3.08 vs 3.42 vs 3.73 gas mileage? - Chevy Impala SS Forum
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Old 09-28-2010, 07:51 AM
STIedge3 STIedge3 is offline
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Default 3.08 vs 3.42 vs 3.73 gas mileage?

Considering gear swap out when I have my R/E O/H for the True- Trac
Setup as follows:
1995 9C1
137k
3.08 rear with True-Trac soon to be installed
275/50/17 on OE Impala rims
Rammit, (CIA Tri-Y headers soon to be installed likely), Dynomax Ultra-Flo 2.5 catback breathing parts . . .

Figure me a daily driver on flat, straight Eastern NC roads
Probably no racing. Lots of driving 1 hour stretches-mostly highway.

Want to keep good gas mileage, and I realize there will be some difference.
I've read the sticky under drivetrain section, but the opionion seems inconclusive.
Stay with the 3.08 or go to 3.42 or 3.73?
I do like to go fast . . . . Don't mind losing some, but don't want to lose a lot of MPG.

Just trying to get back up to speed on all the tech and mods after 10 years absence, and interested in the comments from those that have been there and done that.
Thanks.
-back in the NTRSSPTR . . . .
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Old 09-28-2010, 07:59 AM
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KLASS1C KLASS1C is offline
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3:73's will be just fine but I've learned that my gas last much longer when I keep my foot out of the floor but I also rarely let my tank fall below the half mark....
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Old 09-28-2010, 08:15 AM
kiddpitt kiddpitt is offline
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On my 100 mile daily commute (all highway), I get 4 days out of a tank with my 3.08s & only 3 days with my 3.42s. That's cruising at 80 and occasional speeding up to pass and avoid congestion. I fill up before the fuel gauge dips below 1/4 tank. Around town driving, the 3.08s & 3.42s get about the same mileage.

Fuel mileage aside, I like the 3.42s better around town & the 3.08s better on the highway.
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Old 09-28-2010, 08:28 AM
sherlock9c1 sherlock9c1 is offline
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Stay with 3.08s. GM didn't pick those gears for no reason. The one-hour highway driving stretches is the key factor. 3.08s puts your car's sweet spot at the 30-80mph range, IMHO. Engine rpm, exhaust noise, occupant comfort, and gas mileage are all factors. Plus, you'll be spinning the accessories 20% faster on the highway with 3.73s which will lower their lifespan.
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Old 09-28-2010, 10:15 PM
BALLSS BALLSS is offline
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My $02 is 3:42

I went 3:08 to 3:42 then 3:73....I did a T56 swap but would have put the 3:43's back in if I kept the 4L60E

I would think the lower the gear ratio the more rpm = less gas milage

there are RPM calculators online. tire size/rear gear ratios showing RPM
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Old 09-29-2010, 10:34 AM
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Cmdr. Flash Gumby Cmdr. Flash Gumby is offline
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Default 2nd the 3:42's

Judging from your intro, anyway, the 3.42's seem like a good choice.

I love my 3.73's and feel that they would have made an *excellent* option to a 3.42 standard rear. IMHO the 3.08's are too high a ratio for an automobile this hefty unless it's going to be driven at 80+ MPH for looooong periods of time. The 3.42's are great (if less leggy, for all around use). The 3.73's are perfect if you want extra snap, drive in-town most of the time or tow frequently. 4.10's are great if you take it to the track more than once or twice a year. I'm surely a bit off in my numbers, but my general feeling is that each jump in ratio sheds about 400 lbs of weight from the car in seat-of-the-pants feel (the weight of two adults in the rear seats), about 2 MPG and 7 MPH in the cruising speed on the freeway. My 3.73's feel "right" at just under 75. I had trouble keeping it under 80 with the 3.08's when I was in O/D.

But that's just me.
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Old 09-29-2010, 11:16 AM
STIedge3 STIedge3 is offline
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Thanking you all for the replies . . .

Right now the car will get 20-21 mpg combined, but weighted a little more to the HWY . . .and thats with me with a heavy right foot . . .
I've seen right at 25mpg from my home to Charlotte with all highway at 70mph.

Congested city about 18mpg..

Did not want to get too far off 20-21 combined with weight toward HWY.

My thinking was that with the intake/exhaust and perhaps a 3.42 ratio would make up any "loss" in being able to move the car more easily from stops.
That all things considered, everything might equal out in the mpg area and the car might have a little more "snap".
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Old 09-29-2010, 01:23 PM
dogma dogma is offline
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Quote:
Originally Posted by kiddpitt View Post

Fuel mileage aside, I like the 3.42s better around town & the 3.08s better on the highway.
The gears are not the problem. The 4 speed transmission is the problem. You need 3.73 gears and a 5 speed tranny. Then you would be geared just right for city and moderate speeds but shift into 5th at 80+ if the RPMs get too high for your taste and get the best of both worlds.
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Last edited by dogma; 10-04-2010 at 10:35 AM.
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Old 09-29-2010, 01:38 PM
dogma dogma is offline
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Quote:
Originally Posted by sherlock9c1 View Post
Stay with 3.08s. GM didn't pick those gears for no reason. The one-hour highway driving stretches is the key factor. 3.08s puts your car's sweet spot at the 30-80mph range, IMHO. Engine rpm, exhaust noise, occupant comfort, and gas mileage are all factors. Plus, you'll be spinning the accessories 20% faster on the highway with 3.73s which will lower their lifespan.
That's true, 3.08 shines at 80mph and up but doesn't do so good at 60 and even worse at 50mph, where it's kind of sluggish. The RPMs are just too low at these critical speeds. I spend much more time at 50 than at 80. Given OP is on the East coast, in NC, it's obviously different from a more flat terrain than say Arizona. I would factor that in the equation and go with a lower gear.

I spent a while trying to figure the best compromise, despite my skepticism, I came to realize that 3.42 is the best for sedan and 3.73 is the best for station wagons. These are big, heavy vehicles and they could use all the torque you can get. Especially in hilly terrain and loaded. If in doubt, go with 3.73. You can always put a large tire to change the ratio numerically down.

http://www.miata.net/cgi-bin/tirescgi

Now the OP has almost 28" tires which are 3% bigger than stock 27" tires. So if you put 3.42 in, you are not going to get 3.42 but more like 3.32.

I have 3.73 gears in this wagon but I can control the gear ratio with tires. I have a winter and summer tire. If I want more of a highway gear, I can put in a 28" or a 29" tire which will really bring it down numerically into the 3.50 range. Right now I have a 26" tire, which aside from looking stupid on the wagon raises my ratio to at least 3.80. Believe me, that beast needs it.

In most daily driving, the MPG difference is less than you think. Unless you do 100% highway driving on a flat, straight surface at 80+mph. 80% of the time, IME, 3.73 is more useful than 3.08.
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1996 Buick Roadmaster Grand Sport wagon
LT1, Comp Cam, Comp Cams Pro Mag Full Rockers 1.6, custom headers, Bee Hive valve springs, LT4 valve covers, 3.73 Motive gears, Eaton Posi, 2400 stall, Front F-Body sway bar, Rear Hotchkis sway bar, Moog 5552 springs, Monroe HD shocks, 9C1 gear box, Z28 cluster, high-output alternator, 160F Thermostat, Bonnies, PCMForLess 93 Octane tune.

Last edited by dogma; 10-04-2010 at 10:39 AM.
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  #10  
Old 09-29-2010, 02:17 PM
Bigfoot2222 Bigfoot2222 is offline
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In my ERE 383 with 3.73 with cruise set at 75, I was getting 21mpg avg traveling the interstates here on the east coast.

I concur with dogma about using tire size to control gear ratio and resisting the ever present urge to smash the go pedal will help with gas mileage.

Dialing in the tune and converter lock up were also key points in achieving that mpg.

Curtis
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