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What Do The Numbers And Codes On Gasoline And Tires Mean

There’s a lot of shopping people do in the automotive aftermarket, from tires to what you put in your gas tank. What do the different numbers on some of these products mean? What are octane ratings? What are those codes on the side of tires? Purchasing the wrong “number” of one of these items may not only be unnecessary it may be harmful to your car or even make driving your car dangerous.  

We’ve all seen the different numbers posted next to the price and description of gas at the gas station. You see 87, 89, 94 even 104, the octane rating. The question is what do they all mean and is a specific one needed for my car or unnecessary? An octane rating is simply is a performance measure of gasoline. The higher the octane rating the more compression the fuel can withstand before detonating, performance engines typically have higher compression, and they obtain more energy out of the engine’s fuel and air mixture. For example a Ferrari 458 has a 12.5:1 compression ratio as opposed to a Toyota Corolla at 10:1.  If an engine requires a higher octane but a lower octane is used it will result in engine knocking. Essentially an improper detonation occurs outside the normal controlled combustion process. Continuous knocking can result in engine damage, so if a higher octane gas is absolutely required it should be used. At a cost of about ten cents more a gallon between regular, premium and super octanes, make sure it’s necessary. A vehicle’s owner’s manual should advise you as to what octane gas your car requires. Using a higher octane when not required would essentially just be wasting money, but if you do notice “knocking”, try a higher octane to see if the problem ceases.

Tires have a lot of numbers and letters on them, P215 55/R17 M+S, a pretty confusing series of numbers and letters. In this example P means passenger, the tire is designed for a passenger car, as opposed to a light truck, or a temporary spare, ie “T”. 215 is the tire width in millimeters, the area touching the road.  The 55 is an aspect ratio relative to the width of the tire, from the top of the wheel rim to the top of the tire, it is 55% the width of the tire. R signifies that this is a radial tire; the design of the interior of the tire is manufactured as a radial ply tire. 17 is the wheel rim the tire fits on, from 14 past 20, passenger car rims run the gamut in size. The M+S abbreviations stand for mud and snow, meaning these tires have been approved as all-season tires, helpful in mud and snow. The tire of a Corvette may be considered a high performance tire, built for speed. An additional letter in front of the R may be a Z, W or a Y on performance vehicles meaning these tires are made to withstand speeds up to 200 MPH. What do you need for a tire size and category? Right off the bat you can disregard the need for high speed tires; nobody should need a tire to perform over 150 MPH. A wider and lower aspect ratio tire can be viewed as a style taste; people prefer the look of a low profile tire. Cars with a 35% aspect ratio are not uncommon with a wide width of say 255; the top of the wheel to the top of the tire appears incredibly narrow. The problem here is the cost, tires this size with such a low profile are very pricey, and are meant to handle extremely well under hard cornering conditions, which is rarely necessary. These combinations can also result in a very rough ride, obviously if the tire appears that thin it’s not going to result in a cushy ride.  Finally, if the combination is not appropriate for the vehicle in mind or not installed properly, it can result in alignment, brake and suspension problems. If you’re not going to the race track purchase a well rated, all season affordable tire, nobody needs to drive 100, and corner at 45 miles per hour. Buy for necessity and need, and safety and value.

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