40MPG at 100MPH?

Question

Around 1987-1988 I had a Ford Fiesta/Festiva. I was in the Army and regularly made weekend trips, driving from southern California to Central Washington a trip of about 1200 miles. I would leave in the evening at the end of the work day, and drive straight through (I was younger then). This little car would do well over 100mph, but was very happy strolling along at around 100 and it would get 40mpg doing it! at the time, there was almost no police presence on I-5 in Oregon at night. I would make it across the state 308 miles in 3 hour plus or minus a couple minutes, with a gas stop.

The car is long gone, the memories of the trips and that fantastic gas mileage are still with me. I am pretty sure it was a Ford Fiesta (first generation) I bought it used in about 1987, and drove it until I blew a head gasket (I recall it was of European decent).

30 years later, there are some cars that make claims of 40mpg, but in the real world I don't think any pure gas cars really do. As an example The 2016 Honda Fit is advertising 41mpg hwy.

30+ years later it seems reasonable we should have entry level cars, that easily meet and surpass this. It seems like I should be able to buy an inexpensive car that will cruise at 75mph and get 60mpg, why can't I?

Answer 1

One of the reasons is loads of extra weight added by safety features and options. 8 airbags, 12 speaker stereo, 14 way adjustable seats, tons of insulation all around, power windows, power locks, 18 computers with hundreds of sensors, etc.

Also consider the size/power output of the engine. It looks like your car had a 4 speed manual. Most cars today have 6-9 speed automatics. Engines today are more complex with DOHC and variable valve timing, which makes the vehicle heavier.

At least where I am (Eastern USA), gasoline today is mixed with 10% ethanol, which has less chemical energy than straight gasoline.

Answer 2

There's a few things to consider.

1. Vehicle weight: The argument has been made that weight as little to no effect at constant speed, and that it true. However, vehicle weight will determine how much power will be needed to accelerate at a decent rate, and that will largely determine engine size. Generally speaking and all other things being equal, larger engines are more powerful while smaller engines are more efficient. Your 1st-gen Fiesta had a curb weight of 730 kg, while the 1st-gen Honda Fit you are comparing it to has a curb weight of 1091 kg, almost 50% more, while the Smart car has a curb weight of 940 kg, still 29% more. Because of crash safety tests and safety equipment, cars do tend to be heavier today, meaning they tend to have larger engines too.
2. Engine size and efficiency: as stated above, larger engines tend to be less efficient. Generally speaking, engines from the past 10 years tend to be more thermally efficient than previous engines, meaning they can extract more energy from the same amount of fuel. Before that (late 80's to late 90's), efficiency gains were mostly made from electronic fuel injection (as opposed to carburaters, like your 1st-gen Fiesta). How significant is this? Well, in 1991, Nissan's GA16DE (1.6 L) engine won a prize for fuel efficiency. In 2007, the Honda Fit came with a 1.5 L engine that had the same power as that Nissan engine but slightly better highway mileage (33 mpg vs 34 mpg), while the Smart car claims 39 mpg. Progress in engine technology will allow cars to have smaller engines, therefore even more efficient engines.

If manufacturers would make a car today that is as light as your 1st-gen Fiesta was, modern engines would undoubtedly produce better highway mileage than back then, probably 45 mpg, maybe even 50 mpg.