I have a 95Z A4 with a flowmaster exaust, CAI, 160 stat. I've heard they take off 2 to 3 mpg. I want to run 13.0s will 3.73s get me there faster.

 

I have been trying to figure out which ones I want to go with also.
I will probably go with 4.10's but I really haven't made up my mind.
I am probably going to wait and see what cam I decide to go with; which depends on the whether I go with Nitrous or not.

If you don't plan on going with nitrous then 4:10 seem to be the most popular....with traction being the biggest issue.
4.10 are minorly weaker than 3.73 (just because of the number of teeth required for that ratio) but with a proper install the difference in strength is practically nill.

A lot of guys run 3.73 when they have a Nitrous setup because having Nitrous and 4.10 seem to equal no traction.

I don't know from personal experience with a nitrous setup but the 4.10 on an NA application are killer.

I hope the info I have gathered so far helps out.

EDIT: After rereading your post I see that ....All the info above really has nothing to do with your post. I think that most go with 3.73 because 4.10 really cut down on drivability in the A4. Sorry for all the extra info.

 

Gears, tires and suspension is the cheapest/easiest way to drop your ET.......

 

A rule of thumb I see around here a lot is 3.73 for A4 and 4.10 for M6, but that's just popular opinion. On an A4 4.10s will obviously reduce your highway mpg, but if you want fuel economy, buy a 4 cylinder. I plan on going with 4.10 in my M6, if that helps you any. As far as dropping your ET, it depends on your tire size really. You want to be crossing the finish line when you are as close as possible to your max power in 4th, so you may want to check out the gear/rpm/mph calculators that are all over the web. Also, do a search on this topic. I'm sure you'll find a ton.

 

Number of teeth required for that ratio? There is only a one tooth difference between 3.73 and 4.10, and it's on the pinion gear.

If ur going with N20 on an automatic, 3.73 and 4.10 are both rather high, and u'll have a better gain spraying on a 3.42 setup.

3.73s are very difficult for daily driving, just as a 4.10 setup. If u still plan to use ur car as a daily driver, i wouldnt go higher than a 3.42. If u go higher and do a decent amount of highway driving, ur gonna have TERRIBLE gas mileage.

 

4.10's!

Frank