Speedo and MPG check

[Fbks_Cruiser]

It seemed like my speedometer was off a bit. I decided to check it and the odometer as well as true MPG by running a GPS for a tank of gas.

Turns out the speedometer/odometer is off by about 10% (indicates faster than true speed). When GPS speed was showing 55 mph, speedometer was at about 60-61 mph. Odometer also registered about 10% too many miles.

MPG check with true GPS miles was 11 mpg in mixed town and highway driving.

Here is my setup:

1972 GMC K2500
350/350TH/NP205
4:10 gears (per previous owner, I haven't checked but seems reasonable)
235/85R16 tires
HEI distributor set to 8* base
"RV" cam (per previous owner, I don't know the specs but seems fairly mild)
Weiand 8004 intake
Edelbrock 1406 carb with one step richer than stock metering rods