I think it is hilarious that my 2001 Blazer 4x4 (4500#) gets 14 mpg combined, 17 highway - runs so fine, starts with just a touch, obviously controls fuel mix, has new calipers & brakes and factory size tires properly inflated. Put the same vehicle on a 2800# trailer and pull it with my 2004 Avalanche 4x4 (6500#) and I get 14 mpg on the Interstate running 65mph with the A/C on. Just did a trip to near Detroit and back for my yearly Krown rustproofing and did the Blazer too this time. 400 miles RT at 14 mpg. It just doesn't add up! One case is a 4.3 V6 pulling 4500#. The other is a 5.3 V8 pulling 13,800# It's not wind resistance. Anyone know what the 1998-2000 5.7 V8 pickups got for fuel mileage? Is the LS engine that much more efficient?

 

On my 1999, 4dr, 4wd, 3.73:1 rear end, I'll approach 18-19 mpg on long cross state highway trips but combined I'm more like the 14 mpg.

Although only 3 years different in model year, perhaps the engine management for the Avalanche engine is just much better due to it being a new generation of software? It is amazing what the engineers have been able to do with both fuel economy and power over the last few decades.

 

Between computer settings and gear ratios I actually think there isn't much one can do to gain any in millage. After building my Morf the curb weight has dropped by close to half, millage has not changed from being a s15 Jimmy - 14 / 15 town 16 / 18 Hiway - One would think there should have been a noticeable gain .....

 

Do you think a tune would be worth it?

 

It would prob help some, depending on the cost of the tune the gain may not be enough to compinsate

 

Yeah, I too can approach the 18-19 with the AC off and slow it down to 65. Also, I can hit 21 with the Avalanche running down the interstate by itself at 75mph with the AC on. I can make many excuses for the Blazers poor fuel mileage, but what I have a hard time with is the fuel economy of the Blazer at a steady highway speed. AFAIK each system should be controlling fuel mix to optimum, which should be pretty close ratio to each other. And it doesn't take much engine management on either vehicle to do this at a steady highway speed in overdrive with the converter locked up - I imagine this type of highway driving is a best case scenario. Furthermore, both vehicles have the same rear end ratio 3.73:1 and it seems both are running just fine. The Avalanche simply has to be doing a lot more work (making way more power) per cubic inch, per amount of fuel used to do this. Avalanche seems to have a much more efficient engine design in general, even though the Blazer also has sequential MPFI. I did not realize an engine design could be that much more efficient. Especially when being compared to what amounts to the good old SBC with the front two cylinders missing.

I'm trying to remember... I think what I want to compare is the BSFC (Brake Specific Fuel Consumption) of the two engines as a measure of efficiency. I'll see what I can find and how it might be applied to our vehicles.

 

Possible explanation.

Once I had my 94 4.3 V6 taken apart I was appalled by the pistons and compression ratio it really had.
Measured out at 8.8 : 1 compression, and non existent squish. Might be a possible explanation.
I'll find out once the build comes together if the engine is knock limited and therefore running rich.

Used to see the same effects on all engines with very small venturis vs. displacement.
Running fine at very low rpm, even with decent fuel mileage but increased fuel consumption at higher rpm.
The rear end ratio might be less important than the total ratio in each gear vs. wheels and resulting rpm at the same speed.

 

My Blazer with the 4.3 V6 always got around 16 mpg at 70 mph on the highway. My Jeep with the 4.7 V8 gets 18-20 mpg at 70 mph.

 

Wow I wish I got 15+

My 04 averages like 11-13 around town or on the highway...

But I guess that's what happens with you lift it, put on oversized tires without regearing, and ruin the aerodynamics by cutting off the front lower valance for a winch and install gigantic LED bars on the roof...


But I never expected it to get great mileage. That's why I drive my old Saturn around daily. That 21 year old beater averages about 30mpg lol

 

Same reason I own a '13 Dodge Dart. 34 mpg.