RPM vs indicated speed vs REAL speed vs odometer accuracy

[Rocketman]

Mind you wouldnt the speed and milage change as the diameter of the tyre decreases with wear and therefore everytime you ride you will get a different reading as each ride removes more rubber

You're absolutely right.  However, the change is so small, that it's not going to be noticeable.  The amount that tire diameter can affect speedo/odo readings is calculated with a ratio of "original tire radius" to "new tire radius".  What is the tire radius?  20 inches?  Take off 1/4 inch of rubber, and it's now 19.75 inches.  ~1% change.  The tire radius is going to have more (or at least comparable) difference from tire to tire than from new tire to worn tire.  Or, even more applicable, the difference between a properly inflated tire and an underinflated tire is a lot more than that 1/4 inch.

[sugerbear]

does a gps measure up and down distance too?
elevation.
 

[BradValk48237]

There's a Speedo?

B

[Tropic traveler]

I think I asked a similar question concerning mileage vs. speed differences a while back. Someone smarter than me said that the Valk does split the two {speed & distance} at the speedo head. I asked about the difference in two identical bikes, my '97 & Kim's '98. We have the exact same tires, run at the exact same PSI, & ride nearly exactly the same distances in long rides & our odometer readings are exactly the same. However her '98 ALWAYS shows 3-4 mph more at the same cruising RPM. 
Made no sense to me either until the concept of 1 input {speedo cable}, 2 different instruments {odo & speedo} was explained to me.