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Discussion Starter #1
Hey y'all. I just picked up a used 2000 GMC 1500. It's running on 37/13.5/20 with 4.88 gears. When I got the truck i was given an older model Diablosport Predator along with. The speedometer was reading approx 10mph faster than actual vehicle speed. After advisement from the DS forums I plugged in the tuner to check out the current tune. I found the gearing to be set correctly to 4.88, but the tire size was set to 300/25/18.(my tires equate to 343/63/20 per a conversion cal). I figured this was causing the inaccurate speedo so I changed the tire size to most closely match my actual tires (345/65/20) and took the truck for a test drive while watching a GPS speedo on my phone. Now the speedo is approx 20mph TOO FAST. Replies on the DS forum seem to come very slowly so I figured I would post up here and see if anyone had any insight. I've read tons of threads with different suggestions on how to fix this issue but none seem to have a definitive answer.
 

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Discussion Starter #2
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.
 

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No. Truck is worth more with less miles
don't listen to this fool. if its reading proper speed then everything else should be good to go.

however, custom efi live tune >>>>>>> handheld tuner crap.
 

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I had this problem. Nice thing was the Superchips does the conversion in inches. What you want is the correct gear input of 4.88. Your problem is the weight of the truck on the tires changes the size and fucks it all up. True 33s turn into 31.75 with a truck on top of them. The handhelds dont take this into account. Just mess with the tires size until you get it right. Eventually ditched my Superchips though and strapped mine to a dyno and tuned it that way after I built the motor. Regardless if yours is stock I would seek out that option at some point.
 

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don't listen to this fool. if its reading proper speed then everything else should be good to go.

however, custom efi live tune >>>>>>> handheld tuner crap.
I got efi and did not correct speedo......it is to my understanding shift points stay same regardless so why not leave it? My radar detector tells me my speed so i left speedo readin 20% lower
 

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Discussion Starter #9
I guess I will leave it for now. In the near future I think I'm going to switch to a custom tune from Black Bear or similar. I would think getting a custom tune from them would also keep everything happy.

This truck supposedly had a mild cam in it (possibly z06?), has headers and bolt ons and a mildy built trans. Does anyone know of a way to determine if the cam had indeed been changed before I were to try and get a custom tune made?
 

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My truck had the same issue with speedo. When I entered everything into tuner it was way off. So I reloaded tune and changed tire size within it. It was still off so after a little math I figured out what size to input. I have 35's that measure 34.5" new, but had to enter 32.5" as the tire size for speedo to be correct.
 

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I got efi and did not correct speedo......it is to my understanding shift points stay same regardless so why not leave it? My radar detector tells me my speed so i left speedo readin 20% lower
i wasn't saying the efi live was better than a hand held because its more accurate at setting the proper speed....

for what it's worth i got mined tuned with black bear and they set it right on for my 33's.

the only benefit of correcting the tire size in the computer is so that your speedo and odometer are correct. so i believe you are correct the shift points and everything else are unaffected.
 
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