The Z needs new tires and I am finding plenty of 305/45-18 for cheaper than the 305/35-18 that is on it right now. What is the difference, would the 45's fit?

 

look it up for the future.

but 45 is the height of the tire.

XXX/xx/yy

the XXX represent the width

xx represents the aspect ratio (sidewall height)

and the yy represents the size of rim the tire is meant to fit on.

 

oh ya. and it should fit. im not sure about the width... but if you already have 305/35 on then a 305/45 will fit as far as i know

 

hahahaha it's not if it would fit... but how much do you want your speedo to be off? you would end up with a 5.4" sidewall as opposed to a 4.2" total diameter will go from 26.4 up to 28.8" and you will be traveling at almost 66mph when your speedo says 60mph... yeah, you sure you wanna buy those tires? lol

 

umm i think ur numbers are a little whacked out.

a friend of mine put 37 in mud grappler tires on his jeep, and they changed his speedo by 6 mph over stock.

im not convinced that a 10mm difference in the height of the tire is going to affect the speedometer by 6 mph.

did you get your numbers from somewhere?

 

What tires do you have in the front wheels?

 

Instead of seeing what 20 people are telling you, find out the facts for yourself. Use a tire calculator. Input your stock size and then input the size of the new tire you want to use.

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

 

I'm going to assume you had the OEM 17s on your car. So, from 235/50/17 to 305/45/18 - you're going to have a very large error 9.7%. So when you're speedo says 60, you're going 66. Meanwhile the 35-series tires are a perfect match, with only .4% error.

And for your front tires: from 225/50/17, you'll have a 2.1% error with the 35-series (60>61.5MPH). You absolutely cannot install the 305/45 tires under any circumstance (11.4% error). The perfect match for the front tire is 285/35/18 - with an error of 0.00%.

 

The sidewall number is a percentage not a fixed number. Like 6SPD Z33 said, those are almost 2.5" inches bigger diameter than stock, do not get them. 305/45/18 sounds like a truck tire size.

Next closest option would be a 295/35/18.

 

305??? that's gonna stick so far out of your fender! WILL NOT FIT!

 

A 305 will fit fine with the proper offset on the wheels and not stick out at all. There are members who have run 315s too.

 

305/45/18 will not fit.

305/35/18 is the correct tire size. The aspect ratio (35/45 etc) is the sidewall height which is the pecentage of treadwidth. A 305/35 has a sidewall height of 35 percent of the tread width. A 305/45 has a sidewall height of 45 % of tread width.

JET