Mileage is progressivly getting worse

kc5deb · February 10, 2007

My 2002 GMC 2500 4x4 is continuing to get worse mileage. About 2 years ago when I bought it, it got around 15mpg. After the tune, I picked up some power, and my mileage dropped to about 13.5 - 14. I also discovered that the hankook tires I put on right after I bought it were slightly oversized than the ones that were on it, and my mph was under the accurate speed by a 2-3mph.

I just recently put on a new set of Michelin LTX/AT's that apparantly are more along the lines of the appropriate size of tires, because my speedometer is now showing accurate both my radar and gps...

I checked my mileage today on the first tank of gas with the new tires, and my mileage is now to a whopping 11mpg...

The only thing I've done to this pickup is put a flowmaster 80 series on it, and put a tune for 92 octane fuel. I don't get into the throttle, I drive 75mph tops on fast days, take off slow, begin stopping way sooner than most... I drive kinda like a granny, as a matter of fact.

Should I put the original PCM back on the truck and see if my mileage increases? I was pretty disappointed thinking I was getting 13.5mpg, and extremely pissed knowing that I'm now getting 11mpg.. This ain't cheap driving 1700-2000 miles a month...

*EDIT* One other thing I've done is turn the torsion bars to give the front some lift from it's factory sag. I am thinking of letting the front back down; I've noticed that in 4x4 now, it doesn't like to turn well without some clunks; guessing the angle is now more than what factory intended it to be for 4x4. Not sure if this affects mpg, but, thought I'd throw it in.

nitro882 · February 10, 2007

I would blame the tires for the recent drop off in milage. Agressive tires kill mileage. I have had LTX at/s in th epast and they have dropped the milage 2-3mpg easy. The worst i have had was the toyo at's i had on my last truck, i dropped close to 4 mpg. im sure the tuner and other stuff isnt helping either but the tires are most liley the killer.

kc5deb · February 11, 2007

While I agree with a more agressive tire killing your mileage, the LTX/AT's are less agressive than the hankooks that they replaced.

But wow, you really saw your Toyo's drop it 4mpg?

pm26 · February 11, 2007

What brand fuel are you using? Try Chevron, and run at least two bottles of Techron fuel injector cleaner through the fuel tank. Also, try inflating the tires 5 psi over recommended cold inflation pressure.

Do you have any accessories on the truck that create extra drag?

kc5deb · February 11, 2007

No accessories. No lift, no grillguard. Only thing added to the outside is an xm radio antennae... highly doubt that affects it..

Bish · February 11, 2007

I would agree with others on the more aggressive tires.. Torsion bar adjustment could have had an effect particularly if you didn't have an alignment done after cranking the t-bars. Could also be o2 sensors. I dropped 1-2 mpg over a year and replaced the upstream o2 sensors and the mileage came right back. At 85K, even though they were not setting codes, they still killed mpg. Unfortunately, expensive to swap if you're not sure.

95Sierra2500 · February 12, 2007

Here's a list of things I try to do when trying to improve fuel mileage in a vehicle:

1. Take out anything that you don't need (tool boxes, bags of sand, garbage, etc)

2. Increase tire pressure 3-5 lbs (I usually run my car around 35psi, my truck around 65psi with my E-range tires, 80 when towing)

3. use cruise control. Many disagree with this, but it DOES help quite a bit if you find it difficult to hold a steady speed yourself.

4. a tonneau cover or topper will help to some degree, but the added price doesn't come back right away

5. replace O2 sensors (upstream). If you've got some miles on your truck, this can help. If not, as stated it's just throwing cash at your truck.

6. Slow down to the speed limit, or 5 mph under it. Because aerodynamic drag is an exponential relationship to speed, as you go faster you exponentially decrease your mileage. Slowing down helps ALOT. Don't do 80-85 mph, but rather 70-75 and you'll notice a drastic improvement in your mileage on the expressway. For example, my '95 gets about 12 mpg if I run at 75, but at 70 I can get 15. 55 mph nets me around 17 mpg. My Grand Prix is only supposed to get 28 mpg on the expressway, but with the cruise set at 65 (the speed limit through Ohio) I have seen as much as 34 mpg.

That's my advice and it's worked well for me. A tune would probably help as well, as many people who get a tune for more horsepower notice an increase in mileage as well. I think it has to do with the fact that the tune leans out the mixture (which usually yields more horsepower in an otherwise stock vehicle), but I've done no testing to substantiate that.

kc5deb · February 12, 2007

The agressive tires shouldn't be a factor in lowering my mpg from where it was at. Like I said, the Hankooks that were on it a few weeks ago were way more agressive than these LTX/AT's that are on it now.

I travel lite. Maybe a 10lb rubbermaid box of "stuff" in the back, and that's it.

Tire pressure is set at or a little above cold pressure (will double check that tomorrow)

I'm a huge stickler for using the cruise control; if I'm going more than 2 miles, my cruise control is on. Nothing annoys me more than to get behind someone constantly going between 65-75mph, and constantly passing me, me passing him, him passing me... you get the picture.

Don't have a tonneau cover, but, have seriously thought about it soon

My truck has 75,000 miles on it... I could see the O2 senors doing this.. might check to see how cheap I can grab me a set.

Like I said before, 75 during the day if I'm in a hurry, and 65-70 at night depending on the traffic. I speed, but, not too horrible.

I do have a tune, and fuel up with 92 octane fuel.

I did put a bottle of fuel injector cleaner in today when I filled up, and so far, I've gone 170 miles and the fuel gauge is showing at or right above 1/2 tank. I'll find out a little more when I run the tank lower and fill up in the next day or two.

BTW, thanks for all the advice. I've thought many times about getting rid of my baby, especially when gas was $3/gal, but, with the price of gas predicted to go up, I'm hoping to not spend my time thinking it again if I can't find some way to get the mileage back up.

95Sierra2500 · February 12, 2007

It's not just how aggressive the tread is, but what the tires are made out of. If your tires are made of a softer rubber, they have more rolling resistance which increases road load, decreasing mileage. I could probably get better mileage in my '95 if I had a set of harder highway or all-season tires, but my all-terrain tires are a much softer compound than all-seasons than that and I have to give up 1-3 mpg.

kc5deb · February 12, 2007

It's not just how aggressive the tread is, but what the tires are made out of. If your tires are made of a softer rubber, they have more rolling resistance which increases road load, decreasing mileage. I could probably get better mileage in my '95 if I had a set of harder highway or all-season tires, but my all-terrain tires are a much softer compound than all-seasons than that and I have to give up 1-3 mph.

Ok, NOW I smell what you guys are steppin in... Makes sense.

sonjaab · February 12, 2007

What size tires are you running? If anything bigger than 245s and you have not

reprogrammed your PCM/speedo/odo to the new size your mileage figures

will be off so your calculations for MPG will be low...................

nitro882 · February 12, 2007

Yep Kc5deb those toyo's dropped it 4mpg. When i replaced them with Uniroyal cross country's i got the 4mpg right back. The difference was amazing.

Overall i was not impressed with the Toyo's. The traction was awesome, possibly the best traction tire i have ever had in an all terrain but the milage drop and the crappy treadlife are the two things that will make me never buy them again.

kc5deb · February 12, 2007

What size tires are you running? If anything bigger than 245s and you have not
reprogrammed your PCM/speedo/odo to the new size your mileage figures

will be off so your calculations for MPG will be low...................

They're 245/75's. The speedo is correct again. The previous were 245/75's, but, apparantly they were a little bigger, cause it was always showing I was running faster than I actually was.

MrLM7 · February 13, 2007

Take your tailgate off. I normally got 365-380 miles to a tank, half highway, half town. I took my tailgate off to go to the track, and left it off for the next tank of gas. Got 410 miles with no other mods or changes in driving habit.