This is kind of a newbie question (it is) but I have seen people talk about upgrading the gears in the vette and I believe I saw someone say it should be one of the first upgrades done on an A4. I know a lot of general stuff but I'd like to know in layman's terms changing the gears really means and also what benefits it would provide to the car/drive, especially for an A4.

Just as a side note - my car is an 02 A4 vert. Stock except for MF exhaust/xpipe and tune for now. In the future i'd like to track the car a bit just for fun, nothing real competitive.

thx

 

what is the cars current ratio ?

http://mikemercury.home.att.net/ratio.htm

 

According to the glove box it would appear it is 3.15 - the G92.

 

Most people would agree that for an A4, you only want to go with a 3.42, unless you want to spend alot of time at the track. I have a 2.73 in my car, and I drove the same car( 03) with a 3.42. Big difference. However, to go from a 3.15, to a 3.42, is minimal gain and probably not worth the expense which at least $1000. I plan on going to the 3.42, when I have the money to do it.

 

What type of gains would moving from 3.15 to 3.42 provide?

 

That is a valid point.

I went from 3.15 to the 3.42... but I had a goal in mind, and wasn't looking for a specific amount of change. And the 3.42 put me right where I wanted to be.

The 3.15 to 3.42 change is the smallest jump one can make (only 8.6%).

here's my rear axle ratio change page:

http://mikemercury.home.att.net/342.htm

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Changing the gear ratio to a higher number makes the tires spin slower at the same RPM, but multiplies the torque. The net effect is that you launch harder, and the car accelerates quicker. The downside is that you reduce top end speed (maybe, see below), and the car will have to run higher RPM's to do the same speed. So if your doing 2000RPM's to do a certain speed, if you went to 3.42/3.73 you'd be turning 2172/2368 RPM's. The other downside is since the engine is at higher RPM, your going to use more fuel.

Because of the higher RPM's, your engine hits redline faster. If you use a gear calculator (just going off my head so don't crusify me), top speed of the C5's for 2.73/3.15/3.73/3.90/4.10 is something like 260/240/230/200/180/160mph. You can look up the real numbers pretty easily. Obviously, your car doesn't do 240mph, because other factors (wind resistance, tire friction) all add up to limit top speed. Because of this, only going to something like 3.90 or 4.10's show any reall world decrease in top speed. And then, it's not much more than 10mph less.

No question though, do it. One of the best seat of the pants mods and makes the car much more responsive. I don't know anyone that made the change and thought it would be a good idea to go back.

3.90 and 4.10 are common for the M6. 3.73's are the practial limit for A4's, although some people swear by 3.90's and some even have gone to 4.10's. 3.42's are usually the most popular option, since you can get a used rear pretty cheap from a M6 car (all M6's came with 3.42's).

While your at it, a torque converter change can really wake the car up by getting it into the power band quicker. Simple matter to replace that at the same time as the diff change. See Mike Mercury's page for an excellent write-up. For the install, search for forum vendor Patches, as he has pictures from the install.