I've been past a couple of the radar set-ups that tell you how fast you are going. Both said my speedo is reading about 5% slow. Can it be adjusted or is it better to leave it alone?

 

you can change the gear but that all I can tell you.
there is a link in the, VSS gear ratio thread.

 

At 5% I'd leave it go, that is pretty close.

 

I went from a 215-75-16 to a 235-75-16 so I'm reading low also.

I use my dashtop GPS to determine ground-speed when I'm on the highway.

 

I was thinking that but thought it would be nice to have it dead on. I remember on our early 80s trucks it seems to me we were able to twist the cable a little at the trans or something. I could be wrong on that though.

 

I caught heck from my boss late last fall because I didn't have GPS and he couldn't tell me the right name for the bridge or the exit for it off I-190 to cross to Ontario. I told him once I can get GPS'd to my house and not the neighbor's place 3 miles up the road, and the first place in the next ZIP code, I'll get one.

 

Generally speaking 5% is well within the manufacturer's tolerance, as long as it is lower than the actual speed. The percentage stays the same, but as the speed increases the mph difference is greater. I believe that this is because, for liability reasons, they cannot have the speedo reading high. I take my gps and because the difference is pretty consistent, It is easy to remember the variance of each of my vehicles and drive accordingly. Mark

 

5% is easy to calculate. If the speedo reads high they are cheating you out of warranty, if it reads low they are giving you warranty.

 

I have found that it is not a linear progression of 5%, but rather that the difference getting bigger from fairly accurate below 50 mph to the 5%, or more, at higher speeds. Imagine the potential liability if the actual speed were greater than the indicated speed. The miles pile up faster, as you said, in their favor. What a convenient coincidence... Mark