While messing around with a gear calculator http://www.teammfactory.com/gear-calculator the other day comparing some different combinations, I decided to see what the factory transaxle (4.375 r&p, 3.80, 2.06, 1.31, .89 with 175/70-15 tire) looked like. I noticed that shifting at 3500rpm corresponded with the shift points of 15, 28 and 45mph as marked on the speedometer. I had a little bit of an epiphany of sorts, as I've asked before on the Samba what people considered the minimum cruising rpm's in 4th gear (with the fan still be turning fast enough to cool the motor) and it was now right in front of me. VW considered 2400rpm that number in 4th gear with a small stock (36 or 40hp) motor, which would be working reasonably hard at that engine speed, but the cooling system was enough to keep the motor from overheating. Top speed of this gearing combo is 66mph (at 3500).
I then put the bus .82 in 4th and engine speed dropped to 2200rpm. This might work in a lightly loaded car on flat ground (since we're being optimistic let's add a tail wind too; a chop top would help as well, but let's stay on topic. Bad Al!! Bad! Bad! There's my medicine...), but as soon as the load is increased (car full of people, or a hill or headwind) there's a 4-5mph dead area where the car is revving above 3500 in 3rd or being lugged in 4th. Now this will probably be ok with a 1600 (or larger), as the larger displacement will produce enough torque so at these lower rpm's (22-2400) the motor isn't having to deal with a lot of heat because the ol' foot isn't as far into the throttle as with a smaller motor. This improves the top speed figure to 72mph, but you have to live with that big 3-4 gap
As VW increased the displacement of the beetle motor and the torque curve got better they were also able to increase cruising speed; with the 1500 they left the gears alone, but went to the 4.125 r&p. The shift points increased to 16, 30 and 48mph with top speed going to 70 (again, at 3500rpm). With the dual port 1600 the dark elves at Wolfsburg changed the r&p to the 3.88 (shift points at 18, 32 and 51mph), but reduced 4th gear to .93 as the beetle's poor aerodynamic shape played havoc with the increased cooling requirements at the increased speeds. Top speed (at 3500) was still higher, at 71mph. The rpm's of the shorter 4th gear (at the shift point) was now 100 rpm higher (2500) to increase the fan speed to deal with the extra heat now generated. I don't know the formula, but as wind speed goes up resistance doesn't go up lineally; every so many mph it CUBES. Remember the increased rpm drop (from 2400 to 2200 when shifting from 3rd to 4th at 3500) with the .82 4th? With the 3.88 it's even more of an issue now, with the increased speeds. The solution (again) is more displacement, but now it takes 2 liters to achieve the torque curve necessary to cover the bigger gap. Although it will do it, you really notice the increased spread while driving. You can probably see what an advantage an extra gear would have.
Interestingly enough, when they went to the 3.88 in the Karmann Ghia, because of it's more aerodynamic shape they were able to keep the .89 4th gear. My apologies for this being so long; I thought a few of you would get something out of it (and I didn't envision it being quite like this...) Al
