Inaccurate Speedo

[72firebird]

So I noticed the other weekend when I was going to a car show that is like 30 or 40 miles away that when I was cruising on the highway with my speedo at 80 mph the scanmaster was reading around 73 mph. At about 30 mph on the speedo it reads 27 mph or so on the scanmaster so it's a pretty linear progression.

Now what I don't get is that both are electric and run through the computer, so how are they reading different outside of the gear setup being wrong in the gauges? Any ideas? Since my speedo is reading high then my odometer is going to be reading wrong. Using the mph differences listed, it's reading about 8.5% off which doesn't seem like much but for the 27k miles that are on the car that puts the car about 2295 miles over what it really has on it.

Also, I confirmed the lower speeds from the Scanmaster being correct with a buddy who was following me with an LT1 formula.

 

[RatFink]

I don't think that any of the gauges are very accurate. We all know the stock boost gauge is for "looks" and is way off, not mention slow reacting compared to a real gauge. I've also noticed a discrepancy between the stock gauge and the scanmaster, and about the same as you stated at lower speed, but at 80 mph on my gauge it's more like 76 mph on the scanmaster. hmmm, no answer here. DontKnow:

 

[Turbota6]

Don't have an explanation but my speedo used to not register until about 10mph. Not a big deal since we're all not usually going that slow but kind of annoying. In comparison my SSEi is fairly accurate when compared to radar signs and GPS. The actual speedo and the HUD are about 1mph different.