Really easy...

Take a tire size like 265/75 R16

So its 265mm across the face. So you take 265mm (10.4 inches) x 0.75 = 198.75mm (7.82 inches) in sidewall height.

So now do a 315/75 R16... 315mm in tread face 315mm (12.4 inches) x 0.75 = 236.25mm (9.30 Inches) in sidewall height.

Just for fun... 235/85 R16 - 235mm (9.25 inches) x 0.85 = 199.75mm (7.86 inches)

Now you see.

So if you want to finalize the math. so on the 315's...

9.30" (tire) + 16.00" (rim) + 9.3" (tire) = 34.6 Inch total height (315/75 R16)

7.82" (tire) + 16.00" (rim) + 7.82" (tire) = 31.6 Inch total height (265/75 R16)

7.86" (tire) + 16.00" (rim) + 7.86" (tire) = 31.7 Inch total height (235/85 R16)

Now it should make total sense being I shown all the math of it...

 

Moparman (Michael), you're awesome! Glad that you're here!

 

It all makes sense now!!! I always wondered how that worked. Thanks for the info. I enjoy reading your posts, you have a lot of knowledge!

 

Now there is more... Now comparing tires.

Since I've got 2 sets to do for this example.

I'm running 235/85 R16 Load Range G's for summer and 265/75 R16 for the winter.

265's left and 235' right.


Weighing the 235's


... and the 265's are 85# a tire.

So now.

235's - 4 x 62# = 248# of rotational mass. (Hence why I gain 2-3 MPG with these)

265's - 4 x 85# = 340# of rotational mass. (Hence why I lose 2-3 MPG with these)

340 - 248 = 92# Difference.

Rule of thumb: For every 4 pounds of rotational mass you ditch you require 1 less HP to keep rolling.

92# / 4 = 23 HP difference between 235 and 265's...

So to compare 315's to 235's.

I dug on the Internet to find rough weight of 315's plug the rims typical are 110-120 pounds. So to be nice I'll use 115# for this...

115# x4 = 460# pounds Rotational mass.

Compare to my 235's...

460 - 248 = 212# pound difference...

212# / 4 = 53 HP more will be used to keep you rolling. (Reduction in MPG)

This why its impossible to get high MPG's with large tires.

Now if you flip the other direction. Racing slicks typically weigh in about 20-25 pounds per just tire for rear tires... This allows for the least amount of rotational mass, least amount of rolling resistance (smooth face), and allows for max HP/TQ to be place to the ground.

Proof...

I was at a dyno event back in 2007 when I ran mine. I was proud of my 381/831 run. My buddy got up on the roller with a 97 Dodge 1 ton. Ran 372/805. I raze and teased him hard core about is truck. He demanded to get off the rollers. He went out in the parking lot and pulled the outer duals off and got back up and whipped me by 5 extra HP. Rotational mass hurts HP/TQ output of the engine. Bigger the tires the less power to the ground regardless of gearing.

So I hope this has been a learning experience...

 

But if you could find some tall skinny tires that weighed less, you'd have a taller geared rig without adding weight. Bicycle tire if you will.

 

i am switching to 255-85-16 tire when i get new ones. its a 33inch tall tire but its narrower than a 285 so you dont get as much hp robbing. i took my 33x12.5x17 tires off and put stock wheels and tires on for winter. wow what a difference in power!!! and mpg went up a little bit too of course