Calculations don't match gear ratio, help?
 

I have 3.07 stamped in my housing. It has been open at least once judging by the silicone sealant used. My calculations say 3.40. Ok, see if I missed something.
engine rpm 3500, actual speed 75, tire diameter 24.5"(estimate, same size as new tires minus tread)
3500 engine rpm x 1:1 trans ration = 3500 driveshaft rpm.
3500 / 3.07 rear end ratio = 1140.07 tire rpm
1140.07 x 60 = 68404.2 tire rph
68404.2 x (3.14 x 24.5) = 5262335.1 tire inches per hour
5262335.1 / 12 / 5280 = 83.05 miles per hour

Why is my number different by 8mph?
Is my torque converter slipping that much?
Is my tach off by 340 rpm?
Is my gear ratio really 3.4?

what size tires are they?

there is a 346 rato which came in na 300s.

tom w

1. Check your speedometer accuracy with a GPS to verify speed or use a stop watch and mile markers.

2. Yes. They can and they will. Although they do not have a lockup clutch, I know they come within 95 - 98% efficiency. Only at maximum speeds, of course.

Interesting, my 300D with 3.07 does about 79 mph at 3500 rpm with 205/70-14 (25.3" diam.), so my numbers are pretty close to yours. My best guess is that we a losing a couple of percent in the torque converter.

Stock size tires 195/70/r14.

The speed numbers are GPS verified.

That's what concerns me. You expect about 3% slip in a torque converter at highway rpms. I've talked to a bunch of guys doing 70mph@3000rpm, who must have about 3% slip. Either one of my specs is different(tire size, gear ratio) or I'm experiencing 13% slip. That would be very bad news.

and or speedo error.

but in general the benz speedos are remarkably accurate.

tom w

My 84 was doing 67mph(gps) @ 3000rpm with the speedo showing 66mph. Thats sounds right to me.

I think you would have other obvious issues if you had 13% slip. I guess the tach could be off by a couple of hundred rpm, and your tire diameter may be a little different than you think. I wouldn't worry about it unless you are having problems.

That's very close to mine.

Yes, I'd expect the transmission to overheat at 13% slip.
How would I test the tach?
I've calculated and measured the tires. Both numbers are within 1/2 of 1% of each other.
I just want to know why.

I get the about the same.

The only way I can think of is to use a hand held strobe tach, you adjust the strobe to get a steady image and read out the speed. Very accurate, probably pricey, I don't know where you could rent/borrow one. :confused:

Autozone?

I don't know, I think they are more industrial than automotive. They're used to measure the speed of rotating equipment (pumps, etc).

i suggest checking the gear ratio.

jack up one wheel and put the car in neutral. turn the wheel one full turn and count the turns on the driveshaft. multiply by two.

i suspect you may have a 346 rear end.

tom w