off by 3 mph?

You not using a radar detector with gps that can display speed, because gps speed calculations is not accurate.

You could use a OBD11 scanner to see what the PCM is saying.

I was in a raptor convoy and we all set our cruise at 78. I kept losing on the ones in front of me. I had to speed up to 81 to keep pace. Everyone confirmed they were going 78. So perhaps I have it backwards but regardless it was off by 3. I turned on a GPS app on my phone and when I was going 81 on the speedo, the GPS was showing 78 and then I kept pace with everyone else.

I am stock wheels and tires. The only suspension mods are an RPG bump stop, WSI HD lead springs and mid perch. I can't see how those would matter.

 

[MTF]

I don't know what to tell you. I'd give Mike a call see what his theory is.

 

[Deleted member 12951]

I don't know what to tell you. I'd give Mike a call see what his theory is.

It was off before the tune. I am thinking I can fix it now that I have a tuner that allows me to change the tire revolution per mile. I will post a pic from my phone on that setting. And yes it does allow gear ratio too but that is in a different section.

 

[MTF]

Are they the original tires or is this a second set?

I'm asking because there are two different BFGs, one for Raptors and one for GMs
The Raptor ones cost more. (Raptor Tax you know) LOL

There's only a difference of 3 rev per mile between the two, but that could explain it.