I picked up a nice 2002 2wd 4dr blazer a while back. 4.3 6 cylinder Auto with 3.42 rear gear - pretty standard. Well I am very dissapointed with the fuel mileage. I have ran chevy impalas 86-90's for years with 4.3 and 5.0 abd got 18-20 mpg - this blazer barely makes 15. did some checking on my impalas - they had @ 2.60 rear gear ratios. I would like to change my blazer to a similar gear ratio - I am not after anything but fuel economy. What would be the pros and cons of this? What would I need to change?
Thanks

 

The Impala has a very different body shape, drag coefficient, transmission, and power-to-weight ratio. I would dare say that you Impalas probably also had smaller tires than your Blazer does. On top of that, the years you mentioned wouldn't have the emissions systems found on an 02 vehicle (which, while reducing emissions, put a parasitic drain on the engine and performance aspects). If you tried running 2.60s in your Blazer, the end result would be dismal. You would lose starting torque due to the shorter rear end gears, your engine would bog down more easily - especially in your overdrive gear, as your engine and transmission were not paired up to be run with 2.60s.
If you have the 4L60E automatic in your Blazer, it has a .70:1 final drive ratio. If you have the manual, I believe it has a .79:1 final drive ratio. If your Impalas were running 2.60s for those model years, then you were probably running a direct drive transmission with a 1:1 ratio - I'll have to look this up, as I don't know much about the Impala. The advantage of taller rear end gears with an overdrive transmission is that you have better starting torque. The disadvantage is that, when you're in the overdrive gear, it creates a parasitic drain on the engine (a 1:1 ratio transmission doesn't have this, which is why you'll see some tractor-trailers running 2.64, 2.79, or 2.93 rears with a direct drive transmission, as there won't be an overdrive gear to create a parasitic drain when pulling up a hill).
People assume low RPMs automatically means good fuel mileage, but this is a fallacy. Run your RPMs too low, and you're bogging your engine down, which does no favors for your fuel mileage or engine service life. Put something like 2.60s paired up with a .79 - .70 OD transmission, and that's exactly what you'd be doing.
If you want to improve fuel mileage, do your routine maintenance - change your engine oil and air filter regularly, ensure your spark plugs aren't fouled, use mid-grade fuel (our low grade here in CO is 85 octane, and it is absolute crap), make sure your tires are properly inflated and moving parts are properly lubricated, etc.
As for modifications, maybe a new ignition coil. A lot of people think simply slapping on a higher performance air filter will make a difference in fuel mileage, but, for that to happen, this is something you need to see through further - gasoline requires a pretty precise fuel/air mixture to burn ideally. If you put a high performance air intake and exhaust on without upgrading the fuel injectors, you get a mixture which is too lean, and you won't gain anything from it.