Picked this truck up a few days ago, driving down the highway on the way home, doing the speed limit, or so i thought, speedo read 80mph, wife calls me tells me i need to slow down, she is doin 90 and cant keep up. Ends up 60mph on the speedo is actually around 80mph

Truck is a 99 sierra nbs, 275/45/20 tires, escalade 120mph gauge cluster, i was wondering if maybe the truck was geared and the tune not corrected.

Any insight to this? What would cause my speedo to read slower than actual mph?

And just for ***** and giggles, here is a pic of it

 

Bigger tires than stock. Stock was most likely either 255/r15 or 265/r16.

 

Stock height tire is 30.05, these are 29.74, actually 3/8" smaller

 

I'll be, it is smaller.

Most likely someone either changed the gears or the needle is incorrect on the gauge. There's also a possibility that it was incorrectly tuned.

Easy way to find is is get someone with an OBDII data reader to plug in and see what speed the PCM is measuring, then compare to the gauge cluster. Any Autozone with a decent code reader will be able to read PCM data.

 

The computer is reading the same as the gauge, i have a scanner that will show live data, thats the first thing i thought off, maybe the needle is off

 

I have a buddy that does tuning, im gonna have him look the tune over

 

If the cluster and OBD are reading the same, then the speedo settings need adjusting in the tune. What RPM are you cruising at?

 

Havent really paid attention, thinkin it was up around 1600-1800 on the highway, but i could be way off

 

I do have a stock ecm out of an 01 tahoe, can i swap em to see if its in the tune?

 

No. The Tahoe might have different gears.

The gear ratio needs adjusting in the tune. It sounds like you might have 3.23 gears and a 3.73 tune.