# Rim and tires diameters... **URL:** \ **Category:** SON240sx Automotive\ **Created:** [August 24, 2006, 9:45pm UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267 "2006-08-24T21:45:02Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![system](https://canada1.discourse-cdn.com/flex034/uploads/speedlife/original/2X/b/b7170c8eddc0b95ece6b88172b2f416a08489a45.png) [@system](https://forums.speedlife.net/u/system)\ **Post date:** [August 24, 2006, 9:45pm UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267/1 "2006-08-24T21:45:02Z")
Ok, I’m sure this is simple for most. But I’ll admit I’m still learning. Anyways, here’s the question: how do you know if one set of wheels has the same diameter as another? eg. - 45/17R and a 40/18R. Are they the same diameter? or eg. - a 35/18R and a 40 17R. etc etc etc… I’m sure there’s any easy way to calculate this. I’m all ears… 😉 Leo. ---
**Author:** ![logik23](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/logik23/32/5548_2.png) [@logik23](https://forums.speedlife.net/u/logik23)\ **Post date:** [August 24, 2006, 9:53pm UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267/2 "2006-08-24T21:53:45Z")
Ok, first you need to know what those numbers mean. Take 255/40R17: -255 is the width of the tire in milimeters -40 is the height of the sidewall measured by the percentage of the width (so 40% of 255) -17 is the rim size in inches So you do this… 255 \* .4 / 25.4 \* 2 + 17 = overall tire diameter 255 \* .4 = 102 This is the height of the sidewall in milimeters. 102 / 25.4 = 4.0157 This is the height in inches. 1.0157 \* 2 = 8.0315 Because there is the bottom sideall and then the top sidewall. 8.0315 + 17 = 25.0315 This is the overall diameter of the rim. 205/60R15 (S13) is 24.68 205/55R16 (S14) is 24.88 As long as you are within 5% of the original size, your speedo will be fine. ---
**Author:** ![S13GG](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/s13gg/32/5506_2.png) [@S13GG](https://forums.speedlife.net/u/S13GG)\ **Post date:** [August 25, 2006, 6:33am UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267/3 "2006-08-25T06:33:29Z")
> **[Tire Size Calculator - Tire and Wheel Plus Sizing](https://www.1010tires.com/Tools/Tire-Size-Calculator)** > > 1010tires.com Tire Size calculator. Use our tire calculator to compare tire sizes based on tire diameter, radius, sidewall height, circumference, revs per mile and speedometer difference. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex034/uploads/speedlife/original/2X/b/b7170c8eddc0b95ece6b88172b2f416a08489a45.png) [@system](https://forums.speedlife.net/u/system)\ **Post date:** [August 25, 2006, 8:02pm UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267/4 "2006-08-25T20:02:29Z")
What can I say? Thanks for all the input! Both posts were very usefull… Leo. ---
**Author:** ![robbbby](https://avatars.discourse-cdn.com/v4/letter/r/b19c9b/32.png) [@robbbby](https://forums.speedlife.net/u/robbbby)\ **Post date:** [August 25, 2006, 8:29pm UTC](https://forums.speedlife.net/t/rim-and-tires-diameters/99267/5 "2006-08-25T20:29:07Z")
This isn’t tire related but has lots of really good technical info regarding wheels if anyone needs it. [http://www.rsracing.com/tech-wheel.html](http://www.rsracing.com/tech-wheel.html)