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In Reply to: How many people have the Close Ratio'dogleg' trans posted by Tim In PA on November 11, 2001 at 20:35:54:
A 3.07 with C/R tranny is roughly equivalent to a 3.73 with O/D tranny. The reason why you'd want to go to a 3.25 is because you'll get even more gearing, without going to a stump-puller 1st gear. On O/D trannies, anything greater than 3.73 will get you a stump puller 1st - even if you get a chip to raise the rev limit. The 3.25 works very well with the C/R in northeast, and even southeast driving environments. Out west? Ehh...stick with the 3.07 if you do lots of highway driving. Ideally, you'd want to be around the torque peak, or just below it, when you're doing most of your highway driving. So say you're average highway speed is 75, you'd want to around 3500 rpm. That is actually what I've found to be the most fuel efficient and best to maneuver around traffic. That is why I want to put in the 3.73. Because if I'm doing around 65, I find myself keeping the car in 4th. If I go over it's in 5th. But it's at that speed that I'm going between gears, and I'd rather have one or the other be the sole gear I'd want, rather than having to go between the two.
If you want to do your own calculations, the math is on my website. Rob A already's written much about this:
"Speed in mph/60/.00122982 x Diff Ratio x Gear Ratio=RPM"
So...at 100 mph, with a 3.07, in 5th of 1:1, gives you a theoretical 4160.50 rpm.
At 100 mph, with a 3.25, in 5th of 0.81:1, gives you a theoretical 3567.60 rpm.
At 100 mph, with a 3.25, in 5th of 1:1, gives you a theoretical 4404.43 rpm.
You can do the calculations for the rest of the gears and modify the equation to find speed at rpm, or whatever you're looking for.
With a 3.73, 3500 rpm would be 85 mph in 5th. Or 4000 rpm in 4th would be 79 mph. Those speeds are right where 5th gear ought to be in a NE driving environment (or NJ roads).
(Note that these are theoretical figures based on stock wheel/tire total diameter, and if you screw around with that diameter (which is roughly 630mm) then you'll have different speeds at different rpms than what I've just calculated. Since you're running the correct tires for the wheels, you don't have to worry).
Now, if you want max speeds in each gear, then adjust the equation (use 6.2k rpm). BTW, you will max out the car with a 3.25 and C/R right at it's top speed, no like WH says...take a look. Your car will be aero drag limited at red line in 5th for top speed. (Remember that a tad over 140 (like 141 or 142) is the actual top speed of the dirty engine with euro aerodynamics, i.e., small bumpers). This is without a chip to raise the rev limit (which is between 6.2k and 6.4k on a stock chip, as from the test reports I've read it wasn't an exact science...don't ask me why).
With C/R, 3.07:
1st: 40 mph
2nd: 62 mph
3rd: 84 mph
4th: 118 mph
5th: 149 mph
With C/R, 3.25:
1st: 38 mph
2nd: 59 mph
3rd: 80 mph
4th: 112 mph
5th: 141 mph
With O/D, 3.25:
1st: 37 mph
2nd: 64 mph
3rd: 100 mph
4th: 141 mph
5th: 174 mph
With O/D, 3.73:
1st: 32 mph
2nd: 56 mph
3rd: 88 mph
4th: 123 mph
5th: 151 mph
Notice how 1st gear with a 3.73 at 6.2k rpm becomes a stump puller? You reduce it's speed range compared to the C/R. The reason is that 1st in a O/D tranny is a lower gear than 1st in an C/R tranny. The good thing for C/R tranny is that the gears are spaced closer together, meaning that you for a given speed range you have more chance to be in the optimal gear. For europe with 100+ highways, the O/D was a godsend. In the US, where you don't see much over 80 or 90, the C/R is effectively the best tranny, coupled with diff between a 3.07 and a 3.25. If you're in Montana, stick with the 3.07, if you're in Jersey, the 3.25 will make life much easier. :-)
Hope this helps.
Hello All,
Just curious as to how many listers have this trans. I am thinking about switching to a 3.25 rear instead of the 3.07, but find that I am already pushing 4150 rpm at 100 mph. Hope all is well.
Tim In PA
1986 M535i
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