I posted this in the other thread. It works well for me. Thought I'd share.
I use 1 factor and 1 factor alone to determine my city / highway split: the average speed on the DIC. I reset it every time I add a fuel up. Now lets get into detail:
Average speed = Distance / Time. For the sake of this comparison.. time is what is important. How much time in the city vs. how much time on the highway.
I use the EPA testing cycle as a benchmark. The average speed of the city test is 21 mph. Therefore, I use 20 mph or lower average speed as a 100% city conditions. This has only happened on 1 of my fuel ups. The highway test cycle puts an average speed of 48 mph I believe, so I round that to 50 mph.
Now here is where I make my distinction. I factor in *true* speed of my highway driving. Back at home in Buffalo, NY drivers are less aggressive because the cops brutally enforce the 55 mph state speed limit on all the local highways. Therefore, I am able to safely travel at 55 mph cruise every time I get on the highway. For simplicity (and to counteract getting on and off ramps), I assume a 50 mph average speed or higher in this scenario to be 100% highway driving. I also realize that most people don't have the luxury of driving this speed. While I'm at school in Delaware, 55 mph on I-95 is not only unlikely - it is dangerous. So when I'm here, I bump the cruise up to 65 mph which is about what people can get away with on most highways. In that case, I use 60+ mph average speed as 100% highway driving.
So calcuating the split, the average speed basically tells you how far your car has traveled in the time it is on since you last filled up. I interpolate between the two values. An average speed of 40 mph is exactly halfway between 20 mph (100% city) and 60 mph (100%) highway. It will show up on my fuelly as 50% city / 50% highway. Back in Buffalo, 35 mph is directly between 20 mph (100% city) and 50 mph (100% highway). (Not) Coincidently, this really does accurately describe the driving split. For calculation simplicity (so that you don't need a calculator to do it), just take your average speed.. find out in multiples of 4 (for 60 mph highway) or 3 (for 50 mph highway) how far it is from either end and that is your split to the nearest 10%. So a 36 mph tank for me equates to 60% city / 40% highway in Delaware and 50% city / 50% highway in Buffalo. For the nearest 5% (what is used by fuelly), just use multiples of 2 or 1.5. And on the couple road trips I've had.. speeds of 64 mph and 62 mph have shown up. 100% highway. . Some road trips you get city like conditions like accidents. I've had one slow my average to 57 mph with a half hour of traffic. That was included in the entry as 10% city / 90% highway. Sound accurate?
By keeping this that detailed, my fuelly displays the exact amount of driving conditions I drive in. As of now it reads 53% highway / 47% city.