DerekStangGT said:
i will definitly try this. it sounds like
tho, but i will give it a go
It's definitely BS - at least as far as working on all cars, with all tires, and all transmissions. Though it's always possible it works for a specific model car with specific tires (for instance, if your mustang has a 1.33-1.35:1 3rd gear, and 25.7 inch diameter tires).
Third gear is not the same ratio in all transmissions...unless they're 3 speeds, in which case it's always 1:1. Just like 4th gear is always 1:1 in 4, 5, and 6 speeds. The highest gear that isn't an underdrive gear is always 1:1. So it might work with some transmissions...but not with others. The other variable is tire diameter. ALL this is going to affect the engine speed at any given vehicle speed. In 4th gear (1:1), or 3rd gear in a 3 speed, with a specific tire diameter, there is going to be a speed where the tach equals the gear ratio, but it's going to vary from one tire diameter to another. It's going to be the speed at which the tire is turning 1000 rpm. 1000 rpm, times the rear gear ratio, through a 1:1 gear in the transmission gives you the engine speed equal to the rear gear ratio, with a decimal between the hundreds and thousands places.
For a modern Mustang with 25.7 inch diameter tires(245/45-17's, for example), you reduce the diameter a couple percent for load compression, and 1000 rpm occurs at 25.7 x .98 x 3.1416 (in./rev) x 1000(rev/min.) / 12 (in./ft.) / 5280 (ft./mi.) x 60 (min./hr.) = 74.9 mph in 4th gear. Obviously, it'll be different for every different tire diameter...but this will ALWAYS work.
In third gear, you'd have to divide this number by the 3rd gear ratio, which you'd have to know before hand, which varies from car model to car model, and sometimes from year to year (not every year, obviously, but occasionally). Whereas 4th gear is ALWAYS 1:1.
Just for clarification.
