I too have only anecdotal information but as I keep very good records of my fuel usage it may be useful. Here are the constants.
Every week day I drive from Huntington Beach, CA, to Corona, CA, in the morning and return in the evening. The commute is 37? miles each way, 34 miles on three freeways. I drive a 1994 Nissan Sentra Sedan, 1.5L 4 cylinder, 3 Speed Automatic Transmission plus OD. I deliberately have chosen a commute time that is between the more popular times so I am before the “rush hour” to South Orange County. On weekends I drive only on local streets and rarely go more than 10 miles on both days combined.
I have not changed my driving habits for my morning commute but I have changed my driving for my evening commute. Two years ago it was my habit to set the cruise control at 65 mph and stay in the number 2 or number three lane, as appropriate. I only occasionally had to turn off the cruise control to slow for the impact of another drivers stupidity. I averaged 28 mpg.
After the first of this year I noticed that, perhaps due to the rise in gas prices, traffic was moving more quickly. I was able to keep the cruise control set at 70 or 75 mph and stay in the number two lane. Since I have done this I have averaged 33 mpg.
I suspect that the drive train of the car was designed for highway speeds. It performs poorly from a standing start, “jackrabbit” starts do not seem to be possible, perhaps due to the EFI that seems to say “Go ahead and floor it but I will only give the cylinders as much fuel as they can consume.”
As I said above, this is another bit of anecdotal information but it is also slightly controlled data.
The problem appears to be that many do not understand lugging, its affect on an engine, and engine control items that are affected by lugging; MAP sensors, manifold vacuum, etc.
Consumer Reports is one of the last places I’d look to for expert automobile advice. CR is the same outfit who states (paraphrased here) “the owners manual is always the final arbiter as to maintenance on your car”. CR is obviously wrong because there are a number of things that SHOULD be maintained more regularly than what any owners manual recites; trans fluid changes, fuel filter changes, valve adjustments on mechanical lifter engines, etc. In many cases air filters may need to be changed every 10k miles depending on the environment.
Constant short hop driving may lead to induction system cleaning more often.
Consumer Reports also investigated, AND backed, a lady who thought she was being ripped off by a dealer who recommended a timing belt change at 60k miles. According to CR anyway, there is no age limit on a timing belt and her 7 year old timing belt was just fine.
So owners of belt driven cars can sleep easy knowing that.
Wonder if CR is going to foot the bill for her if/when the belt gives out at 80k miles instead of the recommended 105k?
I hesitate to write this with so many naysayers writing in. However, I kept careful records from two trips from North Louisiana to Grand Canyon. AZ. On the first trip we traveled in a 2004 Toyota Highlander (4 cylinder)and on the second we traveled in a 2006 Toyota Highlander (6 cylinder). On the first trip we averaged about 24 mpg until we reached Colorado. At that point our milage varied from about 29 mpg to slightly over 30 mpg. We were traveling at higher speeds as we went further west (fewer towns, not as much traffic). We experienced the same thing on the next trip in a different car. We have always wondered if higher altitude was the reason. Most of our travel was on interstates and was never slow – probably 65-75 mph until we reached Dallas and 75-85 after leaving Dallas. I notice that I failed to mention that our average mpg was lower over-all in the 6 cylinder vehicle.
The relationship is not always exacty linear, but it’s true that if you slow down your mileage improves. I travel to the same city a number of times. My mileage ranges from 30 mpg at 75 mph with the A/C on to as much as 40 mpg when the going is slow (about 45-50 mph) due to weather condition and the A/C off. So, in my case I slow down 33% and get 33% better gas mileage.
Here’s my recent experience - rental '08 Maxima, Kansas City to Topeka at 70 mph, got 27.5 MPG, Topeka to Kansas City at 75 MPG got 26.0 MPG. I would expect the general rule “faster = lower MPG” to hold true, with exceptions for a minority of cars, certainly above 60 MPH.