Need a Math Wizard

If you know the engine rpm, the wheel/tire rpm, and feet per rpm the tire travels, and length of a mile, what is missing? I'll try to supply it.
 
I did re read that post and confirmed why I glossed over it.

The Op was after speed difference of 2 different sets of tyres.......

Anyway, I also cannot follow why you mention circumference does not change under load or in the air?

It made me go over to shed and do some measuring.

21 inch firestones on 28 tourer at 35 PSI.

Rear tyre straight across horizontally is 15 5/8 inches from tread to centre of cap.

Vertical from ground to centre of cap is 14 13/16 inches.

If the horizontal (no load) measurement of 15 5/8 X 2 X Pi was used, it would give a far faster travelled speed , given same RPM, than the real rolling circumference of 14 13/16 X 2 X Pi.
 
I was just noting that the circumference measurement of the circle is the same even when the circle becomes deformed, that's all. the OP wanted to know how to determine a speed difference with different tires, but he is the one with the variable information.

I like Havi's solution carry a gps!
 
To solve the OP maths question . all is needed is the loaded measurement from ground to wheel centre of each of (one) rear 16 and 19 respectively.

Don't need Pi nor diff ratio or nuffin'.
 
J Franklin said:
Did anybody actually read my first post? I think you can find your answer. By the way the circumference will not change in the air or under load.
Click to expand...

Unfortunately, it does change. Many manufacturers state a "revs per mile" for a specific tire, and it is not an exact multiple of the maximum circumference. First, all tires are 'crowned' with the tread at the outer contact edges a smaller diameter than the center, and second, the inflation pressure, load, and ground speed will all affect the revs per mile. To make matters worse, different manufacturers who post "revs per mile" data may, or may not figure it at that tire size and ply rating's maximum inflation pressure, maximum rated load, or any standardized speed.

The difference between crown diameter and actual rolling circumference is tire-wearing grinding and internal molecular friction while driving. The grinding and internal friction express themselves in tire life and heat. A tire with the least of each will get an A rating for tread life and and A for temperature. That type of tire will ride like a rock.
 
J Frankin, you are still wrong.

To cut across a circle in a straight line is reducing the circumference.

( I did promise myself I was not going to get into technical arguments on this new ( to me) forum).

I guess I lost.
 
J Franklin said:
Did anybody actually read my first post? I think you can find your answer. By the way the circumference will not change in the air or under load.
Click to expand...

The circumference won't change, but the distance traveled will. If in doubt, just let some air out of the tire and roll it one revolution and measure the distance.
 
Ok, every one has gotten to technical, just tell me if I slip on the regulation size 19 rim and tires, floor the pedal like I did with the 16 rim and tires which read 50 mph on an independant speed recorder. How much faster (if any) given equal engine rpm will the Model A go in regulation foot wear (19 rim and tire).
 
Lets make thing more complex and factor in tread wear too! ! I know we're all saddos talking trivialities but hey, Franklin if you underinflate or severely overload then then your speedo will read oprimistic.
 
This has always interested me too. I want a "Unified theory of tire size vs speed" I came up with this. Placing a 16" whell and tire (inflated) next to a 19" wheel and inflated tire, I find about 2" difference in diameter. This means that the circumference is about 6 1/4" longer in favor of the larger wheel. If the 19" wheel is 29" mounted and inflated, Then the 19" tire has a total circumference of 87" (Rounded off) The 16" inch wheel is about 6" less (81 "). A difference of about 7% or about 3 1/2 miles on the speedometer at 50 mph. My math is all off the top of my head, so I may have made an error or two. I've been known to do that in my dotage. ( Pi= 3.1416, 3 9/64)
Terry
 
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