727 Transmission Efficiency...

Guys,

I have a different sort of question. It's regarding a transmission's efficiency, and specifically any transmission such as the 727, when in top / 3rd / drive / 1:1 gear.

I ask this question because I'm doing some "figgerin" as I'm doubting the accuracy of my tachometer. I'm not in love with it, I don't like its style, it hangs up when accelerating quickly, and I don't like where it's mounted. However, I'm a fairly big proponent of not fixing something if it's not broken. As for the hanging up of the tach when accelerating quickly, I consider this minor as when I'm accelerating quickly, I don't need to be staring at the tach anyway. But when cruising along, I'm tending the think the tach is far enough off, that I want to replace it.

So, my question is: When the transmission (727 for example) is in drive, and say I'm cruising along at 3,000 rpm steady, the crankshaft is turning at 3,000 rpm. Due to the 1:1 ratio of drive (3rd) gear, logically one might think the driveshaft is also turning 3,000 rpm.

But is that true? I ask because of the "slushbox" nature of an automatic transmission. How "slushy" is the 727? I know about the "15% drivetrain loss" rule. But that doesn't directly correspond to a 15% loss in driveshaft speed due to slush. I'm not an auto trans expert at all. Barely a novice. But I suspect that in reality, the driveshaft will be turning less than 3,000 rpm, but not significantly.

Here's my math: 3,000 engine rpm = 3,000 driveshaft rpm = (3.23 diff) 928.8 wheel rpm.
My tires are roughly 24" diameter = 6.283 feet per revolution.
6.283 * 928.8 = 5,835 feet = 1.1 miles = 66.3 mph.

My speedometer is accurate, GPS verified frequently. When I'm turning said 3,000 rpm, the Scamp is running 60 mph almost exactly. The math above tells me the tach should actually be reading less than 3,000 rpm, more like 2,700 rpm.

So that brings me back to my question. When in 3rd gear (1:1) assuming a perfectly operating 727, will the output shaft / driveshaft be turning exactly the same as the crank? Or is there a slush factor? And if there's a slush factor, would anyone have a guess as to the slush factor … or "slushiness?" :D

Thanks!


7milesout