And I'm 100% certain you are incorrect.

 

Oh alright

 

Two completely different things. Yes tire diameter will have an impact on the speedometer if the front to back diameter is changed beyond the stock tolerances.

The diff swap has no impact on the speedo because the VSS reads off the ring gear/axle revolution. Which is not changed when a gear swap is done.

Swimming out of my lane here (not an A4 guy) the A4's require a tune to recalibrate the transmission shift points after a diff swap. Not the speedometer.

 

I see, thanks for the explanation.

 

Shift points is correct I went from a 2:73 to 3:15 and had to retune for shift points. So if you upgrade might as well go with 3:42’s

 

 

I wasn’t aware of that. Wow, that sucks!

 

i had a '99 that saw the 3:73 stock gears go kaput. at the time, i had a&a put in a new upgraded and strengthened 3:42 diff as well as converter and wow what a difference. yep had to tune to re-do shift points, of course.

i forget if anything else had to be recalibrated (speedo, as someone mentioned above). 3:42's were night and day difference. if you have an auto, def. go for the 3:42 if you're headed down that path, as life's good mentioned. make sure you pay up for a quality unit.

 

How do they verify it as being the factory stock tune?

Do they:
Download it and visually compare/verify actual settings?
Only check the name?
Check a date?
Verify a checksum value? (which would typically be an indication of a change made to the file)


I ask because name, checksum, date created/modified/etc can all be edited as desired.

I think a cam is playing with fire because it will be audibly different and if they do any actual sniffer hookup it may also skew results if you're forced to convert to factory tune for the test. Bear in mind any time you change your tune you must go through several on/off cycles and speeds over 15mph until all the emissions checks tick as completed.

I say you score a used blower kit for about four grand with a carb number. Add a smaller pulley for more boost. Get it tuned NICE and enjoy the snot out of it.

If they're checking checksums you can tune the car and spoof the checksum easily. Or you can just save and revert to your factory tune for the check. Disconnect the head unit so it's not spinning free. Toss a normal air filter back on temporarily. Get through and then hook it all back up next weekend. A headache, but a small one for enjoying 600hp in ****-land.

 

2.73 or 3.73 gear ratio? I have already moved from my original 3.15 to a 3.42 (not as much impact as I would have liked), and have been considering 3.73s or 3.90s. After making the 3.15 to 3.42 move I do not think I would do it again - but I did score a built up 3 rib diff which is great for the added strength. However, the swap from 2.73 would be very noticeable and apparently not an issue - so far - with CA emissions.

 

pardon, yes i meant 2:73.

3:15 to :42 doesnt seem like a big jump, youre right.

i now have 4:10 (from 3:42) in my z06 and its quite a difference!

 

Thanks. I guessed it might have been 2.73, but thought I may have missed something along the way.
Yeah - 3.42 to 4.10 is a nice jump and it has to be great improvement especially when in traffic.
With our A4 and a fair amount of highway driving the 4.10s would be too low, but your M6 with the 0.56:1 helps pull your RPMs down. If calculated correctly, at 80mph, you would cruise about 250rpm lower with 4.10s than my A4 with the 3.73s; that is a nice reduction in operating rpm.

 

the 4:10's on my z06 are comically entertaining. def. fun for spirited driving! shifting around town is much, much more frequent because of the short 1st gear, but...whatever. i don't use it for daily driving so it's not an issue. gotta be careful flooring it in either 1st or 2nd because traction is a legit issue. quality problem to have!

i really like the 5th and 6th gear when on freeways. the powerband is right there in the sweet spot at 60-80 mph, esp in 5th gear around 60-ish. instant passing power, the engine just sings, seems happy rather than lugging along. some time back when living on east coast and car was with me (unlike calif., where mods really aren't a viable or practical option), i spoke with mechanic (chuck @ corvettes of westchester) about doing headers, mild cam, etc. he knows how i drive and said go with 4:10's and a tune. he was spot on, and it saved me thousands plus not having to tear into the motor.

similarly, years ago when i had my '99, andy at a&a recommended i go with the 3:42's and beefed up transmission stuff (converter, gears, etc; i honestly forget what was all done and just left it in their good hands) in case i ever went with more power adders. forget the name of the company who supplied all the hardware, maybe it was rpm? either way, quality stuff. not cheap, but ya get what ya pay for. he said start with gears and see how i like that, then maybe go from there (he was developing the 1st carb-certified blower at the time, which was where i was possibly headed for some go-fast fun). ended up selling that car to get my z06.

 

For the record, I LOVE my A&A setup. It's totally worth the money. With that said, I'm not sure I would mod my car at all if I lived in CA. There's always the risk that anything you do could be made illegal. Frankly, I couldn't even live there. I would move. Although, I certainly understand that might not be a viable option for you.

 

There are certain counties in Ca where no smog check is required. If you know someone in those counties the car could be registered to that address....

 

reckon ill go with 4:10 gears