If the odometer reading is entered every time you get gas
then reentering start odometer reading would be redundant,
as it would be the odometer reading from the previous time
you got gas.
You might want to have another column to average 3 rows
since you are not likely to get the same fill up each time.
A1: Odometer, B1: Date, C1: description, D1: gal. E1: cost, F1: MPG G1: MPG3
F25: =(A25-OFFSET(A25,-1,0))/D25
G25: =(A25-OFFSET(A25,-3,0))/SUM(OFFSET(D25,-3,0)

25)
The OFFSET in the formulas makes it easier for you if you
missed a receipt and have to add it later.
If you want MPG from start you could use
H25: =(A25-A$2)/SUM(D

)