I presently have a 3.42 posi in my car. She runs 3,000 RPM when I go 75, and with the high cost of gas the idea of running higher gears has been really tempting me lately. I have a spare rear with 3.07 posi........does anybody have an idea how much fuel mileage I might gain and how much performance I will lose in the 1/4 mile by running 3.07 gears? o No:
Well, changing gears from 3.42 to 3.07 will lower the 75 mph cruising engine rpm about 300 rpm to 2693 or about 10%. Maybe you'll get a 10% gain in mpg if everything else stays constant. o No:
Well Joe, Ya just answered a couple of the ?'s I JUST pm'd to you... PS: Put first on your list if you ever wanna get rid of the 3.42...:laugh:
As mentioned in the 3.42 against 3.07 thread. I did the opposite of what you're trying to do in an effort to save a little fuel mileage. I removed a 3.42 rear and installed a 3.08, thinking it would get better mileage, but it did just the opposite. I went from 13.5 mpg to 9 mpg!!! I rebuilt the 3.42, since I had it out, and then re-installed it. The 3.42 is a lot easier on the engine from a dead stop, gives better torque, and doesn't bog down when I shift into overdrive. If you're running a powerglide 2-speed, or a 3-peed tranny, the 3.42 will only help you around town, in a lot of stop and go traffic, but for cruising the interstates, and other long hauls, it's going to kill your mileage. -Josh
Maybe I'll just keep the 3.42's that I have had in there. It only sucks badly when I want to buzz down the highway at speeds above 75.......I don't like to rev her past 3 grand for extended periods of time. Oh well, the speed limit is mostly 65 around here anyhow. :laugh:
how about installing a taller tire. it will knock down the revs alittle, down the highway. there are gear ratio calculators to help u with the specifics. a 28 inch tall tire at about 70 mph will get u appr. 2700-2800 rpm. give or take.good luck to you.:beer ralph
Wo, I don't know how your gas mileage decreased. I never heard of that. I made the same change and it increased but onl about 2 MPG. Not worth it I think as I lost some low end. Going back to 3.42. As far as buying used gears the rear end rebuilder I know will not let me do it. Never use someone elses old gears in another rear end. Will not set up right he says. I will not argue with him on this.
I wouldn't be surprised to see a decrease in mileage going to a lower numerical ratio if the engine was cammed up quite a bit. I had a similar result when a choice of gears brought the rpm down so much the cam's overlap had the engine running rough at cruising speeds. All it took was another 700 rpm or so to smooth things out. For a relatively stock engine I would expect the usual result, more efficiency with lower cruising rpm. As for installing used gears, I've had good luck keeping sets matched up with a careful setup. They obviously have to be in very good shape to begin with. Devon
I would stay with the 3:42 gears, for some reason that is the best gear set for a big block Buick all around for milage and :3gears: , AL.
308 gears might be better on the highway but depenging on city driving 3.42 could be better. Where is city speed , like 30. On the 308 it might be a higher rpm in 1st while 3.42 might be low rpm in second- just an example, no real clue what gear you cruise at.
I have a 3.42 axle in my 69 Sport Wagon and I installed an overdrive. The mileage went fron 15-16 to 19-20 This gives you the best of both worlds. I also installed one in my 72 GS455 car, and the mileage went from 14 to 17. This Buick has a 3.31 axle
When I swapped the 3.42's for the 3.08, I wasn't thinking about how little I use the interstate. My truck has a beefed up SBC 350, and it hauls ass. But the mileage wasn't what I wanted, so I thought the swap would be a good thing. But I do a lot of stop-and-go traffic, so the engine strained to get going all the time, and I lost a lot of mileage. I have since swapped back, and am experiencing a whole new set of non-related issues. -Josh
