Speedometer Inaccuracies

[TheNewGuy]

Hello, I recently purchased a 1998 GMC Sierra and have been having an issue where the speedometer reads higher than my actual speed. For example, when it reads 80 km/h my actual speed is about 63 km/h. The speedometer also becomes less accurate when the speed increases.
Just a few days ago I learned that the instrument cluster had been replaced and is not the original. It's for a v6 Chevy and my truck is a v8.
I was hoping that someone could tell me if it's possible to recalibrate it so that it's accurate, and if it is, how I may be able to do it.

 

[PlayingWithTBI]

It's for a v6 Chevy and my truck is a v8.
I was hoping that someone could tell me if it's possible to recalibrate it so that it's accurate, and if it is, how I may be able to do it.

Yes, your PCM can be flashed for different tires and/or gears. Do you have a Tachometer too? I'll bet it reads high

 

[TheNewGuy]

Yes, I have a tachometer. I haven't noticed it reading high. My truck is an automatic and I believe it shifts up when the rpms get to around 2000.

 

[TheNewGuy]

I'll check the tachometer the next time I'm in the truck to make sure. I wasn't paying much attention to it before so I could easily be wrong.

PlayingWithTBI, when you said that you thought the tachometer would read high, did you mean by a set amount or an increasing amount?

Also, thank you for your earlier response. I appreciate the help.

 

[PlayingWithTBI]

when you said that you thought the tachometer would read high, did you mean by a set amount or an increasing amount?

If the dash is from a 6 cylinder engine, it'll read 125% higher since the V8 is firing 4 times/revolution when the V6 is firing 3 times so, the Tach will think it's running faster.

 

[TheNewGuy]

Sorry it's been so long, I've been a little busy. I couldn't tell if the tachometer was reading high because I don't know what it's supposed to be at and I don't have a tool to measure it.

I've been looking into finding a replacement for the cluster, but haven't found a place with mine in stock. I was wondering if it's possible to program the mileage into the new one once I get it.

 

[TheNewGuy]

Also, do I need to worry about the other gauges not being correct (oil pressure, fuel, engine temperature, battery)?

 

[TheNewGuy]

It just occurred to me to double check that the cluster is indeed the wrong one, as the mechanic that I went to had said. It is the correct one. I got the code for the original cluster from a GM dealer and it matched the code on the current cluster.

My next guess for what is causing the issue is tire size.

 

[Dariusz Salomon]

They go wrong after a while-I think solution is replacing step motors. Later models did that too.I never bothered with mine-reads way too high. Tire size will do nothing as I learnt-I bought my Tahoe with stupid 20 inch rims and skinnyish tires,thought that was the cause, changed the wheels to just about right size to original,and my speedo got even worse. I started driving with phone speedo(an app or google maps will show you accurate readings) and learnt which speed is where-30 is 35, 40 at 46-47, 50 at 60 and 70 at 83-it goes gradually up. Most cars show wrong readings-usually 5 mph more than real speed. Hence I check the speeds on the maps.
As a bonus-I've had a 94 Stang GT once-and this sucker was showing LESS than real speed. Backstabbing Ford.

 

[TheNewGuy]

The odometer is reading high too. I tested it by driving 6.4 km. My trip meter said I went 6.9 km.