What kind of transmission?

[mcmullinauctions]

So I know I have asked questions about the t5 in the past but...What kind of 4 or 5 speed manual is the easiest (no hassle) swap for gas mileage with a 3:73 or 3:08 rear? I will have a 350 up front but would like to look to stay with a stick. I don't plan on doing very much if any hauling or towing at all. I want to try to get 18-20 mpg somehow, that way I can replace me 05 GMC with it as a daily driver. Nothing would be better than to see your insurance agent drive up to your house in a 63' chevy.

[markeb01]

You’re question is more complex than it seems. If your truck is a 63 and already has a stick, the easiest swap would probably be a Borg Warner T10 or Muncie M21 4 speed transmission. You’d just need to replace the existing transmission. If you currently have the 3 on the tree, the driveshaft should fit without modification, although you may need to change the slip yoke. You may also need a longer speedo cable. If you install a T5, the change would be just about as simple, except you may need to alter the driveshaft and possibly move the trans crossmember, and it might need an electric speedo converter depending on the transmission drive. One advantage of the T5 is the integral shifter. Either 4 speed will require a side shifter at an additional cost of several hundred dollars, unless you have a used one.

Gas mileage is the result of a bunch of factors in addition to the rear axle ratio. Vehicle weight, driving style, how far is a typical trip, at what speed, etc. My truck weighs about 4200 pounds, runs a 2.64 low gear Super T10 with a 4.09 rear axle. Normally this combination would result in something like 10 mpg. I regularly get 16-18 mpg, because almost all of my driving is under light load and around town. I rarely take my truck on the freeway, and when I do the mileage really suffers.

To be really happy with the finished truck, it’s important to calculate launch and final drive ratios – essentially 1st gear x axle ratio (launch), and high gear x axle ratio (final). Mine has a launch ratio of 10.8 (2.64 x 4.09) which is fun. A T5 with a 3.76 1st gear and a 3.08 axle would take off like a bullet and run out faster in 1st gear with a launch ratio of 11.6. A 3.76 1st with a 3.73 rear axle equals a launch ratio of 14. This is great for pulling, but also gets close to where you could start out in 2nd gear if you feel like it.

Vehicle weight and driving style are equally important in high gear. A 3.73 rear axle behind a .72 overdrive 5th gear will effectively become a 2.68 – great for high speeds if your engine makes good torque in the rpm range. With a 3.08 rear axle and .72 overdrive, the final ratio drops to 2.21. The problem with having such a high ratio (low numerically) is that you never get to use 5th gear unless you’re going 100 mph. If used at too low a speed, it can also bog the engine since it’s running below the torque curve, and actually decrease mileage. Depending on the reverse gear ratio in the transmission, it can also be a problem backing up. I once installed a 2.70 rear axle in my 54 Chevy with a close ratio 4 speed, and I couldn’t back uphill out of a parking spot without help from guys pushing the car.

None of this is all that complicated, it just takes a bit of figuring to determine how it’s going to be used and which combination is the best value for your application.