# switches tire size how fast am i really going!!!! **URL:** \ **Category:** NYSpeed Automotive\ **Created:** [July 15, 2011, 8:57am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534 "2011-07-15T08:57:14Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![llink78](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/llink78/32/8037_2.png) [@llink78](https://forums.speedlife.net/u/llink78)\ **Post date:** [July 15, 2011, 8:57am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/1 "2011-07-15T08:57:14Z")
ok had 185-55-16 on the car and went to 205-55-16’s for the fact that they were cheaper and i didnt have to wait for them to be ordered bc WHO THE HELL RUNS A 185 TIRE. my question is now is how fast am i really going say my Speedo says 60? ---
**Author:** ![dftsilvia](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/dftsilvia/32/7447_2.png) [@dftsilvia](https://forums.speedlife.net/u/dftsilvia)\ **Post date:** [July 15, 2011, 9:01am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/2 "2011-07-15T09:01:20Z")
um it should be the same because u changed the width not the height of the tire. ---
**Author:** ![llink78](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/llink78/32/8037_2.png) [@llink78](https://forums.speedlife.net/u/llink78)\ **Post date:** [July 15, 2011, 9:13am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/3 "2011-07-15T09:13:21Z")
with going from a 185 to a 205 and keeping the 55 sidewall it does change bc the 55 represents the % of the overall width. i though the tire looks taller for sure ---
**Author:** ![Freek](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/freek/32/7357_2.png) [@Freek](https://forums.speedlife.net/u/Freek)\ **Post date:** [July 15, 2011, 9:25am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/4 "2011-07-15T09:25:17Z")
> [@llink78](#): > > with going from a 185 to a 205 and keeping the 55 sidewall it does change bc the 55 represents the % of the overall width. i though the tire looks taller for sure The tire shouldn’t be taller at all. Nothing should change 205/55/16 = width/sidewall height/wheel size. Unless you’re massively over inflating them, then tire height will change slightly. ---
**Author:** ![llink78](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/llink78/32/8037_2.png) [@llink78](https://forums.speedlife.net/u/llink78)\ **Post date:** [July 15, 2011, 9:28am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/5 "2011-07-15T09:28:17Z")
> [@Freek](#): > > The tire shouldn’t be taller at all. Nothing should change 205/55/16 = width/sidewall height/wheel size. Unless you’re massively over inflating them, then tire height will change slightly. the tire size goes up .4inch… used the tire calc on [miata.net](http://miata.net) and and going 60 my actual speed is 62.3 [Tire size calculator](http://www.miata.net/garage/tirecalc.html)… i knew it would change some ---
**Author:** ![Blue\_Eyed\_Devil](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/blue_eyed_devil/32/7292_2.png) [@Blue\_Eyed\_Devil](https://forums.speedlife.net/u/Blue_Eyed_Devil)\ **Post date:** [July 15, 2011, 9:38am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/6 "2011-07-15T09:38:43Z")
> [@Freek](#): > > The tire shouldn’t be taller at all. Nothing should change 205/55/16 = width/sidewall height/wheel size. Unless you’re massively over inflating them, then tire height will change slightly. Actually it does because 55% of 205 is taller than 55% of 185. ---
**Author:** ![llink78](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/llink78/32/8037_2.png) [@llink78](https://forums.speedlife.net/u/llink78)\ **Post date:** [July 15, 2011, 9:43am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/7 "2011-07-15T09:43:58Z")
> [@Blue Eyed Devil](#): > > Actually it does because 55% of 205 is taller than 55% of 185. thx i thought so…i had forgot about the [miata.net](http://miata.net) calc ---
**Author:** ![Onyx\_Z32](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/onyx_z32/32/7581_2.png) [@Onyx\_Z32](https://forums.speedlife.net/u/Onyx_Z32)\ **Post date:** [July 15, 2011, 11:12am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/8 "2011-07-15T11:12:45Z")
Yeah llink is right, but I feel like you wouldn’t notice it that much. Check it with a GPS. ---
**Author:** ![JayS](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/jays/32/15375_2.png) [@JayS](https://forums.speedlife.net/u/JayS)\ **Post date:** [July 15, 2011, 11:14am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/9 "2011-07-15T11:14:49Z")
> [@llink78](#): > > the tire size goes up .4inch… used the tire calc on [miata.net](http://miata.net) and and going 60 my actual speed is 62.3 [Tire size calculator](http://www.miata.net/garage/tirecalc.html)… i knew it would change some ^This. I love that site. The info being correct though assumes 185-55-16 was the correct stock tire size. ---
**Author:** ![95dime](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/95dime/32/7514_2.png) [@95dime](https://forums.speedlife.net/u/95dime)\ **Post date:** [July 15, 2011, 11:33am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/10 "2011-07-15T11:33:35Z")
185/55: width-7.28 sidewall height-4.00 overall height-24.01 circumference-75.43 205/55: width-8.07 sidewall height-4.43 overall height-24.87 circumference-78.15 i have an excell spreadsheet i made up to calculate this stuff…havent worked it out for mph yet though… ---
**Author:** ![llink78](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/llink78/32/8037_2.png) [@llink78](https://forums.speedlife.net/u/llink78)\ **Post date:** [July 15, 2011, 11:38am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/11 "2011-07-15T11:38:38Z")
> [@JayS](#): > > ^This. I love that site. The info being correct though assumes 185-55-16 was the correct stock tire size. haha didnt even think of that. my speedo is actaully 3.7 mph off from OE tire size. forgot the car had 195-55-15’s from the factory. and Onyx im not worrird about me being 3.7mph over but rather the cop or trooper that is shooting radar and me saying but officer i was only going 65 10 over but really its way closer to the 15 over mark 🙂 ---------- Post added at 02:38 PM ---------- Previous post was at 02:37 PM ---------- > [@JayS](#): > > ^This. I love that site. The info being correct though assumes 185-55-16 was the correct stock tire size. haha didnt even think of that. my speedo is actaully 3.7 mph off from OE tire size. forgot the car had 195-55-15’s from the factory. and Onyx im not worrird about me being 3.7mph over but rather the cop or trooper that is shooting radar and me saying but officer i was only going 65 10 over but really its way closer to the 15 over mark :)[COLOR=“Silver”] ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [July 15, 2011, 11:41am UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/12 "2011-07-15T11:41:39Z")
Well lets just do some math. .55 \* 185 = 101.75mm sidewall .55 \* 205 = 112.75mm sidewall So the diameter of your old tire was 16" + 101.75mm + 101.75mm = 24.012 inches The diameter of your new tire is 16" + 112.75mm + 112.75mm = 24.88 inches So your diameter changed by (24.88-24.01)/24.01 percent = 3.6% Circumference is linearly related to diameter, so circumference also changed by 3.6%, your speed is calculated by circumference inches per revolution, so your speedometer thinks you are traveling 3.6% fewer inches per revolution. So you are going 3.6% faster than what your speedometer says. AKA at 60mph indicated you are actually going 62mph. ---
**Author:** ![Gus](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/gus/32/5112_2.png) [@Gus](https://forums.speedlife.net/u/Gus)\ **Post date:** [July 15, 2011, 12:05pm UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/13 "2011-07-15T12:05:52Z")
Have we factored in tread wear %? ---
**Author:** ![JayS](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/jays/32/15375_2.png) [@JayS](https://forums.speedlife.net/u/JayS)\ **Post date:** [July 15, 2011, 12:14pm UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/14 "2011-07-15T12:14:14Z")
> [@Fry](#): > > Well lets just do some math. > > .55 \* 185 = 101.75mm sidewall > .55 \* 205 = 112.75mm sidewall > > So the diameter of your old tire was 16" + 101.75mm + 101.75mm = 24.012 inches > The diameter of your new tire is 16" + 112.75mm + 112.75mm = 24.88 inches > > So your diameter changed by (24.88-24.01)/24.01 percent = 3.6% > > Circumference is linearly related to diameter, so circumference also changed by 3.6%, your speed is calculated by circumference inches per revolution, so your speedometer thinks you are traveling 3.6% fewer inches per revolution. So you are going 3.6% faster than what your speedometer says. AKA at 60mph indicated you are actually going 62mph. Way to reinvent the wheel captain nerdness. [Tire size calculator](http://www.miata.net/garage/tirecalc.html) At least you left it round. ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [July 15, 2011, 12:15pm UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/15 "2011-07-15T12:15:45Z")
Give a man a fish and he’ll eat for a day. Teach a man basic kinematics and you get to feel smrt. ---
**Author:** ![Blue\_Eyed\_Devil](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/blue_eyed_devil/32/7292_2.png) [@Blue\_Eyed\_Devil](https://forums.speedlife.net/u/Blue_Eyed_Devil)\ **Post date:** [July 15, 2011, 12:18pm UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/16 "2011-07-15T12:18:12Z")
> [@Fry](#): > > Well lets just do some math. > > .55 \* 185 = 101.75mm sidewall > .55 \* 205 = 112.75mm sidewall > > So the diameter of your old tire was 16" + 101.75mm + 101.75mm = 24.012 inches > The diameter of your new tire is 16" + 112.75mm + 112.75mm = 24.88 inches > > So your diameter changed by (24.88-24.01)/24.01 percent = 3.6% > > Circumference is linearly related to diameter, so circumference also changed by 3.6%, your speed is calculated by circumference inches per revolution, so your speedometer thinks you are traveling 3.6% fewer inches per revolution. So you are going 3.6% faster than what your speedometer says. AKA at 60mph indicated you are actually going 62mph. All that math is nice but the part about going 3.6% faster than what your speedometer says is probably only half correct because speedometers are almost never that accurate. 😉 ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [July 15, 2011, 12:32pm UTC](https://forums.speedlife.net/t/switches-tire-size-how-fast-am-i-really-going/214534/17 "2011-07-15T12:32:44Z")
lol very true. It may very well be spot on now. 😛