[len47merc]

If the car has a 3.78 rear and you routinely drive it at speed (i.e., 55 mph and up on the speedo) definitely count on mpg being at the low end of the range - ~12-13 mpg. Those that represent 20 mpg with a straight face either have a Columbia overdrive unit which reduces the end ratio to the low 3's or high 2's, or have a rear pinion set with that ratio (latter hard to find now). Stock/original was either 3.54 or 3.78. I have a stock '47 Merc which burns the exhaust pipe 'whiteish' grey, all cylinders above 115 psi compression, with a 3.78 rear and it is tough to get 13 or above mpg if driving it at or above 55 mph given the high rpms you are turning at those speeds.