thehazz12

I'm trying to find out if I have the stock 3.46 gears in the rear or if mine have been swapped out for 3.73s... After some talk about top speeds on an FB page and some guys throwing out numbers in some gears that made no sense to me, I decided to do some calculations using this> http://ultimategto.com/art29.htm top speed calculator.
Yes, I'm fully aware that I can crack open the dif to find or or simply jack up the rear, spin a wheel, and count the driveshaft revolutions. The problem with that is: My jacks and lift are at my shop and as I'm a full-time student and working-part time, my GTO rarely leaves the garage. Not knowing the answer to this question is just going to bug me so I figured some of you would be able to help me.
As the title states, it's an '04 6-speed... I bought it last July from a private owner with 61,xxx, but was only told of the K&N, ported TB, Headers, custom catback, and Hurst. I was told it's still got the factory tune on it so I jut assume my redline is right around the 6k RPM so I shift at (roughly)40-60-90-120 which, according to the above calculator, seems to be consistent with 3.73s. I asked a local '04 m6 buddy his shift points and they were the same as mine. He bought his with 14,xxx in 2009 from a guy who used it as a showcar. The only mods he was informed of were a K&N and SLP LMII catback (it's much, much more than that now)...
Enough with this damn novel. '04 m6 peoples, tell me: At what speeds (roughly) are you shifting (If you shift after 6k take that into account)? Do my speeds seem to be consistent with yours? And finally, do I have these freakin' 3.73s or not???

Thanks
Brandyn

Edit: Forgot to mention that I noticed a very slight leak in my dif last time I had it on the lift. That was another thing that led me to possibly believe someone did some messing around back there...who knows.

thehazz12

BPP

iTrader: (2)

Most base the gear ratio by cruising rpm and not top speed. Give an idea of what your rpm is at 60 or 70 and I bet you get responses

thehazz12

I understand that, but still...my questoning should work all the same. I can't really give an accurate speed/rpm because I'm on a 245/45/18 now and haven't changed anything in the tune so my speedo is 3% off.
**** it. I'll just throw it on the lift next time I take it out.

409CISecondGen

iTrader: (2)

Idk if this helps- top of third with 3.91s:




Compared to stock gears:

looks like 91/92mph at 6000rpm on stock gears

vs 81mph after 3.91s



I don't think you have 3.73s

svede1212

iTrader: (3)

Your questioning would still require factoring in your tires.

It's a lot easier to ask "At what RPM is your car at 70 MPH?". (and a lot shorter too!)

Almost everyone can tell you what you have then. Those gears would make a 7% difference in RPM.

Sadly it's pretty common to have untouched diff leaks.

redtan

iTrader: (5)

Sure you can, it's easy and quick too.

Grab a GPS unit that displays vehicle speed, see what RPM you are at 60mph then go plug in all your relevant information into a gear calculator and see at what rear end gearing matches your cruise RPM.

thehazz12

I understand that now, but I still have the same problem...my car won't be out of the garage for a while for me to find out what that RPM is. I was just hoping somebody would be able to give me a quick, accurate answer. Yes or no. That's fine, though. I'll just update this when I've got that speed/RPM.

Nothing with this car seems to surprise me anymore.

Got it. Thanks.
How accurate is a GPS unit really going to be, though?

redtan

iTrader: (5)

Lot more accurate than say a speedometer, especially in your case with the different tires and mismatched tune. If you had the stock tire size I would say the speedo is good enough, but since that's not the case then your best bet is a GPS.

Plus you don't need to be dead accurate, you just need to get a rough MPH/RPM reading to match up to the rear end.

409CISecondGen

iTrader: (2)

If it's something like a vbox, extremely accurate. If it's an iphone app, extremely inaccurate.

thehazz12

No idea what a "vbox" is. All I have access to is my iPhone and Phillis, my Garmin.

409CISecondGen

iTrader: (2)

Vbox is a GPS data logger. That Garmin should be fairly accurate.