gasoline prices/fuel economy

[JanDebL at aol.com]

How accurate are the MPG indicators?
My 91 2C TQ gets an indicated 22-23 mpg while my wife's 87 5KTQ gets an 
indicated 26-28 mpg when driving the same route.  When I calculate the 
mileage based on the odometer readings, the numbers get closer 24-25 vs 25-27 
mph respectively.  When I switch to 205-60-15 snow tyres on the 91, the 
indicated fuel mileage is almost identical (although the indicated 
speedometer reading is ~ 5 mph faster @ 80 mph).  When I  stay with one car 
or the other for weeks at a time, the 87 would get ~ 10 - 20 more mile per 
tank than the 91 so it does get slightly better mileage.
My question - is there a calibration adjustment on the back of the 
speedometer on the 2C like there is on the 5K?  I've had the cluster out to 
repair the CB but didn't notice an adjustment.
TIA, Jan Lahtonen