low gas mileage - advice?

Hello,

2003 Silhouette. My gas mileage is running low - about 3-4 mpg lower than it should be. At 55 mph straight and level, warm engine, I've heard it should be in the high 20's or 30 but it's like 25 mpg. Also, actual mileage for a whole tank is like 17 mpg, which is too low. Should be 20-21 for my driving. I have confirmed that the trip computer is pretty close to actual.

So, any advice as to what to replace to increase the mileage?
What is consisted of a "tune-up" besides spark plugs, air & fuel filters, PCV?

Thanks!

EPA:

16/22(18 combined), for AWD

17/24(20 combined), for 2WD

So you are in the ballpark, depending on if it is AWD.....remember those are numbers for a new vehicle, not a 10 year old vehicle.....

Big difference between highway, and driving around town, back roads, and hilly areas......when the wife and I take a trip to Cape Cod, we fill the tank and we refill when we get back...getting around 33 mpg.....then when she is just riding locally, can get as low as 24-26.....stop and go is a killer for gas mileage, as well as hilly areas, which we live in....

A COMPLETE tuneup(wires, plugs, pcv, air and fuel filter) certainly wouldn't hurt.....would also pull the O2 sensor(before the cat), to see if it's carbonned up, or contaminated....but like I said, your mileage is not that far off....

Mine is FWD and I thought the EPA was 19/26. For the kind of driving I do, I do believe I'm 3-4 mpg too low.

It seems that the best I could possibly get on a highway-only trip is 25 mpg, but you got 33 mpg, so something must be wrong.

I'll follow all of your suggestions. If anyone has any others, that would be appreciated. Thanks!

I'm sorry....I don't have a Silhouette....we have an HHR......I was only giving the gas mileage of ours to demonstrate the disparity you can have between highway and local driving......your combined mileage can be pulled low, because of local driving....the EPA estimates I gave were correct....

For front wheel drive, it is 17 city, 24 highway(20 combined)......so if you are getting 25 on the highway, that's great for this vehicle, since it is heavy and not really aerodynamic......but there's a reason why it's not used as a taxi cab......stop and go traffic, delivering customers would kill gas mileage....

2WD = FWD in this case... and i'm also seeing 17-24 mpg from a few websites.

one site did state it went up to 26 mpg, but with real-life drivers updating their mpg ratings, it was down to 24

http://fueleconomydb.com/specs/2003/OLDSMOBILE/SILHOUETTE%2520FWD

also, after making those preventative maintenance checks/ replacements,

(and using at least double-tipped platinum plugs ~ weren't most vehicles past 2000 require them lately???)

switching to full synthetic on the transmission, engine, drivetrain components can squeeze a few extra mpg

of course, driving more conservatively/ smoothly will also help


doing mostly highway driving will surely push the mpg up no matter what vehicle you drive (i also experienced this with various models)

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I think with the onset of up to 10% ethanol content in our current gasoline mixtures sure doesn't help with the mileage as well :2cents:

since they changed from MTBE to ethanol, in ~2003, more people noticed that they're mpg has decreased slightly...

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I too have a minivan with a v6 engine & is having a slightly lower mileage

(i'm overdue for a "tune up" as well & will need to change out the plugs/wires/cap-rotor, timing belt, water pump, etc when the weather gets warmer in the next few weeks)

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carbon buildup along the egr passageways, ground wires, vacuum/exhaust leaks, old o2 sensor, improper working t-stat and / or Engine coolant temp sensor... just some other stuff that may reduce mileage (some of these may also set your check engine light)

do you have any trouble codes or engine light on? autozone free obd II scan will be helpful...

some people neglect the carbon around the throttle body valve, but that has to do with idling smoothly :2cents:
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dunno if we are still in "winter blend" but many people notice an increase in mpg from winter to summer blend

According to the computer on my 02 Silo I'm getting 21.9 Mpg and it's on the rise as I just did the plugs and wires. I plan on working on cleaning the injectors next.

Cheers
Wayne.