Got a 2005 GMC Envoy 47k and just started doing this recently, speedometer jumps up to 90mph when only going 30 or 40 mph, and when I come to a complete stop it drops to only 30 mph???....any ideas on what it could be???

 

try unhooking you battery termnals then see when i would turn my key on on my old truck the speedometer would sett on 100 then when i got rolling about 30 or 40 it would set back right

 

take it to the dealership they will replace it if your under 70k miles. damn chevy recalls

 

this happens to mine a few years back, i was under 45k miles, it would read 70 mph when im going under 20 for a school zone. i took my truck to the dealership and they replaced it free because they were on re-call

 

If you Envoy isn't part of the 7year/70,000mile extended warranty program look into replacing your stepper motors. You can buy them on ebay and replace them yourself or I think there are guys that have an speedometer exchange program. I hear it's really expensive to have the dealer replace the cluster.

 

had the guages replaced on my 2005 silverado 3 times!! damn gauge clusters suck!

 

Just replace the stepper motors. There are a newer set out on the market that are better the ones GM installed in the clusters at the factory.