Trying to get the best use of the "city/highway" tracking feature on Fuelly... Let me be clear, this is not a question of "what constitutes city driving/what is the minimum speed to define it as highway driving" etc. I'm trying to estimate better, but need some clarification. Which of the following three examples should I be basing my estimation on?
1) Miles: 100 miles total, 25 of which were at city speeds, therefore 25% city.
2) Fuel: half tank used whilst driving on the highway, used another half driving through the city, therefore 50% city.
3) Time: three hour journey, the last hour was within the city, therefore 33% city.
Another vote on method 1. Although it's unclear what I can call 'city'. My practice is leaving out the villages (almost no distraction, just lower PSL), but count the suburban main road on my commute in. Then, at the end of each tank, I make a guesstimate...
I've done both time and mileage and found very little difference in the calculation, it was more for my personal edification and tracking oil changes than tracking mileage data. (I subscribe to the oil changes at 100 engine hours or thereabouts theory of maintenance schedules. The old rule of 3,000 miles was based upon the assumption that the average speed of the vehicle was 30 mph, hence 3,000 miles equaled 100 engine hours, similarly when the manufacturers started accepting 5,000 mile oil change intervals for "highway driving" conditions that was based upon 50 mph average speed but the same 100 engine hours.) So for me it was a proof of theory that with my current commute that around 4,000 miles was a good target for oil changes, and since I was already tracking the data I played with it to see if it was working out percentage wise to my driving city vs. highway and it was pretty close there too, again further validating my theory.
In your case I'd do what is easiest to figure or estimate for you. For me I know how many miles my commute is and I know how many of those miles are on city streets and how many are on freeways so I use that as my base breakdown and then adjust it to account for extra trips be those around town running errands or on the freeway taking excursions. The bigger factor is going to be that you're consisitent in what ever method you choose to use, don't use method 1 for one fill up, method 2 for another, and method 3 for yet another fill up, that might work out to bad data since it lacks consistency, just figure out one that works well for you and stick to it.
Technically, the correct way is to do it by gallons used. The units in the denominator of the unit in question are the ones that determine the percentage. For miles per gallon, you would split up the city/highway by gallons used for each condition.
Unfortunately that is unrealistic to do, but don't let that stop you. It's not like there is any sort of technical definition for "city driving" or "highway driving" either. For these reasons, since day 1 I have been an advocate of using city/highway just for your own use and not for comparison to anyone else's data. There are no conventions, and everybody does their own thing already, thus you can't really make any solid conclusions from it.
Just out of interest, I have noticed the official government figures for cars in the UK, suggest that some, in fact most hybrids get more MPG's on the "city" test than on the "highway" test now. Which I guess is what you want if most of your journeys are short, low speed journeys around town!
How this translates in real life, only time will tell.
It's a subjective measurement, to be sure. But the EPA considers any driving over 48 MPH to be "Highway", and anything under 21 is considered "City" driving.
The 27 MPH in between 21 and 48 is where your percentage lies.
If you want to use those parameters, you can easily calculate Highway percentage so long as your vehicle offers an "Average Speed" measurement.
Take your average speed for one tank, and subtract 21. If your average speed is under 21, or over 48, then stop here; you have either 100% City or 100% Highway, no math required. If your average speed is somewhere in between, then take that difference and divide that number by 27 (the difference between 21 and 48) to get your Highway percentage.
Make sure to reset your average speed reading at each fillup.
At fillup, my average speed reading for this tank is 41.
41 - 21 = 20.
20 ÷ 27 = 74.
My highway driving percentage for this tank, according to the EPA, is 74%.