MrNoodly
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Some tips for those who might not know.<br><br>First, tire size nomenclature. The tires on your van are probably either 225/75-16 or 245/75-16. The first number (225 or 245) is the width of the tread in millimeters. The 75 is the ratio between the tread width and the height of the tire sidewall. Here, the sidewall is 75% as tall as the tire is wide. 75% of 225 is less than 75% of 245. (The 16 is the diameter of the wheel the tire is mounted to—16 inches.)<br><br>When the sidewall is smaller, the diameter of the tire is less, meaning the circumference is also less. A tire with a small circumference needs to turn more times in order to travel the same distance as one with a larger circumference. That means the engine needs to turn more times in order to turn the wheels more times, burning more fuel. <br><br>As your tires wear down, they get smaller. So old tires need more revolutions per mile than brand new ones.<br><br>But there's a second thing to consider: friction. The larger a tire's contact patch (the part of the tire touching the ground at any one time) the greater the friction. The greater the friction, the more fuel that needs to be burned to overcome that friction. Here's where the larger tire from above causes problems. First, it's wider, which means the contact patch is wider. But the longer circumference also means the contact patch is also longer. More friction.<br><br>But it's not just the rubber touching the road that causes friction. The action of rolling the tire from its round shape to the flat area that touches the road then back to the round shape causes friction in the sidewalls. This is one reason it's important to keep tires properly inflated. Not enough air means a larger contact patch and greater sidewall flex.<br><br>Also, tire construction and tread compound affect friction and mileage.<br><br>The weight of the van pushing down on the contact patch causes friction. That weight spread over a larger contact patch means less weight per square millimeter than the same weight spread across a smaller contact patch. I don't have the physics or math to figure out which would generate less friction—more square mm with less weight per square mm or fewer square mm with more weight per square mm?<br><br>When you consider road texture, larger circumference tires require less energy to roll over irregularities than smaller circumference tires.<br><br>Then there's tread pattern. And rubber compound. And tire weight. And load rating.<br><br>So, large tires and small tires both have their advantages and disadvantages. What do we do? <br><br>To complicate things further, even though the numbers on the tires are the same, those tires are not exactly the same dimensions. They just fall within certain industry guidelines. This is where a place like TireRack.com is useful. They list the specs of most tires. Actual tread width, actual tire diameter and, in some cases, to save us the trouble of doing math, they list revolutions per mile. For a 225/75-16, tread widths range from 6.6" to 7.1", diameters range from 29.1" to 29.4" and revolutions per mile range from 709 to 715. Six fewer revolutions per mile might not be much, but it can add up.<br><br>To simplify (or complicate) the decision further, they list LRR tires—Low Rolling Resistance. In other words, tires that generate less fuel-consuming friction.<br><br>So it seems to me, the best thing is to keep the tires you have properly inflated, maybe even inflate them to the top end of their recommended range. Then, when it's time to replace them (or only two), go with a LRR tire with the lowest revolutions per mile. And keep them properly inflated.
