Mickey, here is another thing to try...have your wife, friend, etc drive beside you at only around 10 mph in a different vehicle. See if the two speeds agree. If they do or if they're close at that low speed and if your truck fails to increase indicated speedometer speed as you accelerate, look to your speedometer head. I won't say it is a guarantee but let's see what you discover. Also...fairly important...if your speedometer fails to agree with actual speed by 50% or so, let's try to figure out what ratio you have in the rear end. There are some easy ways to do that but first let's see what we see.
6.00 x 16 tires are going to be 28 inches in diameter. All 3 speed transmissions end in 1:1 ratio (3rd gear). Just for sake of calculation here.
