Are rural roads considered to be Highway? I get much different mileage on rural roads with speed limits between 35-50 mph compared to what I get on actual multi-lane highways with limits between 50-65 mph. It seems bizarre to classify them together. For example, rte. 110 is a rural road with one lane in each direction that has a stretch of 4-5 mi. without a single stop sign or light. Is that a highway or city street?
Decided to google it ... up to 2007, the EPA tested city as " ... starting with a cold engine and making 23 stops over a period of 31 minutes for an average speed of 20 mph (32 km/h) and with a top speed of 56 mph (90 km/h)." The highway test "uses a warmed-up engine and makes no stops, averaging 48 mph (77 km/h) with a top speed of 60 mph (97 km/h) over a 10-mile (16 km) distance". So the latter matches my description of Rte. 110.
In 2008, they added some new tests to try and get a more realistic measure, but still, rural roads without stops seem to qualify as highway.
I look at highway speeds as those where the transmission is able to run in its highest gear for more than a mile (or kilometer). You are going to get your best mpg on most vehicles at 45-65 (70-100k). MPG only goes down as you go faster, and driving less than 45 on many vehicles causes (on many automatic transmissions) the transmission to shift down and up too often.
I achieve my best results between 50-60, once the transmission has achieved overdrive and lockup.
But each individual experience may vary. The percentage can be calculated two different ways:
Percentage of mileage driven at city or highway, or percentage of fuel used while driving city or highway. Problem is, for nearly all of us its a guesstimate. Unless you were tracking what distance you actually drove on each, there is no accurate way to know for sure.
I live in the hills of WV which I consider to be VERY rural. The speed limit on most of the roads around here is 55 MPH and I usually drive around 60. But what I have to put up with are very twisty roads where at one moment 60+ MPH is absolutely no problem and then the next you are faced with a 25 MPH hairpin and this is how it goes for almost anywhere we need to go. Given this, is it city or highway?