I done somthing this afternoon:

I filled the tank up to 3/4, wrote down the milage (71442), drove till the fuel indicator hit 1/2, checked the odometer (71465), all in the free way.
Can not understand that, 1/4th of the tank burned in 23 miles.

Or maybe there is another way of counting the MPG. Somone could give me a hint.

ddavidv: forgot to tell you, the tranny: 4 speed manual T-18 Borg, and rear axel: Ford 3.70-3.3M.

Icicle: all plugs are carboned black( Ithought you meant the colour of the ceramic casing of the plug).