When I had my LS with the 3.4 I got over 32 on highway for extended trips so it's possible to get what the car is rated Whiz. I would check plugs & wires first to see if that helps problem. With your mods those are more performance than fuel economy also.

 

with my 05 with the 3400 first highway trip I made was from St.Louis MO to Springfield, MO and return. lots of hills interstate 44 was the route. keep my cruise on about 70 - 72 for the trip down and avg 29 mpg. car is stock. How am I getting such great highway mileage????



Mike.

 

Mike, depends on when you went. Winter you'll get better mileage usually because it's easier for the engine to keep at the correcttemperature, and if you're not running the A/C that helps too. Having the cruise on helpsbecause the engine automatically uses the minimum amount of fuel to keep the speed constant. It also locks up the torque converter automatically, which will reduce mileage when it's not locked up.

My 3900 has the VVT, so I cansee 29 mpg if I'm going 65 mph or below.I'm going to test the mileage sometime to see if10% ethanol blends really effect the mileage that much. I don't know if you were running the car with ethanol at all, but it is supposed toreduce mileage slightly. It's when I get into the upper RPM range that I really start hurting, because the VVT's second cam is more aimed at producing power, than it is at fuel mileage. When cruising at around 2100 rpm or so the VVT isn't active yet, so the engine is more economic. Thats why the 3900 is rated at 242 hp and the N/A 3800 is rated at somewhere around 210 I think. No way the 100 ml difference in engine size would add 32 hp without the VVT. (Variable Valve Timing) But, the half-litre difference from the 3400 does make the car less fuel efficient when compared at the same RPM. My car could never get the same mileage as yours even if everything elsewas identical (RPM, Load, air temp, intake, exhaust, timing,etc etc)

 

assuming your car is in tip top shape, and want 31 mpg, heres what ya do... drive ona highway, never exceed 55mph, try to drive 50-55mph whenever possible, aviod braking, stopping, idling, hard acceleration, drive smooth/ dont jerk the wheel, keep the windows up and the a/c off.

basicly what gm says to do fora mileage test, i've tested a astro rated for 18mpg and pulled out a 22mpg result, and a vibe rated for 34mpg for a whopping 44mpg result. and i bet i could get more than 32 out of my monte, but NO ONE DRIVES LIKE THIS... so yer outta luck.

 

Naah, MPG is over-rated. I saw 25 overall tank to tank with my 03 LS with a 3400, (mixed highway and city). I now see 21-22 mpg tank to tank with the 06 3LT with the 3900. I would GLADLY trade another 3 or 4 MPG for another 60 HP... Wouldn't you?

(3400 = 180 HP, 3900 = 242 HP)

 

I got 25.9 mpg on the 250 mile trip back to college last weekend. Wasn't too bad considering I was going 80 mph into a 25 mph head wind. [sm=headbang.gif]

 

hey flynhghR I hate to tell you but the 3.9 only has 1 CAM not 2. There is a camshaft phaser in the engine that changes the orientation of the 1 cam.....but there is definately not 2 cams in the 3.9 motor. Properly tuned a 3.4 can get over 30 on the hghwy and it doesn't have to be in winter either. I ran mine at 80 with the A/C running and pulled almost 32 out of it in the summer with 90 degree heat.

 

I get around 430 miles to the tank on the open road with my 3.9L
I normally curise her at 65 mile per hour. That roughly around 24 miles to the gallon. I dont think that is bad mileage.

 

yesterday driving back and forth to toronto2 times, did 288 miles in total and stillover a half a tank of gas

 

35.9 mpg ona 150 mile round trip yesterday! [sm=happy046.gif]