Has anyone else noticed this, or is it just my truck? My speedometer is pretty far off, based on my little experiments. I have calculated it to be reading about 5 miles an hour fast at 75mph. The math says I'm barely pushing 70. Set the cruise and time how many seconds it takes to go 10 miles based on the mile markers on the side of the freeway. Then take 36,000 and divide it by that number of seconds for your mph. Compare that with what your cruise is set at.
(3,600 sec/hr)÷(X sec/N miles)=mph
(3,600*N miles·sec/hr)÷(X sec)=mph

I tried this same experiment on my moms Volkswagon Beetle and the thing was dead on at 75mph. Those Germans don't mess around!

PS - i'm using 16" rims with P245 75's. Stock was P265 70 with the same results. Does anyone know of a way to calibrate the speedometer?

 

http://fastq.com/~sschaefer/speedo.html

Enjoy!
Dr. Z

 

Just an update.
I just completed my deckplate mod and I love it. In a quick experiment while I was enjoying my newly discovered power on the freeway, I tried a one-mile speedometer check with the cruise set at ~76mph. The math result was that I was actually going 75mph. Could the deckplate mod, along with the pulling of the EFI fuse, magically cure my speedometer being off? It was only a 1 mile test, so maybe not the most accurate average. That's why I prefer to do 10 miles with the cruise control, because a longer running average will be a better approximation of true speed. Who knows??