Rear axle ratio, perceived or real?

[cascadia]

I recently switched from a 4.08 rear in my 65 230SL to one from a 63 sedan, it is stamped "D 3/9", which I'm assuming is 3.96.   Shortly after the swap I was curious to see what kind of rpm's I would be getting at freeway speed.  My speedo is in kilometers so I took it up to 100kph, which is roughly 62mph.  At that speed my tach was reading 3250rpm.

Later I entered those numbers into a online gear ratio calculator I found on the web, the other variable was tire diameter (height), which I found to be 22.74 inches.  Tirerack.com actually used the 185/60HR14 tire as their model for how to factor this number, same as my Michelin XZX tires!.  Anyway, the ratio the calculator came back with was 3.55.

Barring the possibility that my tachometer or speedometer could be off, what other variables are out there that could cause this discrepancy?  I've spent a lot of time behind the wheel of 113's and the speed and engine rpm sounded and felt pretty accurate at these numbers.

Bob in Portland

[waqas]

quote:Originally posted by cascadia

... it is stamped "D 3/9", which I'm assuming is 3.96.

Silly question, but are you sure the "/" is not a "6"? If so, that would make it a 3.69 ratio, which to my knowledge is the closest ratio to 3.55 available in these axles...

[cascadia]

Not a bad idea but no, it's definitely a slash.

[Naj ✝︎]

Tyre size shud be 185/80x14 HR whatever :oops:

[cascadia]

Aha, mystery solved.  That gives up a tire diameter of 25.65 and a ratio of "4".  Thanks Naj!  :|

[Naj ✝︎]

quote:it is stamped "D 3/9",

Could be 3,9?

Scrape more for another digit.

On the displaced axle, if you remove the larger end of the boot clip, or split the right casing at the pivot pin, the problem with it could show itself.
Would be great to find out what went wrong...

naj

[graphic66]

Jack it up and turn the driveshaft by hand and count the tire revolutions.

[al_lieffring]

Bob

After changing the rear axle ratio or installing tires with a different diameter, the indicated speed on the speedometer will not change in relation to the tachometer. The actual speed the vehicle is traveling will change, the calculation tables should be used as a way to determine the error in the indicated speed after these changes have been made.

The speedometer cable is driven by the output shaft of the transmission, in high gear the drive shaft turns at a 1:1 ratio with the engine. This does not change when the axle or the tires are changed. 5 speed overdrive transmissions don't change the calibration of the speedometer any more than driving around in 3rd gear would, because again the speedometer is relative to the dirve shaft's rotation speed.

Technology to the rescue!
The easiest way to determine speedometer error it to plug in a GPS and compare the actual speed of the vehicle to the indicated speed on the  speedo dial.

It has never occured to me until I was typing this that I should plug the Garmin in my Pagoda and check my speedo accuracy with it,  I think I'll go out and do it now.

Al

[cascadia]

Here's a pic, in my previous post my thought was that getting "4" in the equation was close enough to what I have to make sense.



Here's a pic of the old axle at the split boot, to my eyes I didn't see much that looked wrong?



Now I guess I should decide what to do with it!

[Cees Klumper]

Bob - clear photo, but in my recollection the ratio numbers are stamped into a rectangular flat surface and are the complete ratio, i.e. in my case 3.69 rather than some short-cut number.
As Al noted, changing the differential ratio will not change the RPM-at-a-certain-indicated-speed-as-per-the-speedo. Meaning that, again in my case, when I changed the differential from 4.08 to 3.69 the speedo still read 4,200 RPM at 120 KPH, except that in actuality I was driving maybe 125 KPH when before at the same RPM I was driving an actual 115 KPH. So there is of course a real difference but it does not show on the gauges.