Never Switching To V4

Gspoff12 · February 23, 2010

Since i got my '07 sierra lifted it will not switch on active fuel management not even on the highway. This is killing my gas milage. Any ideas on getting it back?

Thanks,

George

hockeypro1411 · February 23, 2010

That's not what's killing your gas mileage. The lift is killing the mileage. When lifting a truck, expect a decrease in mileage. There's no way around it.

Edit: PICTURES!!!

rodram · February 23, 2010

you lift your truck and put bigger tires on it... then your gas mileage is going to go down...

... if you want to restore what ever gas mileage you used to have and restore your AFM operation, then return your truck to stock.

Z'06 · February 23, 2010

It's a little premature to rule that lift is causing any problems without knowing more about the OP's truck, like type/height of lift, gear ratio, tire size, type of driving (city v. highway), etc. I know of lifted Tahoes/Suburbans with bigger tires than mine that get better all-around MPG just because their gear ratios are better optimized for their tire size and type of driving.

Congratulations to the OP on losing AFM though. It is the least of your worries for improving MPG. If you haven't already, at least get a PCM tune and the minimum bolt-ons (intake/exhaust).

pm26 · February 23, 2010

So now we know a fully legal recipe for defeating the AFM: lift your truck 12 inches and put monster tires on it.

iminaquagmire · February 23, 2010

Gear ration is everything when it comes to lifted trucks. You need to change out the gears for a lower ratio (higher numerically). If you have a 6" lift and 35's, 4.56 or 4.88 gears would be optimal. AFM is not the answer. When you get the computers tuned for the gears, you can have the tuner adjust the AFM settings to make it stay on with more throttle applied, but you can't do too much because you'll overwork the engine trying to push the mass of the truck with 4 cylinders.

Gspoff12 · February 23, 2010

I appreciate the info guys what should i expect to pay to re-gear?

Crazy Canuck · February 23, 2010

Gear ration is everything when it comes to lifted trucks. You need to change out the gears for a lower ratio (higher numerically). If you have a 6" lift and 35's, 4.56 or 4.88 gears would be optimal. AFM is not the answer. When you get the computers tuned for the gears, you can have the tuner adjust the AFM settings to make it stay on with more throttle applied, but you can't do too much because you'll overwork the engine trying to push the mass of the truck with 4 cylinders.

Regardless of his gear ratio, shouldn't he still see AFM activate when conditions are right (maybe only going down a hill given the fact that he has a lift)? I have seen it come on even with 8k lbs in a tandem axle trailer behind me. (again, going down a hill).

brims · February 23, 2010

Ya, AFM does nothing, it's disabled in many of our trucks and we notice no difference in mileage, look at the lift like others say.

I used to see AFM activate on my stock truck only going down hill, and it would only stay active for a few seconds.

iminaquagmire · February 23, 2010

I just paid 260 for 4.56 gears, the oil, and an install kit. I then paid 360 to have it all installed. My truck is 2wd though. If yours is 4wd you can expect around double that. Labor is also a lot more expensive here. Price really depends on 2wd vs 4wd, how quiet you want the gears (pay more for quieter gears), whether you change out the G80 gov-loc unit for a different more robust one (whole 'nother argument there), and where in the country you are. People in the south pay so much less for aftermarket work than those in the north for instance.

As for AFM, Justin tuned mine so that it works to a higher throttle usage percentage. Stock is 6%. It stayed in V4 mode longer but still not a lot. I think it gives SOME benefit. I've yet to see an argument that I think made sense as to a reason to eliminate it. After I had the gears installed and before sending the computers out to be retuned, I could tell it stayed in V4 mode much more than before, which makes sense because the engine still thinks it has 3.42 gears and didn't have to work nearly as hard to turn the wheel. I'm interested to see what happens when I get the computers back.