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would like to upgrade my rear end gears to get the most out of my mods. I've seen 3.73 and 3.91 gears that replace the oem ones without having to purchase a complete rest end. I know that there is more to this and won't just be a simple swap. What kind of undertaking would this be, and would the results be worth it? Is 3.73 or 3.91 better for and automatic?
I've heard either set is beneficial. Only drawback is 3:91 basically has no 1st gear. You'd be shifting from first to second quicker than stock.
I'm auto. How would it affect that? How much of an undertaking is it to swap them?
Swapping gears not to difficult.
Regarding the GR (gear ratio) - there are several factors in getting the best combination for GR.
Tire size, your transmission type (manual 4,5,6, 8 speed/auto 4, 6 speed etc) and GR of the transmission, For racing - estimate speed at top end.
For street manners - Too tall of a GR and you'll lose mpg.
I wouldn't go higher than 3.73 with an Auto that is going to be mostly on the street and little track. Pair with a good stall converter and tune that 3.73 will give you a nice plant in the seat.
With bigger tires (think trucks) they usually need taller gears to get the torque to move them. Can't remember where I saw or read this but something like a truck with 38 or 42" tall tires with a 4.11 rear GR drives like a 3.73GR on normal size tires. :dunno:
There is swapping the gears and then there is correctly setting up the gears using the right measuring tools. Truthfully, lots of people just swap out the gears hoping to get it right and end up with noisy short lived gears. The right way involves a pinion setting tool set (about $70 bucks for a basic set from Summit) and going through the process to ensure both the pinion and ring gear are installed correctly. Honestly the hardest part is having to press and remove the pinion gear bearing a couple of times and maybe ruining a crush sleeve to get the right shims installed. See this set of Richmond Gear install instructions http://api.richmondgear.cartanium.co...RP_Install.pdf or check out some of the many good and not so good videos on youtube. Gears can be done with the axle on the car, especially if you have a lift, but without a lift, it is usually easier in the end to do it with the axle out of the car on a set of blocks.
I have 3.73's in our car with the A4 -- might have gone just a bit steeper as it does not turn all that many R's running down the interstate. However, this does appear to be the gear set of choice for an A4.
He also has a IRS rear so there's a few different procedures compared to a solid axle.
what would the difference be?
Instead of axles you need to remove the half shafts. It's not really that much more work just a different procedures to pull the diff out.