[Excaliber]

Yeah, about that, you would be in the ball park doing that way. And 2.75 is pretty darn high!!! Anything in the "two's" is what you find in a typical automatic trans application. Anything in the 3's you could associate with a standard transmission. Anything in the 4's would typically be a drag racing application, limited top speed but quick acceleration!

With a BMW rear gear it must be hard to find a ring and pinion with the right ratio?

Run the Gear Calculator, use a 1 to 1 ratio for your final drive in the trans. That would approximate 4th gear. Assume a 25" tall tire. I haven't run the specs but I bet 4800 rpm, 25" tire, 1 to 1 trans ratio and 3.5 rear gear (all fairly standard numbers) would put you around 105 mph top speed. A nice over all ratio for street and SCCA "cone racing". 5th would work OK for the long straight away on a road course (assuming your engine has enough power to pull the gear).

Lets say your modest engine does not have "that much" power. So 105 mph in 4th and then you shift to 5th. The rpm drops a bunch but the speed may stay about the same! Not enough power to increase top speed with the "drag" of the over drive gear. I would not count on 5th for any other than a "cruise" rpm gear on the highway, which is worthless with your current rear gear!