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

Ha. I can just see the looks on the faces of the ppl behind the counter at my Autozone if I came in there asking for that.:1blank:

I befuddle them often. My local autozone is used to it.

This is the kind of instrument I was referring to:

http://www.kerncoinstr.com/461825.htm

I like to go to O'Reilly's because they all recognize me and know my car because almost every guy behind the counter has come out to see my veg system.

It seems like all your calculations are right. Maybe you could pull your tach amp off and clean the contacts? Probably a long shot since it actually works but it couldn't hurt.

Now this is kinda interesting. The speeds pattern by year.
79 - 60@3000
80 - 63@3000
82 - 65@3000
83 - 72@3000
84 - 66@3000
85 - 71@3000

The average for 82 and 84 is about the same, but different from 83 and 85 which are about the same as each other.

In reality, the ratios are the same from 77-81 (3.46?), 82-84 (3.07), and 85 (2.88). What we are seeing here is variations in tire size and speedometer readings.

Ok, I've possible causes I've eliminated:
speedo error - gps checked.
tire size error - measured.
gear ratio changed - I got 3.13, which I'm willing to bet is really 3.07.

That leaves:
tach error
transmission slippage

Ok, if 82-84 have the same 3.07 ratio, and the average 3000rpm speed is 67.6 then I'm off by 6.75% or 202 rpm. A little tach error and/or a little looser torque converter could account for that.

I would agree with that, or maybe a little of both.

I'm hoping for all tach error, naturally. Remember that 200 rpm is the difference between my observed results and you guys'. It doesn't include the 180 rpm(6%) slippage everyone has.

If you used my speed for the 84 model yr. it may be diffferent....It's a cali model:rolleyes:

Sometimes I wonder about the People's Republik of Kalifornia. Seems like they are slowly working their way towards outlawing cars. Was the ratio different? You can chock one rear tire, put the transmission in neutral, lift the other tire off the ground, rotate it twice, and count how many times the driveshaft rotates to get the ratio.

That chock part is important. Really important. Chocking a front tire also just to make sure wouldn't be overkill, IMHO.

If you wonder about slippage, let off the accelerator and note the rpm drop.

In my SDL, at 80 mph it turns 3400 rpm. Let off the loud pedal, its 3050 rpm. Loaded, it slips to 3550 rpm. It consistently slips 10 - 11 % at cruising speeds, lightly loaded. The 2.88 is stock and unchanged. Extremely high stalling TC.

Calculated engine speed for a manual transmission with the data given is 3157 rpm at 70 mph.

An automatic will usually add 250 rpm to this figure........on dead level ground. So, you're looking at 3407 rpm.

The difference between 3500 and 3407 rpm is probably the tach.

My SD has a calculated rpm of 2625 for a manual transmission at 70 mph. The tach reads just a hair under 3000 rpm. The torque converter is not slipping by 375 rpm.

One way to check the tach.
http://www.tinytach.com/tinytach/diesel.php#

Meaning you suspect your tach to be off by several hundred rpm?

Tip to get accurate measurement of tire:
1. Park car at one end of long driveway.
2. Take a 24 carpenters' square and hold it short end on ground and long end flat against center of tire.
3. Mark vertical lines on tire on upper and lower section of tire along vertical section of square. Mark a line on ground at edge of square.
4. Make an "X" on upper side next to line.
5. Roll car dead straight forward as far as possible until "X" is on top and lines on tire are vertical again (check with square then mark ground).
6. Count number of revolutions of tire.
7. Measure in inches, distance between two marks on ground and devide that by number of revolutions.
8. This gives you a very accurate circumference of the tire. Accuracy matters when you are talking 700-1000 revolutions per mile for most tires.

To get gear ratio, do it the hard way: pull the cover and count teeth on both gears. I'm betting the gear ratio is whatever is stamped on the case, but there will always be the oddball situaion where somebody changed something.

My tach reads at least 10% low.

Mine's off a bit more...:silly:
http://i135.photobucket.com/albums/q...h/SUC50512.jpg
John

i vote for tach inaccuracy too.

tom w

Mr. T
 

Just curious, why would you multiply the driveshaft rotations by 2?
Seems like the ratio would be driveshaft rotations to one wheel rotation.
Thanks...Al

It would if you had a locking or limited slip differential. In which case you'd have to lift both tires too.
With an open differential and one wheel on the ground the axle drives the spiders, which drive the ring at half speed... 'cause one tire is not moving. So to immitate both tires making 1 full rotation you have to rotate a single tire twice.
Not the world most elegant explanation, but I'm positive the two rotations thing is right.