W463 manual gearbox?

W463 manual gearbox?

A question to pick up your technical brains....

My 1991 300 GD was fitted by a previous onwer with a MB-exchange gearbox a couple of years ago, at a cost of some € 3,000. The gearbox is silky smooth and it works perfectly well. The problem is that in my opinion the gearing is either too short or both the rev counter and the speedometer are lying. Let me explain....

The truck is fitted with 235/85R16 tyres, which are substantially taller than the 215R16 ones that it was fitted with when it left the factory. However, I have the original invoice from a VDO Kienzle service showing that they recalibrated the speedo after the newer tyres were fitted (at a cost of DM 360!). Results of the test show that after recalibration when the truck is travelling at 116 Km/h the speedo shows 120 Km/h.

The gearing in fifth gear in any G-Wagen should be as follows:

formula should be: 1,000 x (1/gearbox ratio)x(1/transferbox ratio)*(1/differential ratio)x(60x2xPi)x(radius of the wheel).

In my case:
gearbox ratio is 0,799
transferbox ratio is 1,05
differential ratio (37:7) is 5,286
Pi is 3,05 (rather than 3.1415926 assuming "flattening" of the tyre due to the weight of the truck)
radius of the wheel is (assuming size 235/85R16, width is 235 milimetres, profile is 85% of the width and diameter of the rim is 16 inches [406,4 mm] and given that radius equals half of the diameter of the rim plus one lateral section of the tyre)= 235*85%+406.4/2 = 402,96 mm

So gearing in fifth gear must be:
(1000*(1/0,799)*(1/1,05)*(7/37)*(60*2*3,05)*0,40296)*(1/1000)= 33,26 Km/h (20.67 mph) @ 1.000 rpm.

Now I have noticed on numerous ocassions that at 90 Km/h my rev counter gives 3,000 revs. At a indicated 120 Km/h (actually 116 Km/h) the engine is working hard, clearly over 4,000 revs. If this is the case, I have a gearing roughly 10% shorter that it should be and I am wondering whether I am making a mistake with my calculations or the gearbox currently fitted to my truck is not the GL76/27K-5 as it should be but another one with a shorter fifth gear.

I have the "Einbaupass" of the new gearbox and it is listed as "Getriebe 463 260 03 00/80" with serial number 430 781 and "Lieferschein 484631". I have checked these and they match the numbers in the plate attached to the gearbox so there's no mistake to be made here. Can somebody check these numbers with their (working) EPCs and let me know?

Thanks in advance,