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I have a women's Mongoose hybrid bike that is older. The tires are dry rotted and I can't read the tire size info.

How do I determine what size replacements to buy?

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    The easiest is to take the rim to a local bike shop (not a bike department in a big box store) and ask them. They can also help you get white wall tires if you want the classic look. If you’re cheap, just get the one wheel done and order the companion online but I’ve found my LBS to be within $5-10 of online prices and it’s worth the convenience. Note: you’ll also need a new inner tube. – RoboKaren Feb 21 '18 at 16:27
  • Note that the tire diameter is the really critical dimension as the tire has to fit exactly on the rim. Most bicycles can take a range of tire widths. – Argenti Apparatus Feb 22 '18 at 13:04
  • How old is old? Mongoose made many different bikes in the 90s, but only BMXs and MTBs in other decades. A photo of your bike might help us eliminate some possibles – Criggie Feb 22 '18 at 21:15
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Tire sizes are extremely complicated because there are so many different measurement standards. See: http://www.sheldonbrown.com/tire-sizing.html

One strategy will be for you to ascertain what the common tire size of your bike is (more likely than not 700c for a hybrid bike of the 1990s) and order that (I’m assuming you’re ordering online) but that won’t work if someone replaced the rims or wheels on your bike and returning tires would be a pain.

Really, the easiest is to take one of the wheels to a local bike shop (not a bike department in a big box store) and ask them. They can also help you get white wall tires if you want the classic look. If you’re cheap, just get the one wheel done and order the companion online but I’ve found my LBS to be within $5-10 of online prices and it’s worth the convenience. Note: you’ll also need a new inner tube and you’ll likely also need some new rim tape. The bike shop can also ascertain how much corrosion there is and whether or not it’ll affect the new tire.

Note that many older bikes from the 1970s and prior have steel rims which have lousy braking performance.

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    +1 but I'll take issue with your last paragraph -- try 90s and prior! I've just dismantled a 1992 Raleigh with chromed steel rims (the rear hub died). – Chris H Feb 22 '18 at 13:46
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A more direct way to determine the size is to use a measuring tape and measure the diameter of the rim in millimeters. If you are able to pump the tires, you can also measure the width. Put together these numbers form the ETRTO size of the tire. For example, 32 millimeters wide tire on rim with 622mm diameter becomes 32-622.

Don't try to measure inch sizes, as already explained in the other answer they do not refer to any physical dimension of tire or rim. If you have to buy in inch sizes, use a conversion table and make sure you can return the tires if you got the size wrong.

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    Measuring rim diameter is difficult - you need to measure the inside diameter of the bead-seat, not the outer diameter of the rim walls. If you know you have a modern 622mm, 584mm or 559mm wheel a rough measurement is OK, but for older wheels it could be a problem. – Argenti Apparatus Feb 22 '18 at 12:34
  • Shouldn’t it be possible to remove the tires and measure their inner diameter relatively accurately? Assuming they have a wire bead and are not folding tires. Another method could be to measure the circumference of the rim if the bead seat is pretty close to the floor of the rim. – Michael Feb 24 '18 at 18:01

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