Choose LT tires if your pickup gets heavy use

Hey there, time traveller!
This article was published 10/04/2004 (7851 days ago), so information in it may no longer be current.

QUESTION — I need new tires for my Ford truck and need some advice on which tires to buy. The truck’s tire size is now P215/70R15. The tire advertisements I see in the newspaper don’t show this size, but I do see LT 215/75R15 tires for sale. Will these tires work OK on my truck?

ANSWER — The LT type tires will work on your truck, but the speedometer will read a little lower than it used to. There are two major differences between the two tires. The first difference is the application the tire will be used for.

LT tires are designated for light trucks, while P type tires are for passenger vehicle use. Many pickup trucks come equipped with P type tires from the factory because the prime use of the truck will be to carry passengers and very small loads. LT type tires have more reinforcement in the sidewall and can carry more load. The LT tire is also stiffer than P type tires and gives a harsher ride.

I would recommend LT type tires for anyone who uses their truck to carry loads, haul a camper, or pull a trailer. The stiffer sidewalls will reduce vehicle sway and make it much safer when heavily loaded.

P type tires can be used if the truck isn’t used to haul loads. Regardless of the tire type you select, be sure to not overload your truck. The maximum gross axle weight for your truck should be listed on the data plate on the truck or in the owner’s manual. If you can’t find it, check with the dealer.

The second difference is in tire size. Both tires are 215 mm in width. This is measured across the widest part of the tire, not the tread. A wider tire may fit, such as a 225, but installing a tire much larger may cause interference between the tire and the suspension or truck body.

The aspect ratio of the tires is where the size differs. The P type tire has an aspect ratio of 70, while the LT tire has an aspect ratio of 75. The aspect ratio indicates the height of the tire sidewall compared to the width of the tire. The P type tire sidewall is only 70 per cent the size of 215 mm or 150.5 mm in height. Lower sidewalls are used on sports cars to give better handling and performance, but there is little difference between 70 and 75 aspect ratios. The only difference you may notice is the speedometer will be slightly out of calibration if different size tires are used.

Select a tire based on what it will be used for rather than cost and you will have a safer and more pleasant truck to drive.

QUESTION — I drive a Volkswagon Rabbit diesel and recently had trouble with the car. I took it up to get an estimate on a paint job at a body shop and went inside to talk to the owner. When I came out, the engine was screaming away and it wouldn’t stop when I turned the key off. I put it in gear and let the clutch out with my foot on the brake and finally got it stopped. When I restarted it, everything seems fine! I can’t find anything sticking. What could have happened?

ANSWER — You have probably just experienced “diesel engine runaway”. Diesel engines regulate their speed by varying the amount of fuel injected into the cylinders. If more fuel is provided, the engine runs faster. When you turned the key off, you stopped the fuel supplied by the injection pump, but the engine was getting fuel elsewhere.

Diesel engines will run on almost any fuel and the paint fumes from the body shop are a great source of fuel. The air currents were just right and the solvents from the paint were likely drawn into the air cleaner causing the engine to “run away”. You are lucky no apparent damage was done. I have seen diesel engines destroyed in seconds during a runaway situation!

Never park your diesel and leave the engine running where fuel fumes could be present.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca