mileage and performance is dismal-what can I do?

denali_dave · February 25, 2003

Greetings all;

I purchased this Denali as a tow vehicle for my Jeep. It ends up that its now my wife' daily driver as the mileage is dismal. It has the 5.7L. It got 13.3 on the highway, is getting 10 around town. I had a 94 2WD Sierra pickup that got 15 intown and 17 on the highway. Thats the mileage I am aiming for. Is it possible? I have done some reading on this board and many of the intakes appear to have no real effect, in some cases, losing mileage. Aftermarket exhaust as well? I used some combustion chamber cleaner and it helped make the acceleration crisper but its still dismal. I just had an intake coolant leak fixed, oil changed, etc. It has 70k on it. Any help would be appreciated. Realize that I am looking for performance and mileage. I already have major mods/dollars tied up in my jeep so I am not looking to modify this for the heck of it. It appears to possibily have a larger than stock tire size on some aftermarket wheels with less backspacing( i believe they are 265/70/16s Michelin LTXs.

CapnDean · February 25, 2003

Dave:

I am a curmudgeon when it comes to gas mileage mods. I believe that in most cases the $$ you spend for a mileage mod is more than what you wil ever see in return. That being said, I would suspect that your good ole 350 should get you a little better mileage than what you are experiencing.

Some things to check. Tune that motor up and replace your plug wires. Replace the fuel filter, clean the intake with carb cleaner (before the new plugs of course). Check the air pressure on your tires, mine surprised me with a recommended 44 PSI when I had been used to 32. Have midas or somebody similar look at your brakes, are they dragging?

Basically spend $300-$500 making sure that that truck is running as it should be...Then perhaps look at your wife's driving style.... I say that you should be able to get 14 or better around town out of that setup.

CapnDean

TarheelZ · February 25, 2003

Not trying to insult anyone's intelligence here, but do you adjust your mileage calculations for the larger tires? Unless you have recalibrated your odometer, it will reflect fewer miles than you actually drive and make it look as if your mileage is lower than it really is.

Just a thought.

denali_dave · February 25, 2003

I will move in the tune up direction first and see if this helps. Tarheel, do you have any idea how much bigger this tire is than stock? Unless its a drastic change, I wouldnt expect it to vary that much, but who knows.

How about mods to improve that truly give you a bang for the buck? Also, any idea what rear end this may have? It does appear to have the factory towing package.

SilveradoWV · February 25, 2003

Not trying to insult anyone's intelligence here, but do you adjust your mileage calculations for the larger tires? Unless you have recalibrated your odometer, it will reflect fewer miles than you actually drive and make it look as if your mileage is lower than it really is.

Just a thought.

In some cases I dont think changing tire size makes a lot of difference, it does when calculation mileage of course but I dont think it makes a big difference with the odometer. I went from 245/75/16s to a 265/75/16 but according to the interstate radar my speedo was off a few mph from the factory when I had the 245's. I havent checked with the 265's and I would assume the interstate things are not 100% accurate anyway, but I cant remember who it was on here but they have a great theory IMO, although I havent tried it yet. Whenever you are on the interstate or whatever, set your trip thing at a mile marker and then over a period of say 10 miles see how much your mileage on the trip is off compared to the mile markers.

denali_dave · February 25, 2003

I have purchased today a new set of 7MM silicone wires, new platinum plugs a new cap (ouch! $$$) and a rotor button. Will install next week and then see where I go from there. Any other suggestions, bring it on.

Shaners · February 25, 2003

I'd definatly toss in a new air filter while you're at it.

Remember that this winter fuel combined with cold temps affects everyones milage. (Mine is down about 10% in the winter)

Here are the RPO codes for 1999 rear gears. Look at the sticker in the glove box and see if any of these codes match.

GT4 3.73:1 Rear Axle Ratio

GT5 4.10:1 Rear Axle Ratio

GU4 3.08:1 Rear Axle Ratio

GU6 3.42:1 Rear Axle Ratio

denali_dave · February 25, 2003

Thanks for the RP codes, I will ck that tonight. I appreciate the info on the mileage, however I am in Florida and its 80 degress today

I am still hoping somebody is going to tell me that they put on one of those Volant airboxes and a gibson cat back exhaust and their mileage improved like 2MPG. Where are those posts?

Markeaux · February 26, 2003

The stock tire size should be detailed on the driver's door placard. You can use this site to compare different tire sizes and the mph diff:

http://www.secret-secret.com/turbo/DIY/tir.../turbotire.html

denali_dave · February 27, 2003

Thanks to your input, I found out I have 3.73 gears and according to the door placard, it came stock with 265/70s and I have 265/75s mounted; Im sure not enough to through the speedo off. So, mileage of 10mpg in town and 13.3 hwy is very bad. Anyone else getting bad mileage like that? I will install the tune up parts in a couple of days and see what happens but I am not holding my breath. I have to admit, I am not a fan of the push button 4WD on the dash. How do I know that its not really stuck in the AWD mode even though the lights indicate otherwise? I dont necessarily feel and pulling in the front end but I wonder how much drag turning the driveshaft and axles, gears etc is pulling down the mileage? Anyone ever convert one of these ISF front ends to lock out hubs? Do they make them?

Redapple · February 27, 2003

The tire size should make no difference whatsoever. You said it came stock with 265070's and you currently have 75's. That is a wider tire not a taller tire. These tires should have no effect on your speedo at all. As far as the AWD switch. If it was stuck in AWD you would feel it. The vehicle will handle and steer very differently. Good luck on finding the problem. I think you have some very good places to start with the tune-up stuff.

Bill

als1984z28 · February 27, 2003

The 265/75's would be 5% taller than the 265/70's.

The first set of numbers is the width of the tire and the second set of numbers is the sidewall of the tire, that number represents a percentage of the width.

The difference in the tire diameter is roughly an inch.

TheScaryGuy · February 28, 2003

I am still hoping somebody is going to tell me that they put on one of those Volant airboxes and a gibson cat back exhaust and their mileage improved like 2MPG. Where are those posts

In liarsville. Since those claims are just about all false. Only things to improve mileage on an old 350 (i know it stinks, have had 3) is the PCV valve, plugs, wires, cap, fuel filter, a high flow K+N air filter and maybe the elimination of your smog pump and catty converters. Last two might be illegal in your state.