You can get near-instantaneous readout by resetting it and seeing what it says. I've used this to, for example, set my cruise control to 70-MPH on a long flat highway and hit reset. It says 40 MPG. Then I set it to 65-MPH and hit reset. It says 45 MPH (numbers made up for illustrative purposes). Then I start calculating how much more it will cost me to get there X hours earlier.
Yeah, but unlike my trip counters, I only have one mpg counter, so I'd have to give up the tank reading to "test" it that way.
I test drove a few cars that had instantaneous ones AND long term ones. Those seemed fun, but man, it just seemed like something to get obsessive about...
So? You know you can also calculate your tank MPG using basic math?
Did you read any of my posts? It is clear I do this. (Since I was comparing the calculated value to the car readout!) In fact, I have the notebooks from 1998, where I have done this with every tank of gas I have ever purchased, I use excel now though. In addition to MPG, I also like to know my cents per mile. However, one of the things I track is to compare how the car readout compares to my calculated readout. If I reset it midtank, I would have to give up that reading. Then my spreadsheet isn't complete anymore.
I don't keep a notebook, but I do keep a pen in the car (handy in other situations too) and I just write down the odometer value on the receipt (which I need anyway to get the gallons filled).
But your way works too =)
I generally do this, but when my pen is missing, I just take a photo of the odometer with my phone.
My current car also tracks average speed, which is better than me trying to estimate city/highway. So I add that to the spreadsheet now too. My spreadsheet also says what the general weather was like (my mileage is obviously worse in the winter.)