I'm sure this has been covered, but I searched and didn't find it. sorry in advance.

can someone give me a percentage change or better yet, a MPG change. I've been happy w/ my avg. 30 MPG on my old 215/18/35 setup.

thanks.

 

I am running 195/50/15 and the best that I can tell is that my speed is off about 7 to 8 MPH, my gas milage has gone to crap on the hyway b/c I am running at higher rpms to stay at speed.

 

with the 205/50/15 you will be about 4-5mph off and about 250rpm higher than stock at 75mph.

Other than that, i just did a cross country drive in the xb and got 33-35mpg the whole way on the freeway at about 65-75mph.

 

On the internet, go to Miata Tire Size and you can get all the converstion info from your old size tires to your new ones. It is very easy, even I can do it. You just enter your old size and your new size and it will give you all the info.

 

This is a slick comparator-

http://www.discounttiredirect.com/di...TireMathNS.jsp

From my fave tire dealer...

If you run 205/55/15's, there's no speedo correction worthy of mention... just like the 215/35/18's you ran before...

 

It's easy to calculate the changes. The online calculators are good, but its not hard to calculate at all and if you don't understand what the numbers on the tire mean, it is good to know.

First, you want to find the diameter of the tire and wheel.

Diameter = Wheel Height + 2*Sidewall Height

For a tire like a 205/50 R15, the Wheel Height = 15 inches

The Sidewall Height = 205*.50 = 102.5 millimeters. The 205 is the width of the tire in millimeters and the 50 is the aspect ratio. Multiplying the width times the aspect ratio (50 = .50, 65 = .65, etc.) gives you the sidewall height in millimeters.

2*Sidewall Height = 205 millimeters ( it just happens to match the width because the aspect ratio happened to be .5, but that doesn't generally happen).

1 inch = 25.4 millimeters. So just convert the 205 millimeters to inches by dividing by 25.4. The total is 8.07 inches.

So the total wheel height is 15 + 8.07 = 23.07 inches.

The circumfrence of the tire is equal to how much it travels in one revolution. So the % change in circumfrence is the % change your speedometer and odemeter will be off.

Circufrence = pi*Diameter where pi = 3.14159

So the Circufrence is 3.14159*23.07 = 72.5 inches

You want the circumfrence of your new tire/wheel combo to be as close as possible to that of the OEM tire/wheels to keep your speedometer and odemeter correct.

If that was worthless and wasted your time reading, sorry. I just thought some people may not know how it works and would want to understand how to calculate it w/o the internet calculators. I've been at the tire store calculating sidewall heights when choosing tires before with the store's calculator, so it was useful to know.

 

yeah i dunno bout all you but i'm a white guy and SUCK at math. so the online calculators work just fine.

 

thanks for all the help guys. I really appreciate it.

I'm on my first tank w/ the 205/50/15's - hope the MPG doesn't take a dump.

 

well, my first tank on this set up yielded 29.3 MPG. that's down from 30.3. big deal. I love these wheels.