Transmission and rear end • The Dodge Challenger Message Board

by **challengerman** » 16 Apr 2004 1:19

How do I know what transmission I have and the gears that I am running? People ask me all of the time, but I don't know what to say. I have a 71 with a 318 and automatic trans. I know this probably makes me look stupid, but somebody please help.

by **TX70Challenger** » 16 Apr 2004 3:18

No such thing as a dumb question! Only dumb answers!

Take a look under at the pan of the transmission, and then take a look at this http://challenger.mpoli.fi/phpbb/viewtopic.php?t=4682. Side by side the 727 is bigger than the 904. As for the gears, I think there is a method that involves counting driveshaft rotations per tire rotation...don't remember all the details. I'm sure some one will add to this shortly :wink:

.

by **moparmaniac (owner4)** » 16 Apr 2004 4:38

If the ratio tag is missing from one of the carrier nuts, then you need two people one to count the rototion of the drive shaft and one to watch the rotation of the tire. Jack the rear end up so tires are not touching the ground and secure the housing with jack stands. Make a mark on the tire (Pass side) and one on the floor. put the car in nuetral and if I remember right, with one 360 degree rotation of the drive shaft, you count how many times the the marks meet on the tire/ floor. If the tire doesn't make a full revolution, you can break it down in quarters. such as 2.76 is about 2 and 3/4 revolutions of the tire and 3.23 is about 3 and 1/4 revolutions and so on.

by jr » 16 Apr 2004 4:58

Just rotate it ten times and divide result by 10. That way you don't need to guess how much last one was over full turn. I don't see any reason why you couldn't do it alone (that's what I did).

by **dave-r** » 16 Apr 2004 7:57

Only he will find that the rear wheels will rotate in opposite directions because his car will most likely not have a limited slip diff (sure-grip). So If the wheels go in opposite directions he will have to double the result!

by **Doug444UT** » 16 Apr 2004 19:38

Or you could just take a ride, remember the rpms at a given speed (say 45 mph) in drive, and then go to one of Dave R's handy spreadsheets he's created to back-calculate your gears.

by **dave-r** » 16 Apr 2004 20:08

Yeah but he will need an accurate tach for that Doug and we all know how far off the factory tach is (where fitted)... :roll:

by **redelliott** » 17 Apr 2004 5:01

dave what do you mean by double the results with an open rear end ? could you please give an example. thanks i tried counting the drive shaft and had 3.23 but did it by rpm at 60 mph and had 4.11 so i'm out somewhere ichecked my speed with another vehicle and i think the tack is good but i never doubled up when counting the drive shaft. thanks

by **dave-r** » 17 Apr 2004 7:25

If you have BOTH wheels off the ground and a LSD (sure grip) BOTH wheels will turn in the same direction when you turn one. You can then make a direct count of the number of times you have to turn the wheel to get one EXACT rotation of the driveshaft.

On an open type rear end (no sure grip) then when you turn one wheel the other will rotate by the same amount in the opposite direction. So you will only get half the wheel rotation per shaft rotation as you did before.
So you need to double the number of times you turned one wheel to get one rotation of the drive shaft.

Another way is to just leave one wheel on the ground so it does not turn. That way you should get the right result without having to double it. The problem with that is that you should not do that if you have a sure grip.

Did you check the tach in the car at every rpm against a good digital meter? Usually the factory tach is OK at low rpm but then gets increasingly inaccurate (usually reading too high) as rpms rise.

by **72 Challenger (Hans)** » 18 Apr 2004 11:12

Don't cound on the rpm @ mph to translate this into an gear ratio. If I run about 3500/3600rpm I drive about 60/65mph in real but the speedo points at about 75/80mph I remember right... This is caused by the little thing in the tranny (speedo ....) which is still original for a 3.23 ratio with stock tires (what height?) while I'm running 4.30's with 28" tires. Makes kind of a difference!

Does this makes sense...

by **dave-r** » 18 Apr 2004 15:48

Oh yeah. Thanks Hans. Good point. Just a change in wheel or tyre size will throw the speedo out let alone a gear swap.