In the United States, vehicle manufacturers are required to maintain an average fuel economy for the "fleet" of new vehicles they sell each year. Currently, the government Corporate Average Fuel Economy (CAFE) mandate is 27.5 miles per gallon (mpg) for cars and 20.7 mpg for light trucks (includes minivans, vans, and most pickup trucks and Sport Utility Vehicles). However because it's an "average" fuel economy, in order to sell large cars or trucks (that use more fuel), the vehicle manufacturer must also sell small cars and trucks (that are fuel efficient). The vehicle manufacturer can be fined if their annual vehicle "fleet" uses too much fuel, and can earn "credits" towards future years if their fleet's average fuel economy is better than the government mandated level.
A tire's rolling resistance does affect fuel economy. For that matter, CAFE is so important to most vehicle manufacturers that they demand their suppliers develop low rolling resistance tires to be used as Original Equipment on their new vehicles. In order to meet these demands, these tires are often designed with a priority on reducing weight and rolling resistance and are molded with slightly thinner sidewalls, shallower tread depths and use low rolling resistance constructions and tread compounds.
However, in order to understand CAFE tests and the roll that tires play, it is important to recognize that CAFE tests are conducted in a laboratory and not on the highway. Many aspects that affect fuel economy in the real world are reduced to "constants" incorporated into the formulas specified.
A vehicle's fuel economy is the direct result of its total resistance to movement. This includes overcoming inertia (Newton's Law), driveline friction, road grades, tire rolling resistance and air drag. In order to offer the same level of performance, heavy vehicles require more power (and more fuel) than light vehicles. All-wheel and four-wheel drive vehicles require more power than two-wheel drive vehicles; and boxy vehicles require more power than low drag aerodynamic vehicles.
But how much influence does each of these elements have and when are their influences felt? The relative percent of influence that these factors represent during stop-and-go city driving are very different then during steady, state highway driving.
During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%.
Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.
Now, lets explore a scenario where a High Performance replacement radial tire has a whopping 20% increase in rolling resistance over a low rolling resistance Original Equipment standard passenger radial. To calculate the potential change in mpg resulting from using the High Performance tires in place of the Original Equipment tires, we would multiply the tire's percentage of influence in the vehicle's overall resistance (15% in the city and 25% on the highway) times the High Performance tires' 20% increase in rolling resistance.
If the vehicle equipped with standard Original Equipment low rolling resistance passenger tires normally provided 25 mpg in the city and 30 mpg on the highway, installing tires with 20% greater rolling resistance would only drop fuel mileage by a calculated 3% (to 24.25 mpg) in the city, and a calculated 5% (to 28.5 mpg) on the highway. While this is a measurable difference, it probably isn't much more of an influence on real world fuel economy than being stuck in rush hour traffic a couple of times a week or being stopped at every red light instead of continuing through a string of green lights.
Additionally, the easiest way to reduce rolling resistance to enhance fuel economy is to make certain that the tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi (based on the vehicle's tire placard) will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become underinflated to just 28 psi. Therefore, maintaining the vehicle manufacturer's pressure recommended for light load and heavy load conditions may almost be as important as the tires being used.
Old EPA 23/33/27
New EPA 21/30/24
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.
Old EPA 23/33/27
New EPA 21/30/24
High pressure are run in auto-x because tires don't have time to warm up and are doing lower speed sharp turns. In road racing/track events starting pressures are usually lower than what you would think and build up pressure thru use (turning/braking). But for all practical purposes (street) a few extra pounds will help steering response and rolling resistence...tires don't build pressures that much on the street since the majority of time you are driving straight and not turning/braking at the limits of your tires.
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After a lengthy search, I finally purchased the General Ameri-G4S, the second lowest rating in the nap.edu listing at 0.0078. That was on a 215/75 tire, so I hope my 185/75's are even lower . The paper calls it the Continetal Ameri-G4S, but I wrote Continental, who make General tires, and they told me it was the same tire, just a different brand name on it now. My search would have been easier if I had stuck with my stock 195/65's. But I wanted to go narrower for even less RR, and no one makes a 185/70-15. Since the Cavalier came with 14" rims for many years, I was able to pick up a set of 14" rims at the junk yard. I purchaced 185/75's to maintain the same circumference, and hence, my speedo/odo readings (within 1 or 2 rev's per mile, probably less than the difference caused by tread wear).
My old Michelins were rated for 36psi, but I had them overinflated to 40psi. The Generals are rated for 44psi, so I put that in them right away - the heck with the placard on the door. Another interesting difference in the LRR tires is the tread width. 185/195 = 0.95, or 95% as wide. But measuring the treads actually shows the LRR tires are only 81% as wide.
I wish the RRC was standard, published info. It would help buyers make informed purchase decisions, without hunting the nooks and cranies of the web for out-of-date data.