Tire Size Calculator
Enter your stock tire size and new tire size — see diameter, circumference, and speedometer difference.
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I had him plug in his summer size: 225/40R19. Then I gave him a minus-size option: 205/55R17. The calculator showed that the 17-inch tire was actually 0.2 inches taller – within 1% of the original diameter. The speedometer would read almost exactly the same. The tire was 20mm narrower – better for cutting through snow. The sidewall was much taller – more cushion over potholes and ice chunks. He asked where to find 17-inch wheels for his BMW. I told him to check Facebook Marketplace or a junkyard. Many BMWs came with 17-inch wheels from the factory. They’re cheap. He bought a used set for $300, mounted the winter tires, and sold his 19-inch wheels for $800. Net cost for the winter setup: $1,100 instead of $1,600. And the car drove better in snow. He called me back a month later after a snowstorm. “Ryan, I passed a Subaru on the highway. The Subaru was struggling. I was fine. Thank you.” That’s the secret. Wide tires are terrible in snow. They float on top. Narrow tires cut through to the pavement. That’s why rally cars in the winter run skinny tires. That’s why your grandpa’s old rear-wheel drive sedan with skinny all-seasons could get through snow that would stop a modern SUV on wide all-terrains. The Tire Size Comparator can help you compare the footprint.Tire Size Comparator
Compare two tire sizes side by side — see diameter, width, sidewall, and revs per mile.
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Take a 245/40R19 summer tire and compare it to a 205/55R17 winter tire. The winter tire is 40mm narrower. That’s almost two inches less contact patch. But in snow, less contact patch means more pounds per square inch pressing down. The tire bites through the snow rather than riding on top of it. I have a friend with a Ford Mustang GT. He runs 275/40R19 summer tires. For winter, he runs 225/60R17 on a set of steel wheels from a base model. The car looks goofy. The wheels look tiny. But he’s never been stuck. He passes trucks spinning their wide tires on the highway. The winter tire conversion isn’t just about width. It’s about sidewall height. A taller sidewall flexes more, which gives better traction on uneven surfaces. It also protects your wheels from potholes. Michigan roads are full of craters. A low-profile summer tire on a 19-inch wheel will dent the rim when you hit a pothole. A 17-inch wheel with a tall sidewall will absorb the hit. The Load Index Calculator can help you make sure your minus-size winter tires can handle the weight.Tire Load Index Calculator
Enter your tire size and load index — see maximum weight per tire and recommended load for your vehicle.
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When you minus size, the load index might change. A 225/40R19 might have a load index of 93. A 205/55R17 might have a load index of 91. That’s fine – 91 is still plenty for a sedan. But don’t go below the manufacturer’s minimum. That number is on your door jamb sticker. I remember a guy named Tom. He had a Mercedes E-Class. He put 16-inch wheels from a C-Class on his car for winter. The load index was too low. He overloaded the tires. One blew out on the highway. He was fine, but the tire damage was scary. He checked the load index after – it was 89. His car needed 94. The difference was about 200 pounds per tire. Not huge, but enough. He bought the correct wheels and tires after that. Some people ask me: “Can I just use my all-season tires in winter?” You can. But they won’t perform as well. All-season tires start to harden below 45 degrees Fahrenheit. Their rubber compound loses grip. At 20 degrees, an all-season tire is like a hockey puck. A winter tire stays soft and pliable. The Tire Speed Rating Guide is less relevant for winter tires because you shouldn’t be driving fast in snow anyway. But check it anyway.Tire Speed Rating Guide
Enter your tire speed rating — see maximum safe speed and performance characteristics.
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Winter tires often have lower speed ratings than summer tires. H or T is common. That’s fine. You’re not going 149 mph in a snowstorm. If you live in a place with real winter – Michigan, Wisconsin, Minnesota, New York, Vermont, Canada – you need winter tires. Not all-seasons. Not all-weather. Dedicated winter tires with the three-peak mountain snowflake symbol. The guy with the BMW learned that. He went from hating winter to looking forward to it. He said “I used to drive my wife’s SUV when it snowed. Now I take my car. It’s more fun.” That’s the difference. Here’s a quick reference for minus sizing: Find your stock tire size. Go down 1 inch in wheel diameter. Keep the overall diameter within 3% of stock. Go down 10-20mm in width for better snow traction. Increase aspect ratio to maintain diameter. A 225/40R19 becomes 225/45R18 or 205/55R17. Check load index and speed rating. They should meet or exceed minimums. Buy dedicated winter wheels. Steel wheels are cheap. Used OEM wheels are fine. Don’t mount winter tires on your expensive summer wheels. The salt and sand will corrode them. The repeated mounting and unmounting will damage the beads. Get separate wheels. Keep your summer wheels clean and stored inside. The Tire Size Calculator and Comparator make this easy. You don’t need to guess. You don’t need to trust the tire shop’s recommendation. You can do the math yourself. A lot of tire shops will try to sell you the same size winter tire as your summer tire. That’s because it’s easier for them. They don’t have to explain minus sizing. They don’t have to order different wheels. They just check a box. That’s not good service. That’s laziness. You know better now. Winter is coming. Get the right tires. Your car will thank you. Your spine will thank you. And you won’t be the guy blocking the highway with spinning tires.