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BACKYARD MECHANIC: Tire pressure key to ride, load weight

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2003 Subaru Legacy for $4,995

2003 Subaru Legacy

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QUESTION: I own a Ford F150 4x4 and need to replace its tires. Right now I have original-equipment LT 245/75 R 16 load range D tires on the truck. The maximum inflation pressure on the tire sidewall is 65 PSI. The door sticker recommends an inflation pressure of 42 PSI.

I will be keeping the same tire size (LT 245/75 R 16) but would like to switch to a load range C. The maximum tire inflation on the sidewall of this tire is 50 PSI. What should my actual inflation pressure be on these new tires?

ANSWER: Tire pressure plays an important part in determining how much weight can be carried in your truck. Higher pressures can support more load. Lower pressures provide a softer ride. The maximum tire pressure as listed on the tire sidewall is the most pressure the tire can safely hold and support its maximum load.

The maximum tire pressure specification, however, is seldom the ideal for normal driving. I would set the tire pressures to those recommended on the door sticker: 42 PSI. This is a good starting point, but the pressure can be varied slightly to provide the best tire wear. Increase the tire pressure in two-PSI steps if the tread wears more on the edges than in the middle. Decrease the tire pressure slightly if the center of the tread is wearing more.

Dropping the tire pressure more than about four PSI below the recommended setting can cause the tires to flex more and build up heat internally. This can cause the tire to fail, so don't lower the pressures very much. If you're going to be driving at high speeds, during extremely hot weather, or when hauling heavy loads, then the tire pressure should be increased by about four PSI above the recommended settings. Never put more pressure in the tire than the maximum listed on the sidewall.

Switching to tires with a C load range rating will reduce the load-carrying capacity of your truck. It doesn't work the other way around, however. Installing a tire with a load rating higher than D may allow the tires to carry more load, but the limiting factor is then the strength of the truck's axles, bearings and suspension.

You may notice that some trucks the same size as yours use P (passenger) tires rather than LT (light truck) tires. The P-type tires have much softer sidewalls and give a much smoother ride, but the at the expense of handling. The soft sidewalls will flex if a heavy load is carried or a trailer is pulled, and this causes the truck to sway. It could get bad enough to cause the driver to loose control. But if you're going to use your truck as a passenger vehicle only and don't carry loads, then you may want to check out P-type tires.

QUESTION: I have a Subaru Legacy and I put synthetic oil in the transmission and differential at 48,000 km. Now I have 170,000 km on the car and have not changed the oil since I changed to the synthetic lube. If I change oil now, would I damage any parts?

ANSWER: It's okay to change the oil. Changing the oil may not be necessary, but it won't hurt anything either.

Small metal particles wear off the gears as the vehicle is operated. A magnet located either inside the case or in the drain plug traps most of these small particles, but those that are not trapped can cause wear on bearings and bushings. Changing the oil will get rid of them.

Many vehicles are operated far in excess of 160,000 km without ever having the oil changed in the transmission or differential. In fact, some manufacturers are now recommending certain models of transmissions only have the oil changed if they need to be disassembled for repair, or at the 160,000-km interval.

Better oils, better materials and closer manufacturing tolerances have allowed manufacturers to extend oil-change intervals for transmissions and differentials much longer than in the past.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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