Speed vs. economy, or Fd = 1/2pv(squared) x CdA


Diesel engines such as this fine 1HZ are inherently economical, but they still have to work harder to push a vehicle faster Fuel economy is a funny subject. Very, very few people want to admit their vehicles get poor mileage. The overwhelming tendency is to fudge the other way when the subject comes up. I know for a fact there are guys who get 25 miles per gallon at 75 miles per hour in their one-ton diesel pickups while towing their 30-foot Airstream trailers, because I hear it with astonishing frequency. 

I also know they don’t. 

I never say as much when presented with these or similar absurd boasts, because there’s simply no profit in doing so. All I do is raise my eyebrows and say, subtly, “Wow. That would be impressive.” To show skepticism, much less outright derision, provokes the same peculiar outrage one receives doubting someone who claims to have been abducted and studied by aliens. There’s just no point in arguing. 

There’s also no point in arguing with the laws of physics—especially those relating to speed. Note that formula in the headline. In plain English it states that the drag (Fd) on a solid object moving through a fluid medium (which in physics includes air) is a function of one-half the density (p) of the medium times the speed (v) of the object, squared, times the drag coefficient (Cd) of the object times its cross sectional area (A). 

Note in particular the reference to the square of speed. That means that as speed goes up, the drag that results rises on a logarithmic curve. If speed doubles, drag quadruples. 

Here’s an example using an algorithm I found recently, calculated around a theoretical but representative vehicle with a drag coefficient of .30 (very low), and a frontal area of 30 square feet, on a paved road. Our vehicle requires just 2.6 horsepower to move at 20 mph. To reach 40 mph it needs 8.2 horsepower, and to reach 60 it needs 19.6. To cruise at 80 would require 39.9 horsepower—twice that needed to maintain 60. (To give you an idea of the drag acting on very high-performance sports cars, our vehicle would need 521 horsepower to hit 200 miles per hour.) The unassailable fact of highway driving is that beyond about 40 miles per hour aerodynamic drag overtakes powertrain friction and rolling resistance as the chief factor in fuel consumption—and from there on up it wages a rapidly escalating war on the level in your tank. 

Another . . . interesting . . . claim I’ve heard is from guys (mileage fudging seems to be strictly a male pastime) who say something like, “I get better mileage at 80 than at 60 because my engine is in its power band there.” Sorry, but nonsense. There might be situations in which, say, shifting from fourth gear at 55 mph into an overdrive fifth gear and adding a few miles per hour will do no harm to or perhaps even slightly improve economy, but for a given gear, adding speed will increase fuel consumption, plain and simple. No one is going to get better gas mileage at 80 than at 60. 

Algorithms are all well and good, but accurate real-world figures are more difficult to locate. Fortunately one of my best friends, Michael Cox, is diligent about recording the mileage he achieves with his 2006 4WD Dodge 2500 pickup, which has the Cummins turbodiesel, a six-speed manual transmission, and BFG All-Terrain tires. He can just break 21 mpg at 55 miles per hour. At 60 it drops to 19.5, at 65 it’s down to 18, and 70 mph knocks off another mile per gallon. He recorded similar changes with his Four Wheel Camper on the truck: 18.5 mpg at 60 mph, 16.5 at 65, and 15.5 or less at 70. 

The immediate lesson from all this is so obvious I won’t bother to state it. But more subtle contributors to drag are worth investigating. Cleaning up the aerodynamics of a vehicle can have a significant effect on fuel economy, even on one with the base Cd of an apartment building. Removing the tall porthole ConFerr roof rack on my FJ40 increased my highway (i.e. 60 mph) fuel economy by a full one mile per gallon—and going from 16 mpg to 17 in an FJ40 is a blessing. Suspension lifts and wider tires increase drag, the former hugely. So, somewhat surprisingly, do such add-ons as bull bars and driving lights, which create a dirty front of turbulence before the air even hits the vehicle proper. Extreme example: When my nephew Jake exploited his newfound skills at welding and fabricated an exoskeleton roll cage for his V6 Tacoma, his mileage plummeted from 18 to 13 mpg. So, do you really need a fat grille bar to fend off stray kangaroos, or would you be served with a simple winch bumper? Do you need that Paris/Dakar-esque bank of driving lights, or would a headlamp upgrade provide all the safe illumination necessary? With gasoline poised to top $4.00 per gallon and diesel above that as I write this, every bit will help on a long highway trip. 

Ditching the roof rack helps. Brush guard and lights might not hurt the “aerodynamics” of an FJ40, but they can on a more streamlined vehicle.Although rolling resistance becomes less important than aerodynamic drag at high speeds, it’s still worth considering. That one mile per gallon I gained on my FJ40 made up for the exact same amount I lost when I switched from BFG All-Terrains to BFG Mud-Terrains on that vehicle. Long experience with various tread types has led me to the conclusion that there’s not a big mileage difference between street/trail tread patterns such as BFG’s Rugged Terrain and the slightly more aggressive All-Terrain—but jumping to a Mud-Terrain you’ll take a quantifiable hit. So if you put mud-pattern tires on your vehicle just because you like the look, think again. You’ll get better mileage, better handling, a better ride, and longer tread life with an all-terrain pattern, probably equivalent traction in most circumstances, and better in some. 

What about tire width? Wider tires certainly increase frontal area and drag, but I’ve never seen an authoritative study to determine if a wide but short contact patch produces more rolling resistance than a longer, narrower one. You’d need to determine loading per square inch, hysteresis of the carcass, heat buildup, and other factors to make a solid judgement. With that said, I believe narrower tires offer advantages in so many other areas that I invariably stick with unfashionably skinny sizes. I just spent two weeks in Egypt’s sand seas in gargantuan Land Cruiser Troopies riding on pizza-cutter 235/85x16 All-Terrains, and we were never more than nominally stuck despite running near-street pressure in areas mined with the razor-sharp limestone outcroppings called kharafish. 

Narrow tires reduce frontal area and possibly rolling resistance, and they work just fine in sand if aired down properlyThe unique characteristics of individual vehicles can make a difference in fuel economy as well. For example, my mechanic friend Bill Lee had a customer who drove an FJ60 with a five-speed conversion, who complained of wildly varying mileage on the highway in the 60-65 mph range. Bill figured out that the secondary venturi on the 2F engine’s two-barrel carburetor started opening at around 63 miles per hour. If the driver kept the speed just below that, mileage was a full two or three miles per gallon better than just above it. 

Modern fuel-injected engines are unlikely to have such a sharp break in efficiency. Nevertheless, it’s worth experimenting with varying speeds—especially if you have a mileage function on your vehicle’s computer—to see if it has a “sweet spot” above which consumption spikes. 

In general though, you can expect your computer to tell you, “Yep—when you drive faster you use more fuel, stupid.”