[gone-ot]

...OK, I performed an initial GALLONS "offset" calibration; and here are some 'semi-scientific' results (26.2 miles each way):

DAY #1 -- 32.0 mpg at 37-deg.F going TO work at 6:00am.
DAY #1 -- 38.2 mpg at 60-deg.F coming FROM work at 5:30pm.
DAY #1 -- 35.1 mpg avg.

DAY #2 -- 32.3 mpg at 42-deg.F going TO work at 6:00am.
DAY #2 -- 38.8 mpg at 67-deg.F coming FROM workat 5:30pm.
DAY #2 -- 35.6 mpg avg.

DAY #3 -- 33.5 mpg at 49-deg.F going TO work at 6:00am.
DAY #3 -- 38.3 mpg at 63-deg.F coming FROM work at 5:30pm.
DAY #3 -- 35.9 mpg avg.

DAY #4 -- 34.4 mpg at 48-deg.F going TO work at 6:00am (drizzling).
DAY #4 -- 36.1 mpg at 52-deg.F coming FROM work at 5:30pm.
DAY #4 -- 35.3 mpg avg.

DAY #5 -- 33.3 mpg at 45-deg.F going TO work at 6:00am.
DAY #5 -- 40.0 mpg at 68-deg.F coming FROM work at 5:30pm.
DAY #5 -- 36.7 mpg avg.

DAY #6 -- 35.1 mpg at 46-deg.F going TO work at 6:00am.
DAY #6 -- 39.3 mpg at 71-deg.F coming FROM work at 5:30pm.
DAY #6 -- 37.2 mpg avg.

DAY #7 -- 36.4 mpg at 53-deg.F going TO work at 6:00am.
DAY #7 -- 40.7 mpg at 55-deg.F coming FROM work at 5:30pm.
DAY #7 -- 38.6 mpg avg (!).

DAY #8 -- 33.7 mpg at 45-deg.F going TO work at 6:00am.
DAY #8 -- 37.2 mpg at 66-deg.F coming FROM work at 5:30pm.
DAY #8 -- 35.5 mpg avg.

DAY #9 -- 34.1 mpg at 45-deg.F going TO work at 6:00am.
DAY #9 -- 38.5 mpg at 66-deg.F coming FROM work at 5:30pm.
DAY #9 -- 36.3 mpg avg.

DAY #10 -- 34.6 mpg at 43-deg.F going TO work at 6:00am.
DAY #10 -- 40.7 mpg at 73-deg.F coming FROM work at 5:30pm.
DAY #10 -- 37.7 mpg avg.

...which produces the linear regression analysis (for this car):

MPG = 0.23*(temp) + 24, with RR = 0.82

...finally, the tank MPG is the average of the individual trip MPG's, ie:

MPG(tank) = [ MPG(trip.1) + MPG(trip.2) + ... + MPG(trip.N) ] / N

...and, the average of the 10 averages was 36.4 mpg while the actual tank-fill value was 35.6 mpg, which is 98%--that's just a 2% error...close enough!


addendum: SGII indicated 9.7 gallons used while the tank-fill was 9.736 gallons!