How Big Tyres Change Wheel Horsepower

Big, beautiful all-terrain tyres are key component of any off-road build, as they add ground clearance and traction. Plus, they look awesome. But they’re also going to cost you some performance, and you can see exactly how much in this dyno test.

Screenshot: Blake’s Garage (YouTube)

Putting big meaty rubber on Subarus seems to be getting popular these days. We’ve seen some wild hacked-up Outbacks on off-road trails, and this particular CrossTrek owner has a big forum thread dedicated to the farkle they have bolted onto the little hatchback.

One of those mods includes a wheel and tyre change. As the video explains, this car went from stock Subaru wheels with 225/60R17 Yokohama Geolander all-season tyres, to 15-inch Method off-road wheels with 235/75R15 BF Goodrich all-terrain tyres.

The upgrade will definitely add ground clearance and off-road traction. Specifically, the BFGs here are about an inch bigger in diameter than the stock Yokos. They also add weight — the video description says each corner gained 5kg of unsprung mass.

So let’s cut to the results. A stock 2018 Subaru CrossTrek is rated to make 152 horsepower and 196-nm of torque at the crank. On the day this video was shot at this particular dyno, the car made 141 HP and 177Nm of torque at the wheels.

You’ll always see lower power numbers at the wheels than you will at the crank, because of parasitic losses between the engine and the road, so that baseline seems solid.

Throw on the big BFGs and the dyno result dropped to 127 HP and 165Nm of torque at the wheels.

So what the heck happened? The rotational inertia, which factors in the mass of the wheel, is likely contributing to the difference. Most simply put, heavier wheels and tyres means less power realised against the road. My friend Jason Fenske from Engineering Explained went into the science there a little more completely in this 2013 video:

And of course, our own David Tracy wrote a great guide on everything you could possibly want to know about crawl ratios and off-road gearing to complete your knowledge about this stuff before you pick a new wheel for your truck, Jeep or, uh, Subaru. One key point he makes is that, even without factoring in a wheel’s/tyre’s weight, increasing tyre radius without re-gearing yields less force between the tyre and the ground, and thus slower vehicle acceleration.

Now just because the CrossTrek in this video added one inch of wheel diameter and 5kg of wheel weight, then lost 14 HP and 12Nm of torque at the wheels does not mean that you can extrapolate how a wheel size change will affect other vehicles.

It’s also worth noting that the testers only did one pull on this Mustang dyno with each setup. Generally, when you’re sciencing, you’re going to want to get repeatable results to make a conclusion. But this is still an interesting example of how much wheel-measured horsepower can change by changing the wheels of a car themselves.

As for the practical effects of the upsized tyres on this little Subaru, I can promise you that the owner of this Subaru is not going to be making any aggressive passes any time soon. The CrossTrek’s glacial pace on stock wheels is really the vehicle’s most glaring weakness, and I can’t even imagine how slow this car is going to feel after being kneecapped by an extra 46.4 total pounds of unsprung mass. Not to mention what’s going to happen to fuel economy.

But hey, at least it looks bad arse.

If you want to get big wheels without losing acceleration, you’re going to want to mess with gearing. But like I said, if you want to learn more about that, David’s your guy.

This story originally appeared on Jalopnik.

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