Rear End

[digithead]

So in a effort to better understand what I got here in my 73 Convertible, today I jacked up rear end. Bear in mind the PO did not do all the work so he didn't know all the specs of what was done to this car. I marked thedriveshaft with white paint line and turned tires to measure ratios. My Marti report says it was built with a 2.75 standard rear end. I am somewhere in high 3's like 3.75 or 3.83 I can quite get pit erfect. But no where near 2.75. Since both wheels turn forward, did the restorer of car put in posi rear end and change gears? I assume so. It has traction bars so I assume he thought he was building a burn out machine. I am not wild about the bars and would like to take them off as they make a pretty good clunk when you get on it or when you hit a good sized bump. What do you think? Thanks all.

 

[jbojo]

So in a effort to better understand what I got here in my 73 Convertible, today I jacked up rear end. Bear in mind the PO did not do all the work so he didn't know all the specs of what was done to this car. I marked thedriveshaft with white paint line and turned tires to measure ratios. My Marti report says it was built with a 2.75 standard rear end. I am somewhere in high 3's like 3.75 or 3.83 I can quite get pit erfect. But no where near 2.75. Since both wheels turn forward, did the restorer of car put in posi rear end and change gears? I assume so. It has traction bars so I assume he thought he was building a burn out machine. I am not wild about the bars and would like to take them off as they make a pretty good clunk when you get on it or when you hit a good sized bump. What do you think? Thanks all.

Sound like you have the posi 3.73 gears on it. The traction bars are easily removed if you don't like them and they are a personal preference. Yeah I agree, sounds like the PO was building a burnout machine. When on the freeway you are also probably turning 3000 rpm at 65 or so. If you know the tire diameter you can calculate what the motor is turning with a 3.73 rear.

 

[digithead]

THanks a lot. Yes RPM is high like that, that was my first clue LOL. Feels like it needs another gear.

Not sure I love the 373 I mean 1st is worthless unless I want to burn out. 4th is fine at 50 or so and you can lunch it in 4th and pass like crazy but for a Sunday driver cruiser seems like lower/original rear end would be better.

 

[Jeff73Mach1]

Wit the 2.75 rear end your car will be far less fun. about a 3.25 seems to be the best compromise for decent mileage, comfortable cruising and still maintaining some performance characteristics.

 

[barnett468]

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I agree with the 3.25 gear ratio for a non overdrive trans or a car you want to cruise at 75 mph for long periods of time, however, if you want to keep some quickness of the line and decent tire burning capabilities, i would consider 3.43 or 3.50.

you really need to determine whether you have 3.73 or 3.90 etc before you decide what you want to do.

changing the gears by only .25 ie 3.73 to 3/50 or3.50 to 3.25 will only make a minor change . you need to change them by at least .45 or so to make a significant change.

also, if the engine is a high revver, it will be a pig off the line if you make to big of a change in the gears.

here's an rpm vs mph calculator

http://www.wallaceracing.com/gear-speed.php