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What do they have factory?
3.08, GM's typical copout response to '90s CAFE MpG tests that were anachronistic in the 70s.
... maintaining highway drivability is important to me.
Gearing for more aggressive acceleration would impact that quite poorly ...
The biggest bang for your buck mod for Fleetwoods, RoadMasters, and Caprices, including your Impala SS, has always been and always will be either:
3.42 if you want to avoid driveshaft vibes and do lots of mostly highway driving
3.42 has a proven track record of statistically irrelevant highway MpG penalties
or
3.73 if you are ready to upgrade your driveshaft and do more city / metro / urban driving
3.73 has a proven track record of improving city / metro / urban MpG about as much as it costs highway MpG:
1-3MpG, depending on the driver and the qualiy of the pcm tune

Feel free to disagree, but 1Slammed96SS knows what he's recommending.
The ISSF has 13 years of collectively repeated and documented experiences which substantiate the above.
If not for the Great Data Mismanagement of Aug 2009, it'd be 21 years instead of 13.

If you are ever lucky enough to dare to test drive a B- or D-car with 3.42, you'll see for yourself:
More assertive gearing impacts drivability quite favorably.
Otherwise GM would not have used 3.42 for the Fleetwood 7000lb tow package (V4P).
 

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So if I swap gears, would it have an impact on the life of the 4L60E?
Were you to upgrade from 3.08 to 3.42, you'd be doing what GM did for the Fleetwood V4P.
Lesser gearing would not be able to withstand towing 7000lb.
Heavier GM vehicles (trucks / suvs) with similar tow ratings have proven that the 4L60E prefers more gear.
Heck my mechanic's 94 caprice just shat the bed. He said there's plastic discs in the trans, and they broke.
... it's just got me worried to break my SS, as it'll take me a LONG time to save up for a replacement.
To make a long story a bit shorter:
Were you to take
the 3.73 vs 3.42 questionnaire
your score would decisively and handily land on 3.42.
Your 4L60E will appreciate it.

That said, there's other cheaper things to do to extend the 4L60E's life, even if you never tow 5000lb.
1. You can improve on the ATF cooler by:
a) relocating it so air can actually hit it, or adding some baffling to redirect air at it, or both
b) adding an electric fan to it
c) upgrading the cooler itself, which also makes it easier to do 'a' & 'b'

2. Provided your Impala SS never sees winter, disconnect the ATF lines connecting to the radiator
In winter, the ATF is warmed more quickly by the radiator.
Once warmed up, the radiator unnecessarily heats the ATF, especially in the summer.

3. have your pcm reprogrammed.
Among many other more enjoyable things, reprogramming the pcm lowers GM's very high fan-on thresholds.
Lowering those fan-on thresholds improves the lifespan of the entire powertrain.
 
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