BRCLS

is a 18/45 tire taller than a 17/50 tire?

does any one have a 45/18 tire?

I would think 18/45 tire is smaller than our stock tire.

isint there a calculation to figure it out like

50 divided by 17 is 2.9 inches- so the rim and tire would be 17 + 2.9+2.9 equals 22.8 in tall

45 divided by 18 is 2.5 in.- rim and tire would be 18 +2.5+2.5
equals 23 inches
that is pretty much the same .2 diff.

a 18/40 combo would be 22.4 (.6 smaller than stock)
I dont want to have more of a wheel gap.

CAN ANYONE VERIFY THESE CALCULATIONS?

I had 35/19s and they were close to stock heigth.
22.6

TopGum

try this site

http://www.miata.net/garage/tirecalc.html

BRCLS

that helps alot, i always thought that site was just speedometer change.
225/45/18 is closer to stock heigth than a 40 series tire.

BRCLS

I just measured my sidewall it is no more than 3.5 inches(I am stock)
that site says it should be 4.4 so somthin aint right

my calculations are closer than theres.

cartan

With the rim off the car and the tire properly inflated, your tire outside diameter should be:

2 x Sidewall Height + Rim Diameter
= 2 x (Tire Width x Aspect Ratio) + Rim Diameter
= 2 x (215 mm x (50 / 100)) + 17 inches (for Stock Tire)
= 215 mm + 17 inches
= 8.46 inches + 17 inches
= 25.46 inches

Depending on the wear of the tire, your measurement might be off a little bit, but should not be off significantly. With one revolution of the tire you cover a distance equal to the circumference of the tire. The circumference is:

pi x Diameter
= pi x 25.46 inches
~ 3.14 x 25.46 inches
= 79.94 inches

At 63,360 inches per mile, that gives you:

(63,360 inches per mile) / (79.94 inches per revolution)
= 792.59 revolutions per mile.

Based on the above calculations, the referenced site appears to be providing correct numbers.

Where and how did you measure the sidewall height? Don't forget that the sidewall extends below the edge of the rim. The important thing for maintaining the accuracy of the speedometer / odometer is maintaining the outside diameter.

Measure your outside diameter and compare that against the numbers calculated by the referenced site. If those numbers are still way off, check to make sure it is not your spare that you are measuring. (J/K) But if your measurements are still way off, something definitely is strange.

typeR

235/40/180 is closer to stock than any other scenerio

BRCLS

type r thats not what the calculator says a 40 series would make your speedo to fast 3.0% off.
you would be traveling 58.2 when your speedo says 60

a 45 series would be off only .3%

I measured with a ruler and did not take into consideration the tire goes below the rim.

thats how you do it. those calculations make much more sense.
thanks alot you seem pretty smart.

O wait I thought stock width is 225 but its 215

then 225/40 is.5 smaller then stock and 225/45 is .5 larger than stock..

danm thats only .25 on each sidewall so you wouldnt be able to notice.
but its an inch diff .5 on each sidewall when comparing 40 and 45
18s

wich one to get?, cheapest?
40 would give me more gap than stock, maybee making the 18in rim look smaller than stock, but I might be faster and lighter.

45 would give me less gap (what we all want)
and not make a diff in acceleration

thanks

typeR

235/40/18 is 25.4 225/40/18 is 25.1 and 225/45/18 is 25.8
your problem is your question is flawed...it cant be answered w/o the tire width

BRCLS

well which one should I get?
is 225 wide enough?

where can I find out tire weights?

ChadT

Exactly! You can't just say 40 series and 50 series as these are percentages of the width. a 235/45/17 has nearly the same sidewall hieght as a 215/50/17 stock size. So 45% of 235mm should be about the same as 50% of 215mm. I'd check it but I'm too lazy to get up and get my calculator.

The correct calculation for hieght is widthX(aspect ratio (40,45,50)/100))/25.4X2 plus the rim size.