What accounts for this?

[bruceha_2000]

Originally Posted by hobba

Here's a good example of what I'm talking about in this thread.

I recently made a daytrip from my home to Orlando, FL and back. Tanked up the car before leaving and did not top off. I drove to Orlando and when I got there, I found gas to be cheaper than at home, so I tanked up again without topping off. I drove home and again tanked up, same as before.

When I used the prius chat excel sheet to calculate my mileage, I averaged 46.7 on the trip up, and 30.1 on the trip back. All travel was done on the same day with AC.

I'm guessing this is precisely your problem. Different pumps. Some will knock off 1.5 gallons before the amount I KNOW I have used, others just shy. Did the MFD MPG say something close to 46 and 30 respectively? Before you pump, calculate how much you SHOULD have used. If the pump kicks off before that, keep going. You aren't topping off, just accounting for a bad pump. My bet is you got ~37 there and 37 back but the pump in Orlando shut off early, so you got more in on the fill back at home than the actual miles and MPG would dictate.

How far is it to Orlando and what sort of roads? Are you driving with 'elasticity' meaning leaving enough space between you and the car in front that you rarely slow down even though those in front of you are continually jumping from accelerator to brake pedal? 37 MPG would just kill me except in the depths of Vermont winter (OK it still kills me but it is at least explainable).

Plus there is the 'out and back' issue. Not much for mountains in Florida but if you happened to have a 20 MPH tail wind on the way to Orlando it is a 20 MPH head wind on the return. BIG delta in MPG, not 16 MPG delta but it can easily be 5 MPG each leg adding up to 10.