allroadscott

has anyone changed their sixth gear to improve gas mileage? i drive 40-45k /year and the 20-21 mpg is killing me on the highway...

2tuŽbos1mission

Probably not worth the cost, it's possible but will be a few thousand.

LCP

...I did a test using 5th vs. 6th gear for freeway crusing on my S4 and found only about a 1mpg difference using 6th over 5th, despite the difference in ratio (0.684 [~25mph/1000rpm] vs. 0.806 [~21.5mph/1000rpm]).

ShowMeData

on what Audi has already selected. 6th gear is purely for highway economy already - I'd bet lunch that Audi has already optimized that the best it can be. There is too much CAFE pressure on automakers not to.

My factory rep told me that's why we slushbox owners suffer from tip lag so badly - because the trans shifts to 4th to cut fuel consumption as soon as you let off the throttle. A necessary optimization to keep the A8 under the gas guzzler tax threshhold.

allroadscott

it sounds like some of you responding are in the industry or in the know. it just seems to me that the ar is reving too high (3k+ at 75mph) for the amount of engine, horsepower, torque involved. my previous car (passat wagon) hummed along at 80mph, @ sub 3K rpm and averaged 26-28mpg.
perhaps this is apples to oranges, but i would expect the 6th gear to be more like an overdrive economy help, like you describe on the tip. Gears 1-6 are tightly spaced, almost so much so that i only use first gear on uphill, stopped starts. i think that instead of first being so low, sixth should be taller.
Why would it take thousands $ to make 6th taller?
would the 402 mod (lowering) help save a mpg or two?

Milpitas2.7T

80 is around 3K how much better do you want to get?

allroadscott

3k rpm gives me 72mph in my ar.
almost all of my driving is at highway speeds, so i dont really need the grunt as badly in sixth gear. i think a lower rpm at 80 mph, like 2500-2750 might be more fuel efficient.

LCP

...throw a chip on the allroad and you'll find that you hit top speed in 6th gear at the time you run out of RPMs in 6th. 6th is very much a "power" gear just as the 5 gears before it with the 6-speed transmission box mated to the 2.7T motor. The gears are all very close to each other and the gearing is as short or shorter than you will find on most any V6 on the market.

allroadscott

perhaps i just don't understand...
i agree that the gear spacing is very close and that they are shorter than most. That's the problem with fuel economy at highway cruising speeds 75+mph. A motor that powerful doesnt need to be @ 3krpm to push 72mph. I am assuming that a lower rpm at a given speed will use less fuel.
Also, chip or no chip, your statement of "you'll find that you hit top speed in 6th gear at the time you run out of RPMs in 6th" seems intuitive...

yzf996fltri

usually because top gear is an overdrive. I think you might find a small improvement in mpg if you could somehow make the allroads 6th gear an overdrive but personaly I prefer to have decent acceleration at hand without a downshift while cruising in top gear.