Huge increase in gas mileage... What could cause this?

[SAATR]

I though long beds had the 40

Only Suburbans have the 42 gallon rear mount tank. Everybody else has side mount, so they have a place for the spare.

Sent from my XT912 using Tapatalk

 

[great white]

I though long beds had the 40

Nope, but there is a 34 as opposed to the 25...


http://www.lmctruck.com/features/cd/CDGTK.htm

 

[thunderstruck]

OP, please tell me your truck is not your DD.

 

[gmcyukondriver]

And I thought my mileage was bad. I've got a 30 gallon tank. I usually fill up when it's around 1/4 full on the gauge, and I end up putting 25 gallons in. I used to get about 325 miles out of that (13 mpg), but after the s/c, I get 250-300 (10-12 mpg). Doesn't matter how hard or gently I drive in the city, it gets 10 flat. On the highway, if I cruise at 65, I can squeeze 12 out, otherwise it's lower. I used to get about 15 highway.

Just curious, what's your RPM's when going 65?

 

[great white]

OP, please tell me your truck is not your DD.

You're in for a disappointment....

 

[1998K1500]

I was getting 8 MPG city all summer, I'll blame it on a heavy right foot

 

[FastOrange]

I get about 16mpg with my 35's and 3.42's and 18mpg with 31's

I have a light foot though, and I make sure my pressures are always the same.

 

[1998K1500]

I get about 16mpg with my 35's and 3.42's and 18mpg with 31's

I have a light foot though, and I make sure my pressures are always the same.

Is that highway driving included? Because maybe my 8 city with 32's isn't terrible then.....

 

[great white]

Is that highway driving included? Because maybe my 8 city with 32's isn't terrible then.....

No, that's pretty bad no matter how you slice it.

that's 454 territory at 350 power levels.

If that was a constant number, I would get rid of that pig as soon as I could.

Consider this:

I average 15-16 mpg city. That's right around double the MPG you're quoting.

I spend about $115-130 on a normal fill and that lasts me about 1 month.

so per year is 115x12= $1380

If I were getting half my mpg (your 8 mpg) my annual costs for fuel would be $2760.

Double my costs and I already pay too much annually for fuel as far as I'm concerned.

Yep, that 8 mpg truck would be sooooo gone. I need a truck but 8 mpg is too low, especially from a 350. I've got better things to spend 1300 bucks on than fuel....

 

[Horns]

Did you check the tire pressures and put them all the same? Did the outside temp suddenly warm up? I check my pressures about every time I fill up. Also when it gets colder the mpg will get worse since cold air is more dense so it will require more fuel. That's why it feels like it has a little more get up and go when it's cold. Other than that it's mainly driving style since you said you haven't done anything to it. I can't really think of anything else.

I guess I must be lucky compared to you, not sure about other 5.7 Vortecs. I get about 11.5 mpg around town. I have a 26 gallon tank and I'm able to go 200 miles (if all city miles) before it gets to 1/4 full.

It doesn't always get worse when it gets cold. A denser air charge doesn't always require more fuel. Sometimes it actually creates a cleaner fuller burn of the fuel. It all depends on the current conditions your engine is running at. Not to mention your computer controls the A/F ratio and should keep it the same no matter how dense the air is.

Oil viscosity has more to do with MPG in the winter than the cold air does. Not only in the in the engine, but in the differential as well.