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Originally Posted by gumby
145 miles / 54.5 MPG = 2.6606 gallons used.
175 miles / 51.5 MPG = 3.3981 gallons used.
So, 35 miles / 0.7375 gallons used = 47.46 MPG.
Keep in mind that using small numbers (gallons used, here, and 35 miles) limits the assuredness of the calculations, but it's some idea.
Not too bad, really. Especially since she doesn't have the same affliction that you do 
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Thanks, Steve. I haven't gone over the numbers to calculate them and see if that works out but i don't think it is that simple - remember that for the average to drop from 54.5 mpg to 51.5 mpg (or 3.0 mpg) is not straight math. IF she had driven an equal distance of 145 miles and dropped it 3.0 mpg then her average would be a simple 47.5 mpg (or exactly 3.0 mpg below the current average number of 51.5 mpg). She only drove 30 miles which is only 20% of the distance I did but dropped the average by 3.0 mpg so this means that her FE was MUCH lower than 47.5 mpg, right?
In my spreadsheet if I show increments of 5 miles distance and 54.5 mpg average then that makes for 29 rows of 5 miles @ 54.5 mpg for 145 miles of distance at an average of 54.5 mpg. The problem here is we need to calculate in the total miles and total mpg. If I enter 47.46 mpg for the next 6 rows (6*5 miles = 30 miles farther down the road) then this only drops the average from 54.5 mpg to 53.29 mpg. In order to drop this to 51.5 mpg I have to enter a value of 37.0 mpg for those 30 miles. Try this and see if it works out as well in a spreadsheet and not just straight calculations.
Also, remember that I was getting a displayed average of 54.5 mpg up to then so the temp, weather, road terrain, etc. were all the same. And from there home I nursed it back up to 53.1 mpg displayed or (in my spreadsheet) an average of 53.7 mpg for the next 455 miles ... on snow tires and ~50 degrees remember. Maybe we'll go somewhere this weekend and I'll let her drive but not tell her why she is getting the opportunity.
Re: Another MPG question
