Tire Math

[born2saw]

Ok you mathmatics people -- I have a question.

My recently purchased '75 roadster has really cheap (4 for $138 in yesterdays paper, S speed rating) all season radials 225/70R15 (like I need all season - if the weathers bad, the vette is parked!).

I am going to get better tires, that's a given.

My question is: Currently my speedometer reads 70 mph but I am really only going 62-63 mph. If I buy a bigger tire-say 245/70R15 will this help correct my speedometer? I have not gotten under the car long enough to find the rear end stamp, but my guess is I have the base axle ratio (L48, 4 speed) as my car has few options and appears to be mostly untouched. From reading the forum, I think the consensus is with stock rally wheels, the largest I can go without modifications is 255/70R15.

As near as I can tell, the 225's have a diameter of about 27.5", 235's - 28", 245s - 28.5", 255's - 29".

Also, if anyone knows, how close in size are metric to the original GR70x15?

Thanks, math was never my strong suit.

P.S. Any suggestions on what I need to do to post a cleaner picture of my car & stay within the forum requirements?

 

[billybob]

best one I've seen by far

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

Bill

 

[born2saw]

Thanks, that's slick!

 

[geekinavette]

Just FYI...

First number is tire width in mm, second number is sidewall height as a percentage of width (distance between seating bead and tread surface), third number is of course rim diameter.

So for a 225-70x15:
tire is 225mm wide
sidewall height is 225mmx0.7 or 157.5mm
rim diameter is 14 inches.

To calculate overall tire diameter in inches:

convert sidewall height to inches by dividing by 25.4, then multiply sidewall height in inches by 2 and add to rim diameter.

for this example:
[(157.5mm / 25.4) * 2] + 14 = 26.4 inches diameter

On that miata calculator it looks like they are calculating tire revs/mile for the two different sizes and calculating a percentage error based on that result....which is fine and dandy but it also looks like they are rounding off to whole numbers, which will cause a small difference...but not much. By using a little more accurate method (and ONLY a little) of converting tire angular velocity (in radians/second) to a linear velocity (in inches/second), and their example of 185-60x14 vs. 195-55x14...I come up with a actual vehicle speed of 58.948 MPH for a speedo reading of 60MPH, vs. their result of 59.2MPH. (whoopteedoodoo :crazy)

BTW...the two tire diameters for their example come out to 11.2" (stock) and 11.4":
11.2/11.4 * 60MPH = 58.947MPH

Dam it's almost 3am and I'm STILL not sleepy! :bang

Bill

 

[Rob]

Click the Tools tab in the upper right hand corner of this page and scroll down under Miscellaneous.

 

[lytemup]

...and have fun burning those all seasons off of there!! hehe