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Tire Guide

Tire size matters when towing

Published December 5, 2013

Q: I am about to replace my aged Subaru Outback and am considering a Land Rover LR2 (2010).

I am having difficulty finding more aggressive off-road tires for it, due to the pretty 19-inch rims. Can I go down to a 17-inch wheel and tire with a bigger (higher) profile to give me more options?

I will be using the LR2 to tow a trailer with dirt bikes into the forest, and other towing duties — using the 3,500-pound capacity of the LR as it was intended.

I like the BFG series of tires and have no issue putting steel wheels on it, with tires that would also do double-duty in the winter.

A: If you put 17-inch “steelies” on the Land Rover LR2, it will be a first. Nothing wrong with being first, but this is a more difficult project than you might image.

The difficulty comes from the fact you want to use its off-road and towing capacity. The LR2 can certainly do that but the OE tire size makes it very difficult to get a proper load index (carrying capacity) of the tire in a smaller minus two (19 inch down to 17 inch) size.

The OE size is P235/55R19 XL with a load index of 107 and a V-speed rating. That equates to a load capacity per tire of 2,149 lb. The tire is 29 inches in diameter.

When up or downsizing, one needs to stay as close as possible to the original dimensions of the tire. But what is most important is that the load index stays the same or goes up.

In the correct downsizing to P235/65R17, there are only two tires to consider, the Pirelli Scorpion ATR and the Michelin LTX A/T 2.

Of these, really only the Pirelli comes close to the specs we need, it has a 108 load index and is H-rated. The Michelin is only 104 load index and S-speed rated.

The Pirelli’s 108 equals a per tire load of 2,205 lb., so it is actually better than the OE tire. These “P” tires, however are not serious off-road tires to go deep into the woods.

You might be better off going the whole off-road setup route and consider an LT (light truck) tire. LTs are built stronger with more layers in the belt packages and run at higher air pressures.

The ride is bumpy but the payoff is much higher load carrying capacity. For instance a LT245/70-17 has a carrying capacity of 3,000 lb. That size tire is 30.5 inches in diameter, which is a bit tall but will fit the LR2.

In the LT tires this size, I like the Toyo Open Country A/T II, the Pirelli Scorpion ATR and the Bridgestone Dueler Revo AT2.

I have misplaced your article giving the formula to get the equivalent tire size when you go down one inch in rim diameter. The tires on the car (2013 Cadillac ATS) are P225/40ZR18 and I think I have more selection for winter tires in a 17-inch size.

Yes you will get a lower price and a bigger selection if you downsize. Also, the narrower tire will provide better deep snow traction.

When changing wheel diameter, it is important to keep the overall wheel and tire combo as close to the OE diameter as possible. A variance of up to 3 per cent is acceptable on vehicles where only two wheels are driven.

Your OE tire has a diameter of 25.1 inches. So look through a tire catalog to find a 17-inch tire with that diameter.

If you reduce the wheel diameter by one inch, you generally increase the aspect ratio of the tire by 10. You then reduce the tire width by 10 as well.

Using those numbers, you end up with a P215/50R17 downsized tire. It has a diameter of 25.5 inches, which is within the spec we need.

Another way to calculate downsizing is to just change the aspect ratio number. These numbers work in increments of five. So downsize the rim diameter by one inch, leave the tire width the same, and increase the aspect ratio by five.

That gives us a P225/45R17 tire with a diameter of 25 inches. Please note that not all tire widths are available with all aspect ratios.

Send tire questions to: thetireguy_1@hotmail.com. Mail volume prevents personal replies.

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