Confusion on radial tire numbers?

Many of you have probably seen MPH/RPM calculator that gives speed with a certain RPM, gear ratio and tire size. When you plug in tire size (235/75/15) it gives you tire diameter along with the speed; my problem is when I compare the computed diameter to the actual tire I find quite a difference. I have a properly inflated 235/75/15 tire that measures slightly over 26" yet the calculator says it should be 28.9"; what am I doing wrong?

 

2 things I can think of off the top of my head - installed rim width can impact the height and the calculated diameter would be without load. Radials squish a bit when loaded. tirerack.com has diameter measurements and it varies from brand to brand even though they are the same size.

 

Tire brands make a difference.. ie, cheaper tires just don't measure up. That is why they are cheaper. Even matters with tires of the same brand.

 

235mm wide =9.2519 in.
75% of 9.2519=6.9839 in of sidewall
6.9839x2 (2 sidewalls)=13.8778
13.8778+15 (wheel diam)=28.8778 in tall.
Btw.....25.4 mm in an inch.

 

It depends on what is written in the program.
If you were to calculate by hand you should use the "effective" diameter. With the tires fully inflated and heated to operating temperature and car loaded with driver, fuel, etc., measure the perpendicular from the pavement to the axis of rotation of the wheel, multiplied by 2 = effective diameter. Multiplied by pi (3.1416) = cir***ference, distance traveled for each tire rotation.

 

Here is how you read whats on the tire.

 

Thanks everyone for responding, the tire I measured is on my Jimmy Sonoma so it would have somewhat of a load on it but I don't think that would account for over 2" difference. The reason I ask as I'm trying to get an idea of what to use on my '36 with a Ford 8". Someone said they were using 235's on their '40 but I don't think I need that much width. The car will be flathead powered about 130-140 HP.

 

No, you don't need that much width.

 

You dont know how to measure a tire is the problem.

 

Yes it will. Either jack the Jimmy up so the tire is only just in contact with the ground or use a plumb bob mark the ground and then measure again, you will find your missing 2". A 215 or 225/75 will do nicely to fill the fenders.

Scotty

 

try calculator at: project33.com