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AFM affected by leveling kit or larger tires?

Brent Lake

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Brent Lake Enthusiast

Brent Lake

Posted
Posted

Hi all,

I just bought a new 2015 GMC Sierra CC Z71 with the 5.3.

My question is concerning the AFM system and when it should be working. I put a leveling kit on it and replaced my 265/70/17's (31.5 inches tall) with 285/70/17's (32.7 inches tall)

I noticed that my AFM doesn't seem to drop from V8 to V4 at anything over 60 miles an hour. Is that normal, or did my minor mods affect the AFM? I didn'T ever drive it at highway speeds before making changes so I don't know what it did before. Thanks in advance for any help!

HondaHawkGT Senior Enthusiast

HondaHawkGT

Posted
Posted

What's your axle ratio? I have 3.08's and 285/70R17's. It makes AFM completely useless, it was 99% useless when my truck was stock. It doesn't save fuel unless you live in an area as flat as the Bonneville salt flats... or South Dakota.

 

You may need to regear if AFM isn't kicking in and you want it to for some bizarre reason.

Mack207 Enthusiast

Mack207

Posted
Posted

The leveling kit would have a very marginal effect on mpg and AFM. The culprit here is the larger and heavier tire (I'm assuming heavier). By going larger than stock, you're effectively raising the final drive gear ratio (lower number numerically) which makes the engine work harder to get to and stay at speed. If the engine is working above the AFM load range, AFM won't kick in. As HondaHawkGT said above, a re-gear is the only way to get a correct ratio back, but it's fairly expensive. Other options are to go back to stock size, or a tuner may help a little.

 

Remember that because your tires are larger than stock, your speedometer will also be off. Use a gps to see what your actual speed is.

 

 

Sent from my iPhone using Tapatalk

tnchevy Senior Enthusiast

tnchevy

Posted
Posted

My truck with stock tires and the air dam removed stays in V4 alot, especially on flatter terrain and of course going down hill. The larger tires hurts but also how aggressive your tread is also plays a part in this.

BigBadSierra Senior Enthusiast

BigBadSierra

Posted
Posted

My Truck goes in to V4 a little less with a stock size tire but a way more aggressive tread.

 

The first week it didn't even go into V4 mode but the truck seemed to be able to compensate for it an is using V4 again just a little less.

 

It's hard to tell how much my fuel millage changes since these engine get horrible millage once it gets below freezing.

soltys Enthusiast

soltys

Posted
Posted

My Truck goes in to V4 a little less with a stock size tire but a way more aggressive tread.

 

The first week it didn't even go into V4 mode but the truck seemed to be able to compensate for it an is using V4 again just a little less.

 

It's hard to tell how much my fuel millage changes since these engine get horrible millage once it gets below freezing.

And even that's a understatement. I've considered taking my truck to AA because I'm getting concerned about it's "social winter drinking" habit.

BigBadSierra Senior Enthusiast

BigBadSierra

Posted
Posted

GM needs Grill Shutters on these. I bet if I taped off part of the front grill the millage would improve. Not sure if anyone ever tried this.

Brent Lake Enthusiast

Brent Lake

Posted
  • Author
Posted

What's your axle ratio? I have 3.08's and 285/70R17's. It makes AFM completely useless, it was 99% useless when my truck was stock. It doesn't save fuel unless you live in an area as flat as the Bonneville salt flats... or South Dakota.

 

You may need to regear if AFM isn't kicking in and you want it to for some bizarre reason.

I have 3.42 gears, new tires are BFG A/T KO 2's. After reading about numerous AFM nightmares, maybe it NOT working above 60 is a good thing..
99-ls1-ss Enthusiast

99-ls1-ss

Posted
Posted

Considering the tire height difference of 1.2" with the 3.42 gear ratio, it should only be dropping your by 50 RPMs at 70 MPH. I would be surprised if that small of a change had a dramatic affect on the AFM.

royrjs Enthusiast

royrjs

Posted
Posted

The leveling kit would have a very marginal effect on mpg and AFM. The culprit here is the larger and heavier tire (I'm assuming heavier). By going larger than stock, you're effectively raising the final drive gear ratio (lower number numerically) which makes the engine work harder to get to and stay at speed. If the engine is working above the AFM load range, AFM won't kick in. As HondaHawkGT said above, a re-gear is the only way to get a correct ratio back, but it's fairly expensive. Other options are to go back to stock size, or a tuner may help a little.

 

Remember that because your tires are larger than stock, your speedometer will also be off. Use a gps to see what your actual speed is.

 

 

Sent from my iPhone using Tapatalk

X2 on this ^^

Gmcant33 Enthusiast

Gmcant33

Posted
Posted

Please share ur

AFM is a gimmick anyhow. The amount of fuel saved, if any, is a pittance.

 

Please share ur evidence to support this claim

Please share ur

AFM is a gimmick anyhow. The amount of fuel saved, if any, is a pittance.

 

Please share ur evidence to support this claim

fondupot Senior Enthusiast

fondupot

Posted
Posted

Please share ur

 

Please share ur evidence to support this claim

Please share ur

 

Please share ur evidence to support this claim

ive driven the truck several thousand miles with afm on and with afm off and there is no substantial change in gas mileage.

 

 

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DevOps Enthusiast

DevOps

Posted
Posted

ive driven the truck several thousand miles with afm on and with afm off and there is no substantial change in gas mileage.

 

 

Sent from my iPhone using Tapatalk

mechanically you are consuming less fuel if v4 is sustained. YRMV.

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