Getting my speedo to be more accurate

[Rich69shortfleet]

When I changed the truck to 3.73 gears (from 3.07 gears) I did the calculations and changed the speedo driven gear to get the speedo back into calibration. However, a few weeks ago I was traveling with a friend. Our speedometer readings didn't match. I told him I was going to drive 65 and so he did too and he pulled away. So, apparently my speedometer is reading fast, or his is off too!

Anyway, on the way home at 100 miles from home I noted the odometer reading. This 100 miles was covered on the Interstate with never leaving the Interstate. When I reached home, the odometer was reading 7 more miles than it should have given the 100 miles covered. So, my speedometer and odometer are reading 7% fast.

If I recall correctly, on the driven gears one tooth is worth about 5% increase or decrease, depending on which direction you go. So, to slow down my speedometer to more closely match the speed I'm going, I'd want to find a driven gear one tooth larger than what I have now. I still need to pull the bullet and see which gear I have since I honestly don't remember, but I was hoping others here can help check my logic on what I need to do, ie: get a driven gear one tooth larger.

TIA