I know you have been complaining about low MPGs for a while, but given your data, I think it's on par with what it's supposed to get. 21.3 MPG for a car that averages 21.5 MPH indicates to me that you do mostly city driving. You also have a SLS with AWD meaning you have two disadvantages to fuel economy: The SLS has the larger 18 inch wheels which results in lower efficiency and the AWD system also adds weight (always) and drag when in use.ipaqxman wrote:I have accumulated meticulously 13 months of fuel economy on my car, since it's averaging so horridly.
I usually get <20 MPG per tank (calculated) but the associated average speed is usually <20 mph. The EPA city MPG is tested at a average speed of 21 mph (while the htihway EPA avg speed is 48). I had a few tanks of 27-30 MPG on roadtrips with average speed ranging in the 40s mph. I know some people don't believe it, but I feel most of the time the average speed gives a good indication of how much city/highway ratio one drives and correlates well to the MPG, so it'd be helpful if people reset their avg speed with each fill-up and report those figures as well.
Kuro's figures are no surprise in that the MPG exceeds the EPA highway rating, since the EPA is tested at 48 mph avg speed, meaning there's still some slowing down and accelerating in the tested cycle. If you can keep your speed at 60 constantly without much dec/acceleration it wouldn't be surprising you are exceeding the EPA rating. Over 65 there will likely be a decline in MPG?
In 13 months after 28 tanks I am averaging 21.3 MPG, with a overall average speed of 21.5 mph. So I am not sure whether to blame the car or the slow city driving around here.
Given your car's specs, the fuel economy isn't that bad. It's on par with rival cars with AWD, in my opinion.
Regarding the decline in fuel efficiency over 65 MPH: Yes. Going faster meaning the engine is at higher revs and burning more fuel. Wind resistance is also greater at higher speeds. Cars are generally aerodynamically tested at around 55 to 65 MPH so these are the speeds in which you get optimal fuel efficiency.
It's also why there is the famous 55 MPH speed limit on most of America's highways. The 55 MPH highway wasn't a creation out of the government trying to curb highway deaths. It was actually a policy to reduce national oil consumption during the oil crisis. However, after setting the speed limit to 55 MPH, it was observed that highway related deaths also declined. This is why it's still in place today although safety wasn't the original concern of the government.