5.3v8 Getting Really Bad Country Mpg

[99silverado1500]

On my 99 silverado, i have finally been checking MPG, and noticed it isnt very great. On my last 2 tanks i got 11.2 and 10.9 for averages. I dont do stealing accelerations, but i dont always baby it, like on average it will go to 1800-2100 when i am accelerating from a stop, and some days i will really baby it accelerating. My driving consists of country roads, there are 3 hills on the route, but they arent huge hills, and its some stop and go, but 5 second stops, its not like city driving. I got the truck with 87000 miles on it, so really have no idea what the other owner did as far as spark plugs or something. The truck is stock except for a dual exhaust. Air filter is clean, just replaced. Would plugs and wires help out, or is it just one of those things? It currently has 100,380 miles on it. If its going to cost 400+ to get better mpg ill leave it alone, but would like to see what i could do to see if i can help it up to like say 14mpg other then a W4me tune. Its a 5.3 auto. Thanks.

[PhilB]

I would think that you would be getting better mpg than what you are. If it were my truck, I would definitely change the plugs, and maybe the plug wires. I would also change the fluids in the tranny, transfer case, and both axles. If anything, it would be for peace of mind. But if the fluids have never been changed, they could be the cause for the low mpg (with the exception of the transfer case).

 

Phil

[MS3DALE]

I agree with Phil,At the very least change the plugs and wires,But I'd change all the fluids as well.

[99silverado1500]

I changed the diff and coolant, and the dealer changed transmission fluid, and theres bout 800 miles on the oil change, and the transmission doesnt slip, so i dont think that would be wasting my fuel. Is there a way to pull the wires to check the plugs without destroying them? I just want to check a plug or 2 to see if thats the issue.

[Lincoln30]

Your mileage probably isn't as a bad you are calculating since it appears you have 285s on your truck but probably don't have the speedo corrected from when it had the stock sizes. You are driving more actual miles then the ODO is indicating. My 99 with stock size tires was getting 12.5 at 80K miles until I changed the plugs and wires...now I'm getting 14.5 and have 40K miles on the new plugs and wires. I have 4'10s, you don't mention what yours is. BTW: The 100K OE plugs don't really last 100K miles.

[99silverado1500]

I have 3.73 rear axle, and my speedometer when im going 69, will be 70.1 on my gps, so thats not a big deal. I have 265/75/R16 tires on my truck right now.

[punmax]

Be happy dude. I get like 8 mpg on my 07 6.0

[MS3DALE]

Well let me tell you a story on the '99 I had...I bought it new by the way...

I changed the plugs around 62,000 miles,along with the belts/hoses/coolant/T-stat,Before I changed them I couldn't make it from my house here in Pa. to my buddies house in N.C. on one tank of gas,Its a 400 mile trip,After I changed them I could make it on one tank with a gallon or two to spare!!

Even when the truck was new I couldn't make it on one tank,I think it was because the truck wasn't broke in yet.

All I know is that I saw a definite increase in mileage after I changed them,I used the OE AC-Delco plugs.

By the way I had 3.73's with P265/75R16 Goodyear Wrangler RT/S tires that I ran in the summer,and LT265/75R16 load range "D" Goodyear Wrangler AT/S tires that I ran in the winter.

[MS3DALE]

Be happy dude. I get like 8 mpg on my 07 6.0

What?

Dude I get at least 15 with mine around town,You must have a heavy right foot?

[punmax]

Dude 15? Are you sure man? I baby it and I still get s**t MPG. How can I calculate how many miles to the gallon I get?

 

And MS3DALE, are the gas gauges on the new trucks screwed? Sometimes it will show half then go back to quarter. Sometimes full then go back to half.

[99silverado1500]

I fill it completely up on the same pump 2 clicks at the same stationand set my trip to 0 when i fill up, then i drive it as normal not really worrying about the gas gauge and then when i go back to the same pump to fill up again, i divide galons into the miles to get my average mpg.

[MS3DALE]

Dude 15? Are you sure man? I baby it and I still get s**t MPG. How can I calculate how many miles to the gallon I get?

 

And MS3DALE, are the gas gauges on the new trucks screwed? Sometimes it will show half then go back to quarter. Sometimes full then go back to half.

 

Yea man,I swear....15,I've even double checked the DIC with a good old calculator!!

Hell I get 17-18 on the highway And I move along at 75-80 there!! I try and let the truck go into V4 mode as much as possible,that helps a lot,I drove it for awhile with the V4 mode disabled and the mileage went down

And before anyone asks....No you cannot disable the V4 mode,I had a bad Hypertech power programer that disabled it for me without me wanting it disabled.

 

On your gas gauge issue.....Mine seems fine,and we haven't had any customers at the dealership complain about it.

[MS3DALE]

I fill it completely up on the same pump 2 clicks at the same stationand set my trip to 0 when i fill up, then i drive it as normal not really worrying about the gas gauge and then when i go back to the same pump to fill up again, i divide galons into the miles to get my average mpg.

 

Yep,Thats the way to check it.

[99silverado1500]

My gas gauge is really bad. If i accelerate real quick in reverse and jam the brakes, the gas gauge will move to blow empty and back up to the current completely full level that was in that instance. Just you have to deal with some things. Is there anyway to pull a plug wire or 2 without destroying them. I hate to have to replace wires if the plugs arent the culprit in my problem.

[MS3DALE]

Believe me,If you have 100K on your truck you want to replace both the plugs and wires if they've never been changed.

I was able to pull my wires without destroying them,but like I said I only had 62,000 miles on it at that time (The last time I saw the truck early last year the guy had 115,000 miles on it)