Yeah there is some confusion there. The two different methods are yielding three different answers.
To make it easier (heehee), here is
the formula; mph =( rpm x tc )divided by (1056 x R1 x R2)
where
tc is tire circumference, and R1 is the tranny gear Ratio, and R2 is the rear end gear Ratio.
Tire circumference is tire diameter x 3.1416(pi); and is also known as tire rollout,when the wheel is installed.
A 295/50-15 is mathematically (295/25.4 x .50 x 2) + 15 = 26.6 tall, and the tc is then; 26.6 x 3.1416 is 83.57. It's roll out may be different.
Why? because a 295 is only a 295 when mounted on a checking rim of 70% of it's design section width, and it is then rounded to the nearest 10 mm,ending in a 5.
For example ,your 295 was designed for 295/25.4 x 70% = 8 inch rim. Nobody puts a 295 on an 8incher cuz it either wont live long, or won't handle. A 295 needs at least a 10incher, returning long tread life at 24 psi, and still rolling over in the turns.
So a 295 on a 10 incher is no longer a 295( 11.6 sidewall width). I think mine bulged out to 13.2 inches at 24psi. So the math says 13.2 x 25.4 =343, which would be rounded to 345. Get it? It still the same tire right? But the math says it is now a 345 and it is thus 28.58 hi, and it's circumference would calculate to 28.58 x 3.1416 =89.79
So the math method only works when adhering to the manufacturers specs, and then still only ballparks the roll-out.
So let's talk about roll-out. You need to make two chalkmarks on the sidewall of the mounted wheel assy. The marks must be 180* apart,more or less exactly through the center of the wheel diameter. Then you just back up the car until an imaginary line drawn thru them is vertical.Make a mark on the concrete floor adjacent to the mark,then roll the car ahead,one full 360* turn until the marks are again vertical, and make another mark. Measure between the marks and Shazzam!, you have an exact number to enter into the formula.
'Course you could just jack the car up and measure it with a cloth tape
If you are not so good at math, here is the same formula,restated to spit out the gear ratio;
(mph x 1056 x R1) / ( rpm x tc) = 1/R2
Here's an example using your numbers and assuming the 295 is 84.8 roll-out.
(50mph x 1056 x 1.00tranny Ratio@zero-slip) / (2000rpm x 84.80tc ) = 1/R2 the rear Ratio
Which is; 52800/ 169600 = 1/R2
And finally .311 = 1/R2 ,which when restated is; R2 = 1/.311, and again; R2 = 3.21 ; Remember at Zero-slip.
So what is Zero-slip? Well let's say you are cruising along on a smooth level hiway doing 50 at 2000rpm. Then you come to the bottom of a long steep hill. To maintain 50 you will have to lay on some throttle. So halfway up you notice the tach is now showing 2100 rpm. and a bit later you see 2200. The hill gets steeper and you feed it more pedal.Still maintaining 50mph, you now see the tach at 2250. After that it stays at 2250 till you crest the hill.
So what's going on? Well at 2250, the convertor finally quit slipping is what is happening. That thus means the 2000 number must have been a slipping number. And the how much , is, 1 - (2250/2000) = .125 or restated 12.5%
What all this boils down to, is that the 3.21 ratio listed above, while correct for a 4 speed with a 1.00 final ratio, it may be off for an automatic. How much is anybody's guess, but the next size up, a 3.55 is just 1 -(3.55/3.21) =10.6 % away. So could there be a 3.55 in there? I don't think so, but sometimes the math is misleading.
The method shown in the video works, almost exactly, but is only as accurate as the guy performing the operation.
