Speedometer off by 15%

Not sure which section to post this in, but I need help diagnosing a speedometer issue.

I just bought a 97 Sonoma, 4.3 5 spd, only 24k miles. the seller knows nothing about the truck as he bought it off someone else who built it, nor has he owned it very long. The truck has Corvette C5 brakes all around, rear end feels like factory 3.08s, I believe the truck has factory tuning on it as there is still a 96mph governor.

I bought the truck and was driving it home from Vegas and when I was going 80mph I was getting passed by big rigs. Downloaded a speedometer app for the cell phone. 90mph on the truck = 77mph. The fact that governor kicks in at 96mph means the truck can really only go 81mph or so. Regardless, I need the speedometer fixed cause I don't want to be putting an imaginary 15% extra mileage on the odometer.

Where is the speedometer reading taken from? transmission or rear axle? The tires on it are 275/30/20, which is 26.5". The factory Zq8 235/55/16 is 26.2" so if anything the tire difference means the speedo should be 1.2% slow, not 15% fast. It would need to have a 22" tire to explain the speedo reading 15% fast.

There is a fabricated rear suspension, DTS diff cover, so maybe the gears were changed in the past and the speedo was calibrated, and then gears put back to stock but no programming change? They definitely do not feel like 3.73s or 4.10s, feels like stock 3.08s cause the truck isn't that fast off the line and 1st gear is still very useable.

Could swapping to C5 brakes in the rear affected the speedo at all?

I ordered used hypertech on ebay for $70 to try recalibrate via tire size and gear ratio computer changes, but am scratching my head at how it could be so far off?

It sounds like the gears were changed. You can figure out the gear ratio by jacking up the rear end and counting RPMs of driveshaft while you rotate the wheels. Brand-specific tire height differences can do it as well; My speedo still reads ~10% high even though it's corrected in my tune for the theoretical height of a 275/40-17.

Yea, soon as I get home I'm going to do that. I don't think it can be tire size since it would need to be off by 4". and they are only 0.3" difference.

Could the Vehicle Speed Sensor have gone bad? The truck is pushing 20 years old but it only has 24k miles (realistically less if this speedometer has been off for a long time). There is an aftermarket driveshaft on it also, not sure what else could be affecting the VSS.

well I'm a dumbass, jacked her up when I got home from work, marked the tire, marked the driveshaft... about three and three quarter driveshaft turns to rotate wheel 1 time.

so it's already been swapped to 3.73s

That's good cause I dont have to cough up the money to do something I wanted to do.

That's bad cause I thought it would be a lot faster with them 

I have a hypertech coming off ebay on Friday so that should solve my speedo problem. I wonder if the computer thinking I'm going 15% faster than I really am has any affect on how it runs, or is it just rpms that matter?

what does the RPOs say the truck should have for ratio? GT# or GU#s I think it was.
if the truck originally had 3.08s and was swapped to 3.55s that's 15% off.
if the truck had 3.08s and was swapped to 3.73s is should be more like 21% off.
likewise, 3.23 to 3.73 is 15%

i doubt the VSS going bad has this sort of manner, and it's on the trans output

you are right, it was a lot more than 15%, but I estimated that by using a cell phone speedometer which im sure wasn't that accurate.

with factory governor at 96mph, it was cutting off under 80mph in reality.

they are definitely 3.73s, i put it up on jack stands and counted the drive shaft turns to spin the wheels on rev, it was right at three and three quarter turns.

Looks like you're on the right track. Like Ryo suggested check the RPO codes if you really want to know the actual reason for the discrepancy. Rentedmule also pointed out that not all tires are measured the same. I know some tire companies will post the actual tire height for each of their tire sizes. If you can drop that into the hypertech I'd go that route to get the most accurate speed reading.