lgoodson@pacbell.net Posted February 17, 2010 Share Posted February 17, 2010 I have spent months looking for a clsd and I have fianally found and bought one but now I am wondering if the ratio is the best one for a T56. Right now, with a 3:36 I believe, the RPM's at 70MPH is 2000 and I still don't get very good milage. With a 3:70 I don't know. What does everyone think about ratios and what are you LT1 and LS guys running for street use not drag strip? Larry Quote Link to comment Share on other sites More sharing options...
Sparks280zt Posted February 17, 2010 Share Posted February 17, 2010 I don't think your rear end ration is the reason for your poor fuel mileage. Quote Link to comment Share on other sites More sharing options...
deja Posted February 17, 2010 Share Posted February 17, 2010 I'm running a 3:70 with 25.5" tires and a 4L60E auto which only has a .7 overdrive not the double overdrive your T56 does. I see no problem. Going from my old 3:54 to the 3:70 was not really noticable. Quote Link to comment Share on other sites More sharing options...
lgoodson@pacbell.net Posted February 17, 2010 Author Share Posted February 17, 2010 Yea, I've got to find a good performance tuner for this car and see what is going on in the fuel system. I am still dialing in the speedo. I have the Cable-X box converter. Until I get the speedo right, I don't know what actual mileage I am getting. Irt's good to hear that the diff ratio shouldn't make a big problem for me. Now if I can solve the CV bottoming out problem. I might be ready to put the lsd in. Quote Link to comment Share on other sites More sharing options...
Sparks280zt Posted February 17, 2010 Share Posted February 17, 2010 Yea, I've got to find a good performance tuner for this car and see what is going on in the fuel system. I am still dialing in the speedo. I have the Cable-X box converter. Until I get the speedo right, I don't know what actual mileage I am getting. Irt's good to hear that the diff ratio shouldn't make a big problem for me. Now if I can solve the CV bottoming out problem. I might be ready to put the lsd in. You can use a GPS to accurate calculate fuel mileage and speed. Quote Link to comment Share on other sites More sharing options...
Atilla the Fun Posted February 17, 2010 Share Posted February 17, 2010 Hey, ignore me being new here, I'm experienced with this. But where to start? I guess with T56s. First, which version do you have, or do you even know? The T56 has been built with a bunch of different gear sets. For sixth gear alone, the choices include 0.50:1, 0.56:1, and 0.62:1. Next, assuming a 25" tire, such as a 255/40R17, and your 3.70:1 rear gear, your rpm at 70 mph should be 1741, 1949, or 2158, respectively. and even with 2160 at 70, you should still be seeing 24 mpg. Whether or not less rpm gives more mpg often depends on how wild your camshaft is. Quote Link to comment Share on other sites More sharing options...
lgoodson@pacbell.net Posted February 18, 2010 Author Share Posted February 18, 2010 1993 Firbird T56. I am told that the 94 would be better. I would love 24MPG Quote Link to comment Share on other sites More sharing options...
bearcat Posted February 18, 2010 Share Posted February 18, 2010 A 3.70 gives nice acceleration and is usually the perfect blend of street and performance. Limits your top end though. Quote Link to comment Share on other sites More sharing options...
Atilla the Fun Posted February 18, 2010 Share Posted February 18, 2010 For '93, there were 2 different versions offered. The T56 had standard ratios of 3.36/2.07/1.35/1.00/0.80/0.62 with the standard 2.73:1 axle, or with the optional 3.23:1 ratio that changed to 2.97/2.07/1.43/1.00/0.80/0.62. Please report back on your rear tire size, and your cam specs. Oh, the easiest way to find out which version you have is to do 5000 rpm in first gear, hold that speed steady regardless of what your speedometer says,, but hold that indicated road speed, then shift to second gear. If your rpm drops to 3100, you have the standard ratios. If 3500, then you have the optional ratios. I can give you more info when you report back with your cam specs and rear tire size. Quote Link to comment Share on other sites More sharing options...
lgoodson@pacbell.net Posted February 19, 2010 Author Share Posted February 19, 2010 Tire size 25.6, Cam specs "stock" for a 93 Trans Am Tomorrow I'll take the car out and try that 5000 rpm trick. Thanks Quote Link to comment Share on other sites More sharing options...
Atilla the Fun Posted February 19, 2010 Share Posted February 19, 2010 Assuming that 25.6" is when the tires were new and not mounted to the car, you should be doing 1957 rpm at 65 mph, and 2108 rpm at 70 mph. I had a '95 Z28 convertible, with the stock cam, and it regularly returned 23 mpg at 80 mph turning more rpm than you're geared for. Mine was an automatic, with 3.73:1 gears and 255/45R17 tires. You should be able to kiss 25 mpg any time you take it easy. Also, assuming you have the 2.97:1 first, you're geared just about ferfectly for a quarter-mile dragstrip. You do have enough gear to use the ZZ4 cam without taking a noticeable hit to mileage. Some years back, Hypertech did some testing, and found that the LT1s run best when shifted at 6200 rpm, stick or automatic. The stock rev limiter won't let you. But with the Hypertech programming in my Z, and running it that high for over a year, on a car that already had over 120,000 miles on it, I can ell you I had NO problems. I mention that because GM specifies you should not exceed 5800 rpm with the ZZ4 cam, due to it's Inverted Radius lobes. But I found a noticeable bit more mid-range torque when I tried the ZZ4 cam. If you prefer RPM over torque, you might like the HOT cam. But it's like an on-off switch, I suppose like a Honda VTEC. You can tell when it gets on the cam. And it will hit your gas mileage, around 2-3 mpg. Quote Link to comment Share on other sites More sharing options...
Nealio240z Posted March 9, 2010 Share Posted March 9, 2010 I wen from 3.36 gears to 3.90 and it was great! multiplied my torque like crazy Quote Link to comment Share on other sites More sharing options...
janaka Posted March 10, 2010 Share Posted March 10, 2010 Its funny I was contemplating doing down to 3.7's from my 3.90's when i blew my spider gears due to having a hard time launching with the power. I didn't cave though, insta-torque is fun on the streets Quote Link to comment Share on other sites More sharing options...
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