(MCT) -- Q: I just read your article "Wrong tire size is not a solution," where you advised that tires should all be the same size on a Jeep Liberty. I recently blew a tire on my 2011 VW Touareg and had to purchase four new tires because tire stores said they couldn't sell me just one tire due to the vehicle having all-wheel drive. My VW dealer confirmed this. Why wouldn't this apply to the Jeep Liberty as well?
A: With any AWD or 4WD vehicle where power can be applied to all four tires at the same time, the critical issue is tire diameter. All four tires must have the same rolling circumference, meaning each tire rolls the same distance in one complete revolution.
Why? As I explained in the earlier column, any significant differences in tire diameters can generate significant mechanical stress on driveline components. Think of it this way; if you had a solid axle with a different size tire/wheel welded on each end, what would happen when you tried to roll the axle straight forward? Either the larger tire would skid, or the smaller tire would skip. Can you see how this would stress the shaft or axle?
So why do carmakers recommend replacement of all four tires at the same time? Tires are still manufactured one at a time in a mold, so even the same make, model and size tire can vary tire to tire, particularly when manufactured at a different time.
Carmakers are concerned that a single replacement tire may vary enough in diameter to stress the driveline. They're also concerned that traction due to increased tread depth on the new tire could affect handling and braking. I also suspect that part of the four-tire replacement recommendation has something to do with concerns over potential liabilities.
Would the car know that one tire was newer than the others as long as it's the same exact diameter? I don't think so. It may sense a difference in traction due to tread depth difference, particularly in wet/slippery conditions, but not simply that one tire was newer than the others.
One important point to remember is that tire diameter shrinks over tread life. Thus, replacing just one worn/flat tire with a new tire of the same make, model and size could create stress on the driveline due to the larger diameter of the new tire. But as long as all four tires are the same rolling diameter, no damage to the driveline will occur.
Q: Last week I stopped at a gas station to put gas in my 2001 Kawasaki 1200 motorcycle. After choosing how I was going to pay, I grabbed the diesel hose without thinking. I pushed the button for the highest octane gas, not the one for diesel, but diesel fuel came out anyway and I filled the tank. I went about half a mile before the engine began to balk and I realized what I had done. A repair shop is estimating a cost of up to $1,000. Does this seem reasonable? Can't they just drain the tank and carburetors and refill it with gas?
A: Considering the age and value of the bike, I'd sure try that first. In addition, I'd open the draincocks on the carbs, route drain hoses safely into a container and completely flush the fuel system from tank through gascock/filter and carbs with gasoline. It may take several flushes to clear the system, but I would think this would get you back on the road. Change the engine oil, as well.
It might be a good idea to keep the first few outings close to home and carry your cellphone.
ABOUT THE WRITER
Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Readers may write to him at: Star Tribune, 425 Portland Ave. S., Minneapolis, Minn., 55488 or via email at email@example.com. Please explain the problem in detail and include a daytime phone number. Because of the volume of mail, we cannot provide personal replies.
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