So. 96 Jimmy SLT 4x4 . Is there a rear end that would swap out to give a bit more fuel economy?

 

I would imagine so. Of course if you have 4wd, you'd have to swap the front too so that the gear ratios matched.

If you didn't already have the locking rear diff, if you do the swap, then would be a good time to upgrade.

What gear ratio are you running now? You can find the code for that in the list of RPO codes.

 

I honestly have no idea other than its the OG rear end. I'm just trying to push as much MPG out of it as possible. Stock everything but brand new.

 

So as for axle ratios available in 99, which I think were the same across all years, at least for the second gens like ours, you had the following ratios with the corresponding RPO codes:
RPO Ratio
GU4 3.08:1
GU6 3.42:1
GT4 3.73:1

According to the 1999 Blazer specification buyer's guide I have, the GT4 was not available for the 2wd models in 99.

This information should be on a sticker on the inside of the glove box door. If it is lost or damaged a GM dealership should be able to print it out for you. There are also VIN decoder sites online that can print out these codes and list what the option that goes with them is. Otherwise if you only have the list of codes, you have to manually look them up in a table.

If what you want is maximum mileage, you want a GU4 option. Again, if you have a 4x4, you'll need both the front and back axles.

I'm only guessing here, but I would think that the GU4 option was likely rare on the 4x4s. I have the 3.73 axle. I'm fine with the lower mileage as I purchased it for towing and going into the woods.

Another way to increase mileage is to drop weight. This is why even with the same axle ratio, the 2wd Blazers got lot better mileage due to not having the extra weight from the front axle and transfer case to haul around.

 

i have the 3.73 with the 4x4 its a 96

 

Remember that you already have Overdrive and a Torque Converter Clutch that keep the RPMs way down. Changing the rear end ratio will not help much if any with mileage.

Things that actually will help:
1. Make sure tires are correct size and aired up to the proper PSI (see label on the door). No big tires.
2. Jack up vehicle - all 4 wheels. Make sure each tire will rotate freely, without brakes dragging. Hit brakes and make sure they release right away.
3. Make sure torque converter clutch is working (drive 45+ steady with right foot - then tap brake with left while holding same pressure on accelerator with the right. Do RPMs go up for a few seconds and then back down? If so, the TCC is working)
4. Make sure front axle is unlocking when in 2WD. Jack up just the RF wheel off the ground - can you easily turn it without spinning the front driveshaft?
5. Slow the F*** down. Easy does it. You are pushing a brick through the air. Also, no need to beat the civic in the next lane at the stoplight.
6. Avoid warming engine up to clear frost and ice. Use the scraper.

In a 1996 4WD Blazer/Jimmy, that should get you about 14 city and 17 highway mpg if the engine is running right. They are definitely no economy car. I have a super heavy 2004 Chevy Avalanche with 5.3 V8 (think Suburban) that gets better mileage than my 2001 Blazer.

You can always replace the catalytic converter or the S1B1 and S1B2 O2 sensors if you pass 1-6, still can't get there, and want to shoot the parts cannon with some fairly expensive parts. FYI my O2 sensors and converter are still original. But I see no need to replace them as my fuel trims are perfectly normal.

Hear is the EPA estimate for a BRAND NEW 1996 Jimmy 4x4. Yours is no longer new.
https://www.fueleconomy.gov/feg/Find...n=sbs&id=13221

 

Actually this 1996 has all brand new parts with less than 10k
https://www.facebook.com/media/set/?vanity&set=a.122155482176196908But if you didn't know that that's okay. There is always something that can be done to tweak MPG but I do understand that the Vortec is a difficult beast.