How accurate do you think the GPS speed reading is?

I took my hand held in my truck on a long drive last week.
I compared the truck speedo with the handheld, they were way different. I know the truck is off a bit cause I changed to a larger tire, but the difference was alot. How accurate is that handheld WAAS and all??

I use my Raymarine chartplotter GPS for my speed in the water.
I don't trust the fishfinder paddle speedo at all.

Thoughts ???????????

there is some data here
http://gpsinformation.net/main/gpsspeed.htm

I would trust the GPS over the truck speedo. Off the production line your truck could be off 10% at higher speeds (hence the 7 mph over the speed limit rule). I have a GPS speedo on the boat and it is always exactly the same as the chartplotter speed. The transducer wheel speed is always going to be off at low speeds due to the current. You could hold your position in some current and the wheel would tell you the speed of the water going by you.

Rock On, shouldn't you be in MD right now...?

Van...GPS speed is the most probably the most accuracte measurement over ground you can get. I took my hand held this winter on several trips and beyond telling me when to turn it gave me an a speed that was very close to my my speedometer. I had the truck in crusise control @ 75mph and it was saying the same on the gps.

Where you reading knots?:D

Speedometers read the rotation rate of a tire. If the tire is non-stock there is your problem. A small increase or decrease in diameter will result in a measurable difference in rotation rate. Even 1 inch will make a measurable difference.
I think you can get your speedometer tuned to match your tires but I am not sure... never did that. The last time I had non-stock tires it was on a 1973 Camero 454 that I sunk in a TON of money to....I knew the speedometer was off but I was 18 and look much at that guage....Boats are more fun to waste money on.:D

Van,
Your gps is realy acurate. I did the same test and its probaly your truck.

My depthsounder, boat speedometer, and gps are all different. I just look at the one that's faster.

I don't have alot of faith in the paddle wheel, or water being forced into a tiny hole, I have more faith in the gps. If it can get me to my favorite spot in pea soup fog, it must be great.