That method is good, but flawed. It does not take into consideration actual stop & go traffic(real city driving). One could drive all 35-45mph roads in town with very few stops & still get a high city % based on average speed. Does that sound right?
A real city MPG number needs to have many stops & idling to get an actual realistic number, something average speed does not take into account what so ever.
That's all fine & great, but I will take my usual drive as an example. Based on my average speed every single tank would be 70%+ city, which is the exact opposite of actual(my average is more like 70% hwy).
My last fillup/top off was 190miles with most of the drive with the cruise set at 65mph. 10 miles or less was city driving with lots of stop & go(at least 20 stop lights). My DIC indicated 46.9MPH average speed, which by your method would be something like 60% hwy, when in actuality its probably more like 85%+ hwy. Does that make sense if less than 5% of my actual miles driven are city that I rate my drive anywhere near 40% city based solely on average speed? I think not.
On my usual driving route I have lots of small towns I drive though with no actual stops, but the speed is reduced to 35-45mph. Those lower speeds bring down my average speed but I use a bit of common sense & say anything with a constant speed & no stops is still hwy(above 40MPH average).
As my example above shows it really doesn't take much to bring an average speed down, which makes the city/hwy split based on those numbers inaccurate. I do use your method but also take into consideration what percent of the miles per tank are hwy. Any average MPH above 40MPH on the DIC is mostly highway based on the data I have collected.
Well, that is a good point. This is just one way you could potentially tackle it. The rationale really is just what I explained before. My commute this summer was 11 miles, 9 were highway, 2 were in the city. My commute took roughly 19 minutes door to door. For 11 miles, that is an average speed of ~35 mph. That corresponds to about 50/50. If I went by distance, it should be 80 hwy / 20 city. My MPG for tanks usually hovered around 30-31 MPG, and the car is rated 24/36/28. It just is kind of hard to rationalize me concluding that it is more highway when I spent 10 minutes in the city, 9 minutes on the highway, and my MPG was only 30 MPG in the summer. It sounds really close to what the EPA found for the vehicle so the method works for me!
On a road trips when encountering traffic / small towns, your average speed may drop from 65ish to 55ish, which still would be 95%-100%ish highway based on time. If you peep your FE at the end of the trip, it should be right at the EPA estimate.
Wouldn't it be better to just use the average speed of the EPA highway test? Most cars get their best eonomy, aka what most people consider "highway" mpg around 55 mph. In addition, increasing the "highway" speed in the calculations makes it more biased towards city driving percentage.
For example, in theory, if your state had 80 mph highways, you would enter 80 mph in the max highway speed box. This means that even at an average speed of 70 mph, you are still doing 10% "city" driving? That doesn't make any sense. It would be more accurate to stick with the EPA test cycle numbers of 20 and 48 mph average speeds city and highway respectively.