rear end ratio question

mjmacqua · #1 (**permalink**) 07-28-2008, 07:47 AM

Guys,

This may sound like a simple question, but I need some help making sure I get this right.

Jag Rear

I am trying to determine what rear end gear I have (without opening it up and counting if possible). Currently the tranny (and engine) is out of the car. Isn't there a way of turing the rear wheel and counting rotations.

Reason I am trying to figure this out is, I have a tremec 5 speed and at 65 mph I am around 3K (in 5th), I am hoping to change that...

Thanks Mike

trularin · #2 (**permalink**) 07-28-2008, 07:49 AM

Verify your tach before you do anything else.

Just a suggestion.

mjmacqua · #3 (**permalink**) 07-28-2008, 07:52 AM

Simple answer is Tach sounds right

G_Edmonds · #4 (**permalink**) 07-28-2008, 08:01 AM

Turn wheel one time and count revolutions of the driveshaft(yoke) will give you approximate rato.

-Greg

native1 · #5 (**permalink**) 07-28-2008, 08:06 AM

Jack up rear end. Put tranny in N. Rotate tires 1 complete turn and count how many times the driveshaft rotates. Will not be exact but will get you close ex. 3.1 3.25 3.5 3.75 I have 3.64 gears but would prefer 3.73s. Hope this helps. Be careful

Ron61 · #6 (**permalink**) 07-28-2008, 08:12 AM

G_ Edmonds is correct. If you know the range of gears available for the rear end you have, then you can figure out which you have that way. I used to set mine so one ear of the U-Joint was on top and mark it with chalk, then mark the rotor and where I started. I would turn the rotor one full turn and count the times the drive shaft turned. I had 3:27 gears for example and I would get about 3 1/4 turns of the drive shaft for one turn of the rotor. In my other car I have 3:90s and I get just short of 4 turns of the drive shaft.

However with that tach reading I would also consider what the others said about checking it. I ran a Tremec 5 speed and at 70 with my 3:27s I was taching 1900 RPM. I also checked mine with the CHP and their radar at 50, 60, 70, and 80 MPH.

Ron

mjmacqua · #7 (**permalink**) 07-28-2008, 08:19 AM

I really think the tach is right, but will check it.

Thanks for everyone's input, I thought that was the way to do it, but really wanted to make sure I was right. I am in the middle of a rebuild on the engine so everything is out of car and I am making changes now. I am hopeful to get it back shortly and I want everything to be right this time. Man that is a good one.....

Cobra 29 · #8 (**permalink**) 08-04-2008, 07:02 PM

My Jag rear end is stamped with 43 / 13. 43/13 is equal to my 3.31 ratio.

Excaliber · #9 (**permalink**) 08-04-2008, 08:48 PM

3:31 is VERY common rear ratio for the Jaguar. 3000 at 65 mph in OVER DRIVE sure aint right. It should be like Ron61 says above, around 17-1900. 3000 would be about right for a 3:31 rear ratio with 1 to 1 (direct drive), like 4th gear. Some five speeds are not over drive, 5th is actually 1 to 1, just like 4th in a top loader would be. Something fishy here...

CHANMADD · #10 (**permalink**) 08-04-2008, 10:03 PM

What diameter Rear Tires do you have and then we can work it out exactly? and/or what is the tire size?

xlr8or · #11 (**permalink**) 08-04-2008, 10:53 PM

I'm thinkin his speedo is wrong. Which Tremec do you have? The 3550 uses a .68 and the TKO 500 and 600 use a .64 with an option for the .82 on the 600.

Excaliber · #12 (**permalink**) 08-05-2008, 12:17 AM

.82 OD is rather uncommon, I'd guess it's in the .6 range. One could assume a 25 to 26" tire size. Even with .8 OD 3000 at 65 is still to high for a 3:31 ratio.

3000 rpm at 65 mph with a 26" tire and a .8 over drive would be a 4.5 rear gear ratio. 4.5? Man THATS radical, thats a freakin' serious drag racing ratio and totally uncommon. A .6 over drive would put the rear ratio in the 5's, virtually impossible.

No, something fishy here, the numbers just don't make sense. Tach, speedo, trans type, something aint right.

wtm442 · #13 (**permalink**) 08-05-2008, 02:33 AM

I've got a an 0.68 for 5th gear and a 3:73 ratio rear end. With 15 inch tires and a very accurate speedometer and tach, and I'm doing 100 MPH (not 65) at 3000 RPM.

As many have said before me .... "something is wrong".

Here is a good calculator ...

http://www.cobranet.com/rpmmph.html

Richie18 · #14 (**permalink**) 08-05-2008, 09:32 AM

Quote:

Originally Posted by wtm442

I've got a an 0.68 for 5th gear and a 3:73 ratio rear end. With 15 inch tires and a very accurate speedometer and tach, and I'm doing 100 MPH (not 65) at 3000 RPM.

As many have said before me .... "something is wrong".

Here is a good calculator ...

http://www.cobranet.com/rpmmph.html

Maybe he is wondering why everyone is driving so slow!

Excaliber · #15 (**permalink**) 08-05-2008, 10:30 AM

Maybe it's not a Tremec with an over drive? Maybe it's a 1 to 1 final ratio in what ever trans were talking about? If so, THEN the numbers make sense, or if the numbers are actually being calculated in 4th gear (1 to 1) instead of 5th (possible OD).

Ron61 · #16 (**permalink**) 08-05-2008, 12:20 PM

I have a 69 NASCAR type Cobra and it has 3:91 gears in the rear end with a 4 speed. With 25" tires on the back I tack just over 3100 at 60 MPH.

If you can, get one of the little ISKY Ratio Computers. They are just a round cardboard disk with smaller wheels on both sides that will let you set any combination of stuff and read the other. And they are amazingly accurate. I think when I was a kid everyone had them. Once you get your readings in 4th if that is your straight through 1 to 1 gear, then it is easy to use one and determine the RPM, Speed, and gear ratio in 5th.

Ron

Dwight · #17 (**permalink**) 08-05-2008, 04:14 PM

buy or borrow a GPS and check it. Your tach and speedo could both be off. Some of our guys had their speedo off by 14 mph. Best way to check is with a GPS.

Dwight