Best drivetrain option for fuel economy

[Jimmy87T]

Hi, guys. Here's what I'm working on...a 1979 Buick Century wagon. I've gota NA 3.8 Buick V6, a 1985 Monte CQ 200-4r with 1800 rpm stall, and 2 rear end options. One option is to build a 3.23 posi, the second is a low mile 2.73 posi bolt-in. Tires are 26" 215-65-15. I am looking for highway fuel economy and towing up to 1000 pounds in 3rd gear while still getting decent fuel economy.
If I install the 3.23's I'm pretty sure the 1800 rpm stall converter is fine, but if I go with the 2.73's is 1800 too high? Should I go with something around 1000 rpm? If my math is right, the engine will be doing around 1600 rpm at 65 mph in OD with the 2.73's and 1900 at 65 with the 3.23's. I'm thinking the 3.23's would make a better driver and not sacrifice too much fuel economy. Any input is appreciated.

 

[clean8485]

I would go with the 3.23, for the reasons that you mentioned.

 

[Peter]

3.23 would be my choice as well. great gear with the overdrive trans

 

[online170]

A built up chev 355 with a T56 is pretty unbeatable for fuel economy. I would wager that the BOP engines would be even better, but the T56 swap is not as friendly with them, and parts add up real quick.

My buddy's Regal gets 10 L/100 regularly on highway driving. Its his daily driver in the summer. TPI engine, with Trickflow heads, and roller cam. 4.10 rear gear. Not bad on city driving either. Its as good as most 90's early 2000's cars.

My old firebird managed a hand calculated 34 US-MPG, with a Holley 750dp carb. It had about 8.8:1 compression. TRW 11:1 pistons, 74cc heads, dual plane intake, HEI ignition, and a 3.42 rear gear. It was also much more aero dynamic than G bodies. It only did 16mpg in the city, and my 750dp crapped out (4 corner idle), so I used a 650dp, and I couldnt sustain the car in 6th gear anymore. The MPG dropped closer to 20 at that point being stuck in 5th. The 4 corner idle, and the timing being on the hairy edge of "too advanced" was cruicial for these numbers.





At the end of the day, you need to figure out how much money you plan to save by spending on parts to get fuel savings. If you own something now that gets 14mpg, but does the job, then you spend another $5000 to get that up to 25mpg either in parts or a new vehicle, you should come up with a figure for how long it will take for you to recoup your cost. It will YEARS not months or weeks.


At that point, you need to ask yourself is it really worth it? You can buy alot of fuel for $5000.

 

[Jimmy87T]

Oh, yeah, cost is definitely a factor. I'm doing this on the cheap. The 25K motor was $100, the 65K trans $170, and either rear end option is pretty cheap. The 2.73 axle is $75! Putting the 3.23's from a buddy's Z28 will mean GN 28-spline axles that I can get used. I'm also using this car as a recipient of parts from my '87 Turbo Regal...new exhaust for Regal, old exhaust for Century. I have extra T-type wheels and tires, might as well reuse them. Thanks, guys.