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?
um it should be the same because u changed the width not the height of the tire.
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.
the tire size goes up .4inch… used the tire calc on miata.net and and going 60 my actual speed is 62.3 Tire size calculator… i knew it would change some
Actually it does because 55% of 205 is taller than 55% of 185.
thx i thought so…i had forgot about the miata.net calc
Yeah llink is right, but I feel like you wouldn’t notice it that much. Check it with a GPS.
^This. I love that site. The info being correct though assumes 185-55-16 was the correct stock tire size.
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…
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 ----------
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”]
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.
Have we factored in tread wear %?
Way to reinvent the wheel captain nerdness. Tire size calculator
At least you left it round.
Give a man a fish and he’ll eat for a day. Teach a man basic kinematics and you get to feel smrt.
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. 
lol very true. It may very well be spot on now. 