I always suspected my speedometer was incorrect and today I confirmed it. While my speedo read 60 MPH a buddy riding along side confirmed that I was really going 65 MPH. I have 27" dia tires (225R7015 slightly worn). I believe my rear end is a 3.08 and I have a neutral color gear in the muncie 4 speed. Does anyone have any idea what could be going on?

The speedometer could be a bit out of calibration.   This is very common on older cars, even when they were new.  The head can be recalibrated.  You may also be able to change the speedo drive gear in the trans.  But the correct gear I will defer to Joel (Adams Apple).  He has those codes/specs.

I found out my speedo was off with my GPS Navigator. It's pritty much on the money at 85 mph. But at 110 mph I'm really only doing 105 mph   I guess I'm OK.

5mph off can be fixed with a gear change. Take the speedo housing/bullet out of the trans, and count the teeth on the driven gear there. Then get a gear that has one less tooth on it to speed up the reading at the speedo.
I do have a speed-o gear tooth count chart posted here somewhere....I'll look for it. It will be just as easy to count your gear, and then change it to what you need, as what it has may be what it is supposed to have to start with, but the tire size difference is throwing the speed-o off a bit.


                                                         Speed-o Gear ChartAdams' Apple2008-06-21 18:52:36

3.08 rear end, 27 inch dia tires, 2300 rpm in 4th =  60 mph. My speedometer is off also. I created a little correction chart that I keep in the car...cheaper and easier than replacing the speedometer drive gear.

I've never owned a pre-84 car that was on the money..  My 81 is 5mph optimistic..

Does anyone know how I can confirm my rear end gear ratio?

IF all else fails, pick up both rear wheels, put the car in neutral.  Mark the drive shaft and the wheels.   Turn the wheels exactly one full turn, and count the number of turns at the drive shaft.  This is your rear end ratio.