Tires and improving fuel economy

[john70t]

Drove a Prius last week. Flat roads were good with warm temperatures. Easy driving. Consistent conditions.

The dashboard said the milage was 43mpg.
Too low.

Since the front tires felt like they were dragging in turns, I checked the tires. They are rated for a maximum inflation of 44psi.

They read 36/36 front and 36/40 rear.
I filled them all to 41-42psi. Never mind the door sticker.
The car felt like it drove "normally" after that.

The milage immediately went to 50mpg.
The next day, the milage went to 55mpg.
Curiously, my leadfoot girlfriend got 55mpg as well.

Although the computer calculates the milage in 1/10ths of a gallon, I have some suspicion of its accuracy, but was suprised at a dramatic 12mpg change from changing tire pressure 4psi.

Could high-pressure, harder compound tires be part of the key to reducing national fuel consumption?

[peppy]

John, I run 41psi in my jetta and it seems to be the magic number.

[Noah930]

I run about 42 psi in my non-hybrid Civic. I can get about 40 mpg on the highway with judicious use of the throttle. The ride is a bit rougher, but it's just me in the car usually so I don't mind.

[slodave]

You wanna feel the road? Throw some Hoosiers on and get ready for a bumpy ride!

[masraum]

Hmm, interesting. I knew that skinny hard tires with higher than normal inflation was good for mileage, but I'd not have expected an increase of 28% based on 4-5 psi. Honestly, I suspect there was something else going on. I wouldn't believe that sort of improvement without more testing.

I may try it in my car. If I remember, I'll try it next week and post back.

[Noah930]

Originally Posted by slodave

... and get ready for a bumpy ride!

Yeah, those would be little orange traffic cones in my case.

[slodave]

Originally Posted by Noah930

Yeah, those would be little orange traffic cones in my case.

Dude! 3 timed runs, 0 cones hit....

[Porsche-O-Phile]

I also experimented with this a while back - definite improvement with tires inflated to maximum rated pressure, but definitely not the best for ride quality or cornering performance.

[Tervuren]

When I have a tires a few PSI low - I can feel it with my right foot on the interstate.

[LWJ]

I might agree. I have a crappy old suburban that I run skinny high pressure tires. I have gotten 17 on a road trip and get 15 on the highway commonly. It is a 3/4 ton 4x4 and I have a lead foot. Tires do make a difference.

Larry

PS - it beats the heck out of you unless there is some payload!

[kaisen]

The actual tires make a big difference too. Not just the PSI they run.
I bought low rolling resistance Kumho 'Eco Solus' KR22 tires for my wife's Camry and picked up 2-3 mpg over the Michelins that were on there, running at the same PSI. And the KR22s have a 100,000 mile warranty. And they're half the price of the Michelins.

Some tires take more energy to push along.

[john70t]

Originally Posted by masraum

Honestly, I suspect there was something else going on. I wouldn't believe that sort of improvement without more testing.

I was a bit shocked, and just wrote what I saw. The computer calculation results are definitely suspect. Same ~1/2 tank of gas was used on both days.

Just had a thought: The one rear tire was 4psi more than the others. Could the traction control system be braking a wheel that much?

[304065]

I own four tire gauges and get four different readings from the same tire. These things ain't exactly traceable to NIST. I have a big Intercomp that I use as the "reference" but who really knows, it's not traceable to NIST and has never been calibrated, and even if it was, a good drop would take care of that.

I bet a cheapo digital piezo type is more accurate than a bourdon tube. Have to do some tests.

[Heel n Toe]

It should be no mystery that a dashboard mileage readout is showing significant gains in MPG when tires are inflated to higher PSI's. Think about it; it is calculating that the engine isn't having to work as hard.

The readout will indicate a lower MPG if you are going up a grade (engine working harder).

It will indicate a lower MPG if you have more people in the vehicle.

Likewise if you are driving into a strong headwind.

Neither of our cars have this readout, but I have seen one on TV.

[Noah930]

I've never had a car fancy (or gimicky) enough to have a fuel mileage calculator in it. Rather I do it the old fashioned way, taking the miles I drive between fill-ups and dividing by number gallons it takes to fill the tank back up. I might be a little bit off for any single calculation, depending on if I overfill or underfill by a smidgen, but over time I have an idea of the mileage I'm getting for any given vehicle.

[speeder]

Originally Posted by kaisen

The actual tires make a big difference too. Not just the PSI they run.
I bought low rolling resistance Kumho 'Eco Solus' KR22 tires for my wife's Camry and picked up 2-3 mpg over the Michelins that were on there, running at the same PSI. And the KR22s have a 100,000 mile warranty. And they're half the price of the Michelins.

Some tires take more energy to push along.

This is interesting. Is there an accepted measuring standard for rolling resistance of tires? I think that most people, (including me), would consider it a strong factor in comparing tires, just like traction/speed/tread wear ratings. On a DD or commuter vehicle, it might be my first consideration, especially since it would seem to correspond pretty directly to long tread wear life.

[kaisen]

From Tire Rack Wholesale (they may have the same tech articles on their retail site as well):

In the United States, vehicle manufacturers are required to maintain an average fuel economy for the fleet of new vehicles they sell each year. In 2009, the government Corporate Average Fuel Economy (CAFE) mandate is 27.5 miles per gallon (mpg) for cars and 23.1 mpg for light trucks (including 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.

A tire's rolling resistance affects fuel economy. Most vehicle manufacturers demand their suppliers develop low rolling resistance tires to be used as Original Equipment on their new vehicles to help average out their CAFE-mandated mpg. In order to meet the manufacturer's 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 role 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? Once you eliminate the fuel converted into heat by the engine, the relative percent of influence that these other factors represent during stop-and-go city driving are very different then during steady-speed, 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%.

Let's 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 on 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.

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.

[kaisen]

Defining Rolling Resistance:

Tire rolling resistance is defined as the force required to maintain the forward movement of a loaded pneumatic tire in a straight line at a constant speed. And just like the laws of physics and forces of nature, it is an obstacle every vehicle has to overcome to transport passengers and cargo to their destinations.

Tire rolling resistance is caused by the natural viscoelastic properties of rubber along with the tire’s internal components constantly bending, stretching and recovering as they cycle between their loaded (where the tread footprint flattens against the road) and unloaded states. The final contributor to tire rolling resistance is the tread’s interaction with the road.

The tread area represents a new tire’s single largest and heaviest region and is the greatest contributor to tire rolling resistance. The tread and its underlying plies typically account for about 2/3 of a new tire’s rolling resistance, while the sidewall and bead area represent the remaining 1/3.

Larger tires require more rubber and longer reinforcing cords than smaller tires. Therefore within a single tire model line, there is typically a relationship between tire size, weight and the resulting rolling resistance force where larger tires have more rolling resistance than smaller tires.

The most common laboratory test measures the force required to rotate a tire at 50 mph against a large diameter steel drum. Multiple samples of each tire size/model are tested to establish an average rolling resistance value. And since tire rolling resistance typically declines moderately as tire temperatures rise from cold to normal operating conditions during the first 30 minutes of driving every time the vehicle is used, values are recorded in the laboratory after operating temperatures and rolling resistance values stabilize.

Tire Rolling Resistance Force is measured in pounds or kilograms of resistance. Comparing Rolling Resistance Force provides a direct way to compare tires of the same size, as well as offers an accurate means of comparing differently sized tires to one another.

Tire Rolling Resistance Coefficient is calculated by dividing the measured rolling resistance force by the tire size’s prescribed load during the test. Comparing Rolling Resistance Coefficients only allows comparing tires within a single size. Tire Rolling Resistance Coefficient does not allow comparing different sized tires.

As noted earlier, larger tires generate higher Rolling Resistance Forces than smaller tires. However the larger tire’s greater overall diameter, circumference and rolling radius allows its tread area to bend, stretch and recover more easily as it cycles in and out of contact with the road. Larger diameter tires also revolve fewer times per mile and cycle at a slower rate than shorter tires for any given speed. While this essentially reduces Rolling Resistance Coefficient (larger tires will often have a lower Rolling Resistance Coefficient than smaller tires), it still does not change the fact that a larger tire actually generates more Rolling Resistance Force that the vehicle’s engine has to overcome.

Unfortunately comparing tire Rolling Resistance Coefficients is somewhat like comparing the fuel efficiency of an 8 passenger, 15 mpg sport utility vehicle to a 4 passenger, 30 mpg car based on miles per gallon per passenger when fully occupied. While both vehicles offer the same miles per gallon per passenger fuel efficiency when fully occupied, the sport utility vehicle will always use more fuel than the car (as well as more fuel per passenger anytime the sport utility vehicle is driven below its maximum passenger capacity).

Regardless of their calculated Rolling Resistance Coefficients, a large tire generating 30 pounds of Rolling Resistance Force will require more energy to roll than a small tire generating 15 pounds of Rolling Resistance Force.

Tire rolling resistance has an impact on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving. The engine and driveline is estimated to consume 80% of the fuel, while the remainder is used to overcome inertia, wind resistance, converted into heat by the brakes or consumed when the vehicle is idling. The automotive industry estimates a 10% reduction in tire rolling resistance will result in a one to two percent improvement in vehicle fuel economy. While that might not seem like a lot, it can reduce fuel consumption by a couple of tanks per year and make the purchase of lower rolling resistance tires a better value over their lifetime

[kaisen]

Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires


New, Full-Treaded Tires Generate More Rolling Resistance Than Shallow-Treaded, Worn Tires

Tire rolling resistance gradually drops by about 20% during the life of a tire as the tread wears from its original molded depth to worn out. This can be attributed to the reduction in tread mass and rubber squirm, as well as subtle hardening of the tread compound during years of service and exposure to the elements.

While this gradual reduction in tire rolling resistance and minor increase in fuel economy may be too subtle to register during the tire's life on a tank-by-tank basis, the virtually instantaneous switch from worn tires to new tires (even if they are the same brand, type and size) will typically result in an increase in rolling resistance of about 20%. Since the automotive industry estimates a 10% increase in tire rolling resistance will result in a 1% to 2% decrease in vehicle fuel economy, drivers should expect to experience a potential 2% to 4% decrease in mpg.

New, Full-Treaded Tires Travel Farther per Tire Revolution Than Shallow-Treaded, Worn Tires

Vehicles are programmed with their Original Equipment (O.E.) tire's revolutions-per-mile to allow their odometers to calculate the distances traveled. Unfortunately vehicle odometers aren't always 100% accurate and the tire revolutions per mile will change as its tread wears.

The diameter and circumference of a new, full-treaded tire is greater than that of an old, worn-out tire. Considering that many passenger car tires are molded with beginning tread depths of 10/32" to 12/32", the tires will be approximately 1/2" to 6/10" shorter in diameter when they've worn to the minimum tread depth of 2/32".

In order to determine how much odometer error this dimensional difference might cause, the Tire Rack team drove one of our 2008 BMW 328Ci test cars on a set of new, full-treaded 205/55R16-sized tires, as well as another set of the same tires shaved to worn-out (2/32") tread depth. We used a Global Positioning System (GPS) to measure the 100-mile test distance on a dry Indiana Toll Road and maintained an average speed of 70 mph. We then compared the number of miles traveled indicated by the vehicle's odometer to the GPS receiver and highway mile markers.

When the GPS indicated exactly 100 miles had been traveled while the vehicle was equipped with new 12/32" deep tires, the vehicle's odometer registered 99.4 miles. When the vehicle was then equipped with another set of the same tires shaved to 2/32" of remaining tread depth, the odometer indicated 101.0 miles. While the 100-mile test distance didn't change, essentially the vehicle's odometer overstated the distance traveled by about 1.5% when equipped with the worn out tires.

Since drivers traditionally monitor their vehicle's fuel economy by dividing the number of miles traveled as registered on the odometer by the number of gallons used to fill the tank (or by simply letting the vehicle's trip computer handle the task), the accuracy of the vehicle's fuel economy calculation is dependant on the number of miles indicated by the odometer.

This means that the vehicle's fuel economy (whether calculated by the driver after filling up or the trip computer) would instantaneously appear to drop by about 1.5% when fitted with new tires simply because the vehicle would actually have traveled 1.5% farther than it did when equipped with its recently removed worn-out tires.

Not All Tire Dimensions are Created Equal

Even though tires may be branded as the same size, their specifications may vary slightly by manufacturer and tire line. Tire Rack has seen the overall diameter of a single passenger car tire size vary by 2/10" from the smallest to the largest.

As an example, we'll compare the differences between an O.E. tire with two replacement tires in the 185/65R15 size used on the 2008 Toyota Prius,

Diameter Tread Depth Tire RPM*
Goodyear Integrity (O.E.) 24.4" 10/32" 855
Yokohama AVID TRZ 24.4" 11/32" 850
General Altimax RT 24.5" 11/32" 843

* Tire Revolutions Per Mile

As shown above, there are slight differences between the tires' published diameters, tread depths and tire revolutions per mile. However, if a tire rolls fewer times per mile than the tire it replaces, the vehicle will again actually be traveling farther than is indicated by the odometer. Calculating the influence of the different tire specifications on the vehicle's odometer would indicate the Yokohama AVID TRZ would travel .6% farther than the Goodyear Integrity, while the General Altimax RT would travel 1.4% farther.

Conclusion

While many of these individual differences may seem insignificant, it is easy to understand that when they are added together, the new tires may appear to reduce vehicle fuel economy. It also means that a Toyota Prius appearing to get 50.0 mpg just before replacing its worn-out tires with new tires of the same brand, type and size, might be reduced to registering just 47.25 mpg afterwards, even if all of the driving conditions were identical.

Remember, "your mileage may vary."

[speeder]

So what you're saying is that I need to run bald, over-inflated tires, right?