Kumho 722

Well, it was time for me to get some new tires. With gas prices being the way they are, I figured I’d try something to increase my fuel mileage. What I ended up doing was getting a tire that was slightly thinner, and slightly taller than the stock tires on my car. For those unfamiliar with tires a tire size usually looks something like this:


The first number, ’235′, is the width of the tire in millimeters. The bigger the number, the wider the tire. The second number, ’40′, is the sidewall aspect ratio. The height of the sidewall is the sidewall aspect ratio multiplied by the width. In this case, the height of the sidewall would be 0.40*235 = 94mm.

Okay, so, I got it in my head that a taller, thinner tire might help my gas mileage. Since I needed new tires anyways, I went and got a set of 225/45 tires to replace my original 235/40 size. That means my new tires were 4.3% thinner, and 1.6% taller. What was the result? About a 9% increase in average fuel mileage from 22mpg to 24mpg.

Now before you all go out and buy new tires, let’s talk about this a bit. There are a large number of factors that could have contributed to this increase in fuel mileage, and to be honest, I don’t know which one is driving the increase.

  • Tire Pressure: being somewhat of a car nut, I’m pretty anal about my tire pressures. if you don’t already own one, go and buy yourself a half way decent tire gauge, and keep tabs on your tire pressure. Under inflated tires will eat gas. Anyhow, I kept my tires at 40psi before these tires, and they are at 40psi now. So I think that I can rule out a change in tire pressure being the cause
  • Tire weight/inertia: having a lighter tire means that your engine will need to spend less energy getting your wheels to rotate, and consequently less gas getting from point A to point B. In this case, my mileage actually should have suffered slightly, as the old tires were worn out and weigh less than when they were new. But then again, the new tires were thinner. So all in all, it was probably a wash in terms of the weight change from the old tires to the new ones.
  • Rolling resistance of the tire: okay, this one is a bit complicated, and I don’t really understand it myself fully. As far as I can gather, rolling resistance is mainly a function of tire pressure and tire width. The higher the pressure, the less the tire deforms and the less rolling resistance the tire has (that’s why you see some road bicycles running like 200 psi tires these days). Similarly, the wider the tire, the less the tire deforms, and the less rolling resistance the tire has. Given what i said in point 1, this means that with these assumptions, my wider tires should have had less rolling resistance. But I’ll be the first to admit that I think there’s something else going on here that I don’t fully have my head around. The compound of rubber will make a tremendous difference. My previoius tires were very high performance tires (Toyo Proxes TS-1), which means they had very sticky rubber. My new tires are less performance oriented (Kumho SPT) and consequently have harder rubber. I think that this may be the main factor in my increase in mileage.
  • Gearing: as you may or may not know, between your car’s engine and the wheels there is a set of gears that transmits the power. Shorter gears allow you to accelerate faster, and taller gears allow you to cruise at higher speeds. Taller gears also result in better gas mileage because it allows the engine to run at a lower RPM which will have less losses to the friction inside the engine. Well, there’s one final gearing, and that’s the one between the drive shaft going to the wheel (the axle, or half-shaft) and the road. The taller the road is, the taller your final gearing ends up being. So in this case, by getting a taller tire, I have made my final gearing taller as well. This results in better mileage.

And there you have it, more info than you wanted to know about me trying to figure out why I got better mileage. At the end of the day, I’m surprised at how much better mileage I get (9% is a lot). I may not totally understand it, but the facts don’t lie. I’m getting better mileage, plain and simple. So if you’re in the market for new tires (if you have good tires, it’s not worth it to replace them), and could use a boost in mileage, give this prescription a shot:

  1. Get a harder tire: All tires come with a Treadwear gauge. Now these aren’t the most accurate and consistent ratings in the world, but look up your old tires and find their UTQG Treadwear Grade. When you get your new tires, look for a tire that has a significantly higher treadwear, and it will end up being made of a harder compound of rubber.
  2. Get a thinner tire: Don’t go nuts. I’d suggest deviating from your stock tire size by no more than 10mm
  3. Get a taller tire: Again, no need to go crazy here. As I said, my tires only ended up being 1.9% taller. It takes a bit more math to figure out what sidewall aspect ratio you’ll want given the new tire width, but it’s not rocket science. You really don’t want to go too tall, because you might end up rubbing the tire on something in the wheel well.

Whatever you guys out there do, you do at your own risk. I take no responsibility for any misfortune you might come across for trying this out. Always consult your mechanic before doing anything to your car.

And now for quite possibly the most literal use of this acronym on the internet: YMMV!

[image source: Tirerack.com]

Filed under: Cars | Transportation @ 7:32 am

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