I am going to be rebuilding my rear end this week and am looking for advise on the ring and pinion ratio.
Currently I am running 3.42 gears, t5 trans, 26 in tires, with a 5.7L engine.
I don't like how I run through the first 4 gears so quickly, in 4th 40 mph at 2000 rpm.
On the other hand I don't like how low the highway 5th gear rpms are, 70 mph is 1750 rpm.
I'm considering 3.23 gears or 3.42 gears. The car is just a daily driver that with a built racing engine. It has a high rpm power band for racing purposes. I'm currently getting 12 mpg city, 28 mpg highway.
Just sharing my build setup and asking for recommendations if I should change the ratio to 3.23.

 

Sounds to me like you need a different 5th gear, and the 3.42 rear would then be just fine.

Find yourself a .73 5th from out of one of the high-perf V8 T-5s (LB9 or L69), to replace the .63 LG4/L03 grocery-cart one you have now.

 

I believe the 5th gear is 0.50, The trans only has 20000 miles on it so no need to change it

 

Fifth in a T5 is either .63 or .73. Sixth gear on a T56 is .5.

 

Sorry, I just redid the math, the 5th gear ratio for this t5 is about 0.56

 

Sorry, just physically look at the gears themselves, the ratios are .63 and .73. For yours the same as everybody else's.

 

I'm not trying to sound rude or argue. It's 0.56 here's the link to the 5th gear
http://m.ebay.com/itm/WORLD-CLASS-0-...-/151019821128

I'm starting to lean towards getting 3.23 gears

 

Close ratio T5 has a .73 5th gear, otherwise the 5th gear is .63.

 

That's for original cars. I specifically remember rebuilding mine with a 0.56 OD

 

so when you changed to the different 5th gear ratio, did you take the rear end ratio in to account ?

 

Your 'racing engine' isn't gonna like the 3.23s as much as 3.42s. Hell, I'd go with 3.73s.

 

If you have MS Excel this spreadsheet that I made will let you look at rear axle, tire, and transmission calculations to compare RPM at a given speed.

http://www.12bolt.com/gearing_calculator


If you have an engine with torque band shifted higher the RPM range than OE you will want a lower axle gear (numerically higher - at least the 3.42 maybe 3.73) for the engine to be in its happy place on the highway. Going to the 3.23 will make a very marginal improvement to the first 4 gears (you'll hardly notice) and be a big disappointment on the highway if it revs too low now. JMO

 

I decided on 3.23 since I want more top end out of the first 4 gears. 5th gear is pretty much useless for accelerating but it's a great highway OD. Basically 4th gear will be a little more useful for cruising between 40 and 50 mph.
5th gear basically gets me about 30 mpg at 75 mph with a carbureted race engine 350

 

Your car, up to you.

The above numbers don't add up. Are you sure you don't already have 3.73s??

 

100percent sure I have 12:41 ring pinion

 

Hmm, perhaps your tach or speedo or both are out a bit? 4th is 1:1, with 26" tires at 40mph should equal about 1750-1775RPM

Edit - 235 60 R15s?

 

Speedo is correct. I used the gauge as well as gps, not sure about the tach

 

Not trying to say you need to change your plans, just wanted to be sure you had the right data to decide with.

I spent a lot of time trying to decide what the perfect combo of axle and 1st/OD was for another application. It always seems to be a bit of a compromise with a 5 speed, especially in a heavier car (I was looking at options for a Chevelle, similar size tire also with 3.42s) so I relate to the challenges. A big part of it is if you like shifting a lot or not. If not, going to the 3.23 or further is the right idea. I hope the OD still works well for you.