New (to me) truck, need help with tuner

New to the foum here, and just picked up a 2002 GMC. It's the half ton 5.3L. The speedometer in the truck reads around 10mph too fast at highway speeds. (displayed speed 70mph, actual 60mph) The truck does have some modifications.. It has about 10" of lift, and sits on 37" tires on 20" wheels with 4.88 gears. The engine also has a mild cam and some bolt ons.

I received what appears to be a Predator programmer with the truck, and the seller was not exactly sure why the speedometer was incorrect. I am totally new to programmers on gas trucks and the only thing I have is the actual tool itself and the cable that runs to the OBD port in the truck.. I did not receive any software discs, or cables to attach to a computer or anything.

Looking for suggestions on how to figure out which version of the Predator I have, as I see no part numbers on it, and also how I could go about correcting the tune that is on the truck.

[mikel]

load the tune and go to tire siZe and set it correctly, try again

Alrighty will do won't have time until tomorrow but I'll report back then.

I found an older thread on here where someone had an issue when swapping gears and tires too and ended up having to have a custom tune written for it. I suppose if retrying doesn't work I may be headed in that direction?

Ok, so I finally got time to plug in the tuner and poke around.. First off, it appears this tuner is very old (menu says build date 09/02) and the software version is 1.03 and hardware v1.00. I cannot update it because I got the tuner passed along to me with the truck used and all i have is the tuner itself and the cable for the OBD port, no cable to attach to a PC.

On to my issue, first I checked the current settings and found the gear ratio to be set correctly to 4.88. Next I looked at the tire size, and found it to be set at 300/25/r18. THE TRUCK HAS 37/13.5/R20. As stated in my original post, this configuration was making my speedometer read approx 10mph faster than actual vehicle speed. Thinking I had found the issue, I used a tire conversion calc to figure my 37's should be 343/63/r20. I then went in and changed the actual tire size to 345/65/r20 (settings went by counts of 5 so this was as close as i could get). Using a GPS app on my phone to check speed I took it for a test drive and the speedometer is now reading approx 20mph TOO FAST. I'm having trouble figuring out why with the correct sizes it made the problem even worse? I understand this unit needs to be updated, but it still has the options to do what I need so it should be capable of solving my issue? In another thread I saw mention of someone saying you had to find the calculations for a smaller tire/gear combo that would produce the same RPMs or something in order to get the Predator to be able to get an accurate speedometer reading?

I've owned a similar truck in the past, with a very similar set up (2001, 38" tires, 4.88s, and had a hyper tech and speed was accurate and no break lights on or anything) I did not build this other truck but it seems the Hypertech it had was able to correct all the changes.

From what Ive seen most people consider the Predator to be the best of the handheld programmers for these trucks, I remain skeptical of this as researching this issue I seem to have found many many other people with my similar issue and no single fix for it.

Any guidance would be much appreciated!

Another update..

For reference, the ACTUAL set up on the truck is 37/13.5/R20 with 4.88 gears.

After more trial and error test runs, here are my results:
(These are the settings in the tune via Diablo Predator, and the +/- of the GPS speed compared to vehicle speed while traveling at 50mph according to truck speedometer.)

4.88 300/25/18 +10mph (Speedo reading +10 over GPS actual speed of 40mph.)
4.88 345/65/20 +20mph *** THIS IS THE ACTUAL CONFIGURATION THE TRUCK IS EQUIPPED WITH.

3.73 345/65/20 +15mph
3.73 265/75/16 +8mph (This would have been the stock set up on my truck)
3.73 285/125/16 +20mph
3.73 285/35/16 -8mph
3.73 285/50/16 0mph (With these settings inputted the truck speedo matches the GPS at all speeds spot on)

Now my question is, if I leave the truck programmed as-is, (3.73 gears with 285/50/16 tires), what else will be affected as a result of this, odometer/trip, etc..? Maybe shifting patterns/driveability? It seemed fine in my short test run, but it was only a mile or so.

Also does anyone know a way to fix all of this the correct way? Is my only option buying something like the Hypertech in-line speedo calibrater and running it separate from the Predator tuning? I would prefer not to spend the money if it's not necessary.

First, that definately is an old version of the tuner. I'm not sure if it is updatable or not. Mike r Lew can probably answer that. If it is, all you need to do is get a serial to USB cable to hook to your computer, and a 12V power supply to power up the tuner, and you are good to go.

From my experience, I wouldn't put too much weight on the actual tire size/gearing that the tuner says. As long as it is set so your speedo is reading correct, you should be good to go. For example, my truck is on 37x12.50-R17s, but the tuner thinks it is on 34" tires, and stock size is 35". I just kept adjusting the tire size in the tuner until it read correctly with GPS, and I also checked it with a radar. As long as your speedo is reading right, then your odometer will be accurate as well.

Your shifting should be fine too. I believe that MPH is one of the facters read in shifting, which is why with bigger tires it seems all confused and shifts at the wrong points. But if the speed reading is correct, then all systems should now be happy!

Also, if you did not know, they no longer even make the predator. It has been replaced by the inTune.