This week, we capitalize on last week’s basics of tire rolling resistance, how it translates to passenger vehicle fuel economy, and non-Newtonian and non-linear nuances.
Tires by the Numbers
Figure 1 shows the nameplate for one of my tires. Left to right:
- M+S indicates the tire is suitable for mud and snow. Mud? Don’t try it.
- 225 is the tire width in millimeters, sidewall to sidewall. 225 millimeters is about nine inches.
- 40 is the sidewall height, from rim to tread, divided by the tire width. 40 means this ratio is 0.4, so that height is 0.4×225 = 90 millimeters.
- R means radial tire.
- 18 is the rim diameter.
- 92 is the load rating; in this case, it’s 1,389 pounds.
- H is the speed rating, discussed extensively below.
Figure 1 Tires By The Numbers
Other ratings on my tire include 560 for treadwear, A for traction, and A for temperature. These ratings are all relative to a test case.
- “A” treadwear rating of 560 means the tire would last 5.6 times as long as a reference tire over a 7,200-mile test drive. According to my research, 560 is pretty good.
- The traction rating indicates the friction coefficient of a locked, skidding tire. I mentioned last week that all mechanical engineers calculated this in their sophomore dynamics class. An A-rating is second of four, so it’s pretty good.
- The temperature rating indicates how well the tire can reject heat. Hot tires are bad for performance, as we can see. An A rating is the best.
Rolling Resistance and Passenger Vehicles
Speed Rating
The speed rating introduced above indicates the highest safe speed for the tire—not to be confused with the highest safe speed for the vehicle or driver. Figure 2 provides a handy list of speed ratings. My OEM (Original equipment manufacturer, or in this case, the tires that came with the vehicle when it was new) tire in Figure 1 has a maximum safe speed of 130 mph. When they need replacing, I will choose tires with a Y speed rating and AA temperature for less deformation, which wastes energy.
Figure 2 Tire Speed Ratings
What does the speed rating mean for tire physics? The higher the speed rating, the less deformation the tire experiences as rotational speed and driving forces impact it. Tire deformation wastes energy in the form of heat – something we don’t want for fuel efficiency. Also, I’m not too fond of it because deformation results in a squishy, sloppy ride. I like tires that ride like rails. Higher speed ratings have lower rolling resistance and are therefore better for fuel efficiency, per Figure 3 from x-engineer.org.
Figure 3 Rolling Resistance Coefficient Versus Speed Rating and Tire Speed
Unlike bicycle tires I examined last week, passenger vehicle tires experience significant increases in rolling resistance at high speeds.
Tread Wear
New tires have more tread for better traction, but the cost is more rolling resistance and reduced mileage. I always experience this with new tires, but staying out of the ditch is also convenient. TireRack.com says that mileage can drop 2-4% with new tires. In my experience, it’s significantly more than that, like 10%, minimum.
Sometimes traction is more important than efficiency. Two years ago, as I casually made my way to the UW-Madison for an electrification presentation, I rounded a paved corner that was freshly greased by light rain. I fish-tailed right, left, right, and into the ditch. Was my first thought “OMG, I could have been killed“? No. “How do I get to campus in time for the presentation?“ The show must go on! I made it, no problem.
Weather
Wet, snowy, packed snow, and ice all decrease mileage and/or increase rolling resistance because the surface is not hard and smooth. In some cases, roads may be slippery, and slippage wastes energy. Wet roads can increase rolling resistance by 50% – at least, I would say. A more frequent factor in rolling resistance may be cold temperatures, which, like wet roadways, can increase rolling resistance by 50% per Figure 4.
Figure 4 Rolling Resistance Versus Ambient Temperature
Tire Pressure
Tire PressureReaders know by now that higher tire pressure reduces rolling resistance and increases fuel economy. Figure 5 shows the relationship between tire pressure and rolling resistance. For reference, a bar is roughly 15 psig. Car tires are typically inflated to 30-35 psig.
Figure 5 Rolling Resistance v Tire Pressure
Electric Vehicles
Electric VehiclesReturning to where we started, in Wired.com’s article, The Race to Create the Perfect EV Tire, “Michelin claims that, on average, EVs have 20 percent more tire wear than comparable internal combustion engine vehicles.“ Uh-oh. How does this affect maintenance costs, which is one selling point of an EV? No oil changes are often cited.
A local oil change runs $60.
The Tesla Model Y is said to be the most popular car in the world, so let’s go with that. One problem with Tesla is its tire sizes are large (Figure 6), uncommon, and expensive. My tire of choice at this size runs $346 each for the front, and WHOOP (cue, Chris Berman) Continental doesn’t even make that size of my tire of choice for the rear. A comparable Continental tire runs $460 retail.
Figure 6 Tesla Y Tire Sizes
Putting these numbers in a calculator and using 40,000 miles per set of tires for the ICE vehicle, we have arithmetic results shown in Table 1. Tire replacement costs dwarf oil change costs – but fuel costs dwarf oil and tire costs. The Tesla comes out ahead by about $5,400 over 120,000 miles for fuel, tires, and oil-change advantages.
Table 1 Tire and Oil Change Comparison
The Big Picture
The Big PictureI provided an abysmal dive into tire characteristics, but tires are only one part of fuel economy. Vehicle weight matters, and so does drag, aka wind resistance. At a mere ~ 35 mph, drag overtakes rolling resistance as the leading cause of fuel consumption for propulsion. Drag forces increase exponentially with speed. The energy required to overcome drag (force times speed) increases with the cube of speed. Drivers should notice a clear difference in mileage driving 78 mph on an Interstate versus 70 mph. Consider that!
Figure 7 Drag and Rolling Resistance
Tire Shopping Summary
Here is my Christmas tire-shopping guide:
- Be safe! Traction first!
- For fuel efficiency and savings:
- Higher speed rating: H minimum, Y or W preferred
- Longer tread life: 500 treadwear rating or better
- Traction rating: A or AA
- Temperature rating: A
- Maximum tire pressure: at least 50 psig
- Inflate tires to 40 psig
