Gear Ratio RPM issue

[Orange Terror]

Well, I assumed I have 2.73 gears in my car. It's the stock 10 bolt, I changed the fluid in the diff this past spring and never thought to count the teeth on the ring gear, didn't have a reason to. Also when I took the cover off the bolts and everything looked like they've never been touched since the factory.

I've jacked the car up twice, both tires off the ground and counted the turns of the driveshaft to 1 turn of the tires, its less than 3 which leads me to believe its a 2.73 ratio gear.

Well, the problem is, running down the highway in 4th/OD at 80 MPH I'm at 2800 rpms. I've used several different gear ratio calculators online and even have the actual formula for calculation at home and I've worked it out long hand. Every time I get 3.73. Now, I would love for them to be, but when I work it backwards I get around 2000 rpm's @ 80 and 2800 should work out to 112 MPH!!! So something doesn't add up, somethings off. I know the factory tach isn't exact but I doubt its 800 rpm off. I know I need to pull the cover and count the teeth to be sure, but either way I'm missing something or somethings off.

Another thing, I have an aluminum driveshaft, which from what I've read, only came with the performance gear ratio of 3.23. I haven't checked the code's on the door but I just found them on LS1 tech and will be looking tonight. But that gear ratio doesn't fit what I'm finding.

Also, yes, from what I can tell my converter locks up, you can see it over 40 mph in OD the rpms drop just a bit, and its never acted like its slipping. So I don't think its that, and if it was, I'd have bigger problems slipping 800 rpms.

Anyway, just curious what RPM's you other A4 guys run with 2.73 gears and if you had any thoughts.

[kammi10]

i don;t know if the gear ratio's are the same between a 700r4 and your tranny but my camaro would run about 2100 at 80mph. im willing to bet you either have 3.23's or 3.08's in it. i didn; think 4th gens came with 2.73's

[shiftinda93z]

RPO code will tell you... just open up the glove box

Sounds like you have some other issue like not going into OD or something though...

[Orange Terror]

i don;t know if the gear ratio's are the same between a 700r4 and your tranny but my camaro would run about 2100 at 80mph. im willing to bet you either have 3.23's or 3.08's in it. i didn; think 4th gens came with 2.73's

Most of them came with 2.73 gears. WS6's, SS's, and some specially ordered came with 3.23's. That's the A4's, all the M6 cars came with 3.42's. 3.08's were not an option on any. Check this thread out.....

http://www.ls1tech.com/forums/showth...7&page=2&pp=30

[pHEnomIC]

I had 2.73s for sure in the Z, and the rpm method never worked for me. I got the numbers off the gearset so I was positive. My ds spin did the same thing as yours. It is still possible that you have 3.73s but the rpm method wasnt very accurate. It is possible that your speedo is off as well. And another thing, any play in the driveline whether it be in the trans or gears will throw off your readings.

[Orange Terror]

Yeah, guess the best thing is to just get under there and pull the cover and find out. I got all winter. lol.

[shiftinda93z]

make sure you turn the driveshaft, and you will not need to pull the cover. Just mark it out with chalk on the tire . You can also tell what type of differential you have without pulling the cover too...

[wezeles]

dont forget that rims and tires are also a factor when you calculate... when did you buy it and who did you buy it off of? 3.73 is probably one of the most common upgrade gears so its possibal... i doubt your tach is off that much they seem to be something that works or doesnt... speedos have failed for me in the past slowly wearing out but never a tach... check the axel too... just cause your RPO code says one thing doesnt mean thats the rear end in the car... there are 3 or 4 spots you can get the axel id from... both tubes off the pumpkin left and right sometimes a little tag on the cover under a bolt.. would be nice if it had 3.23's or bigger in it though one less thing you would have to upgrade.

[Orange Terror]

dont forget that rims and tires are also a factor when you calculate... when did you buy it and who did you buy it off of? 3.73 is probably one of the most common upgrade gears so its possibal... i doubt your tach is off that much they seem to be something that works or doesnt... speedos have failed for me in the past slowly wearing out but never a tach... check the axel too... just cause your RPO code says one thing doesnt mean thats the rear end in the car... there are 3 or 4 spots you can get the axel id from... both tubes off the pumpkin left and right sometimes a little tag on the cover under a bolt.. would be nice if it had 3.23's or bigger in it though one less thing you would have to upgrade.

Yeah I know, they are the stock wheels and tires, 16" GM 5 spokes with 245/50-16 tires. Stock size. I'm really hoping that I do in fact find 3.73's in there, that would rock. I bought the car off of a dealer, I'm guessing that I am the 3rd owner, looks like from a couple papers I found under the seats that the last guy didn't have it a year. It was spotless, so my guess is the first owner took good care of it. But hard to know for sure, this is just something that's been bugging me lately and its getting to the point where I need to know!! lol.

[1BADAIR]

depending on the rpm my tach is off by as much as 400-500 rpm.
http://www.f-body.org/gears/

[SlipStreamZ28]

If you still have that sticker with all the codes on it ( mine was on the drivers side door , near the jam) . Look up the final gear codes online and then look at the sticker till you find one that matchs and thats your factory gear ratio . I think most V8 camaro's came with something around 3.43 ( thats what mine has), but thats the LT1 ones I dont know about the LS1's

[Orange Terror]

If you still have that sticker with all the codes on it ( mine was on the drivers side door , near the jam) . Look up the final gear codes online and then look at the sticker till you find one that matchs and thats your factory gear ratio . I think most V8 camaro's came with something around 3.43 ( thats what mine has), but thats the LT1 ones I dont know about the LS1's

LS1 A4's were 2.73 or 3.23 which was the performance ratio that also came with the aluminum driveshaft. I think the LT1's A4's came with the same or similar ratio's since they used the same tranny. The M6 cars only come with the 3.42 gear, so I'm assuming yours is an M6.

Yes, the door sticker is still there, I'll be checking it tonight.

[1BADAIR]

you mean 5.13's didn't come stock

[wezeles]

sure the did..... in most semi's lol

[JoeliusZ28]

this is the same problem ive been having with mine.

my car finishes the quarter mile like it has 4.10s. cruises like it has 3.90s, and turn tests like it has 3.73s. ive been very confused about it since i bought the car, and im not gonna ultimately find out until pull the cover again most likely.