G'mornin',

What color is your exhaust??
Have you done "the Purge"??

I would think with that large a difference in mileage, if it is fuel going thru the engine...you would be having nice black cloud following you around. Or you have a heavier load on the engine than you had before...drag in transmission, wheel bearing (but I doubt it would add enough load),......

Assuming that you are measuring fuel added to the tank vs odometer reading....is your odometer working, 100% of the time.....I got very eratic initial mileage readings, using my odometer......found that my odometer works about 90% of the time.....but on occasion, after reversing the car....my odometer quits for awhile, sometimes going to reverse again corrects it....and sometimes just a reset on the trip odometer does the trick.

Try measuring fuel vs a known distance......I will be driving to N'awlins for Turkey Day...1503 miles from here.....so if I stick to my known route a 15 mile difference for fuel stops and such....will be 1% of my distance and should calculate out with a 1% error factor by my way of thinking.....but a fillup and a 100 mile trip on the interstate, and another fillup.....should get you close to your actual mileage....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG