Why isn't 6th gear super tall?
 

Just got a 03 Audi 6 speed v6 A4. mpg is 27 going about 80 mph all day.
RPMs are 3000 going 80 when V6 horsepower can easily move the car in any gear at 2000 RPM. Why don't car makers make a super efficient 6th gear to drop the RPMs so it get super highway MPG?

They build it to get good number on the EPA test cycle and the EPA test cycle doesn't go that fast. They want the sweet spot to be at the test speed. There is probably also a point where the gears would be too much of a drop in RPM or just too much strain on the parts themselves. There is also a limit in the size of a transmission between the input and output shafts that the difference in what the lowest gear to the highest gear can be. To make that separation bigger they would have to increase the separation of the shafts. So it's probably a combination of all 3. I bet bottom line, if they could make the EPA number better they would do it even if they had to re-design everything else to work with it.

^^ there's probably truth in that. And I'd also add that customers might complain that the car feels a little "doggy" with taller gearing, which would be a marketing problem for them.

Gearboxes in general are classified as close-ratio and wide ratio. Close-ratio is for sportscars, wide-ratio is for trucks. If sixth is raised at the very least fifth would have to move as well.

Audi might want to sell to people who ski, with lots of friends.

I had a auto two gear ford falcon, a 3 gear mustang, multiple 4 gear trucks and cars . Many 5 speeds. Now I got this 240 hp 6 speed . If I'm just coasting around, I skip gears due to getting bored with all the gears. I say " I want 5 gears for driving, and if there is a a 6th. It better be frickin tall so I save gas on the freeway. gosh darn it!
Yes, I do rev it up and enjoy the heck out of it, but I can't act like a 17 year old all the time anymore.

Permalink, if I drive 40 mph in 6th, I get 40 plus mpg on display, tach shows about 1700 rpm. Engine takes it fine, no knock or lug. I see your point. EPA tests and all. Avg fleet mpg. Still a taller 6th would seem to help make the results better. It would also increase top speed I reckon.

This is among my favorite gripe topics. Your gearing is way better than my Acura. My 6th gear has me doing about 65 MPH at 3000 RPM.

The thing is, 6th is completely worthless as a racing gear. It would take several miles of a straight racetrack for me to top 5th gear out. A jumbo jet runway isn't long enough for me to reach 6th in a drag race. But, it's so short that it isn't any use cruising on the highway at 60 MPH either.

It sucks at everything!

If Acura had programmed the VTEC to kick in at somewhere less than 6,000 RPM (yes, that is when it begins to work), then the gears wouldn't have to be so closely spaced.

I've thought about putting an RSX final into the TSX, but it would only marginally help tame the RPMs.

Other than that, and the car having the turning radius of a train, it's a perfect car.

I put a five-speed in my (normally four-speed) VW Type III notchback. I shifted up into fifth at 85mph.

The advantage came in having one more choice for gearing driving around town.

The new Civic Si has an amazing 6-speed. 1-3 is as short as the old NA Miata's (VERY short). Then 4-5 is fairly tall, and 6th is really tall. I believe 6th at 80mph is 3000rpm. I know 6th at 70 is 2700, which is about what the honda insight is in 5th. Guess that's why it's rated for 41mpg highway.

Most of the times a 6th gear is worthless at all, and sometimes a 5-speed transmission may actually lead to a better mileage compared to a 6-speed. But anyway, instead of a taller 6th, if I had the choice I'd get a wide-ratio transmission with a higher gear spread and match it to a higher differential.