Speedo reading slow!!

[jess t]

So I drive my 92 Silverado 100 miles last night and my speedo is reading around 50 mph at about 75 mph??!!! It has worked fine until last night. What gives? jess

 

[AuroraGirl]

What about other speeds, is it at 0 while moving 15, 20 at 35? or just starts to differ at 50?

A progressively increasing problem is usually tire size cable or similar issue. A certain speed and up issue is usually gauge.
A jumping around gauge is either a cable or gauge, but depends mostly on what kind of gauge.

you dont have a cable right? Vss?
does it work in reverse

check for codes

 

[jess t]

You're right, VSS, no cable. Seems slow but steady until 50-55, then won't go higher. Haven't checked for codes. I switched rear axle ratios about 10 years ago and had to have the board behind the glovebox recalibrated, it's probably the culprit.