Hi Guys!

Ive been checking out the forum for a while, but finally decided to post. I have a 2003.5 5AT Sedan thats giving my wallet a lot of grief at the gas station right now. I have replaced all the spark plugs with denso iridium's (the old ngk's were fine), replaced the pcv valve (old one was fine), replaced my air filter, cleaned by throttle body (was pretty dirty), and also ran a can of sea foam though my brake booster. I did get an improvement from 14L/100KM or 16mpg (On the Highway) to 12L/100KM or 19MPG.

However, im still way under what the car should be getting on the highway from what im reading, and I do not floor the car in anyway and go around 60mph with cruise on. My city mileage is also horrible but I do not have a long commute to university from my house so I am attributing that to the car heating up ect. I also have checked for vacuum leaks and found nothing with my propane bottle today.The car is completely stock and is not running any error codes and runs great besides the crappy mileage. Also I always run 91 octane (the max I can get here in my part of Canada) and use all synthetic fluids.

Any help would be appreciated! Thanks a bunch!

 

How are your tires? (thread left) (proper PSI inside the tires along with good thread amount to better MPG)

Is the Car pulling in a certain direction? (if you are pulling the wheel constantly to one side you better bet that it'll hurt mpg)

Have you changed the differential oil? (old oil more friction)

How are the wheel bearings? (any clicking noises while driving) (hard to tell) (clicking, = bad wheel bearing = more friction)

How's are the catalytic converters? (clogged cats = fuel consumption)

Another thing since you have a 03/03.5 which is a VERY important question..

When was the last time you changed the thermostat and fan clutch? This plays a huge roll because if the engine is running warmer than usual it will trigger the ECU to dump more fuel and pull timing to cool down the EGT's which in turn burns more fuel because the car will feel like it's lacking power thus you giving it more pedal than actually needed.

 

Hi Eric,
I have 35 PSI in my winter tires and they have about 60-70% tread left. My car doesn't pull and the alignment is solid. I haven't changed the diff oil but I got my transmission serviced a couple weeks ago and the mechanic said the diff oil looked good. Would you recommend that i drain it and put in some royal purple? Also I don't believe the cats are clogged and I havent noticed any loss of power with the car, I also did a throttle body relearn the other day after cleaning it and the car idles at 650 after its warm. The thermostat and fan clutch have not been replaced since I have owned the car but it has been dealer serviced up to 140km's and I am at 180km's at the moment. The car doesnt seem like it runs hot but I could monitor the temp with my scanner. Also after all the tune ups ect the car actually dropped rpm's at highway speeds and I only go 2000 rpm at 100kmh instead of 2100 before.

 

Your mileage seems pretty spot on actually. I run 50psi in my tires, have perfect alignment, 420 treadwear rating (harder compound, less rolling resistance), and have a manual tranny and get about 24mpg in warm weather.

35psi, auto, winter (softer) tires, stock alignment specs and cold Canada weather, I would suspect about 19-20mpg on the hwy. It makes sense. Especially since your O2 sensors are probably original and they wear out gradually too. Our cars just suck on gas.

 

He should be doing a little bit better on the highway.

Can i ask how you are calculating your fuel economy?


Do you have a long stretch of highway you can really get on and go for a drive and figure it out. As soon as you get off the highway and start city driving, the fuel economy will drop like a rock.

With my AWD sedan, i average 19 MPG mixed. Thats based on my hand calcs as well as the Navi computer. However, I frequently go on long (100+ mile drives) on the highway and get around 23-25 MPG when I do that. However, the minute i get off the highway and start city driving, it drops like a rock.

Oh and it's been consistently like this since new. I have 153K miles now on it.


I always joke about this, but it's sad. I traded a V8 Mustang GT in for this car and the V8 got better fuel economy and ran 87 octane.

 

Hi Guys,

Yeah I actually sold my 02 GT and it got way better mileage, I drove it across the states and ended up getting 29mpg on average during the trip running 93 with it loaded from top to bottom with my stuff. I actually replaced all the O2 sensors with ntk's a few months ago as there was a great deal on rock auto for them (45bucks a piece). Would you recommend bumping up my psi to 40? Will I just burn through the tires?

I actually am in a small university town here and do like 3-4 miles of driving a day in town but every week or so I do a 120km drive to a larger town that's almost all highway. I just get off the highway for max 10km and then am back on the highway and i calculated the mpg's from the gas light. So I add around 20 dollars of fuel, figure out how many litres I get and then set the trip and calculate it by hand when the light goes on again. When I just do city driving, I get around 90km per 17L or around 13mpg.

Also on a side note, I ordered a takeda cai yesterday because there was a great deal on them and will put it in tomorrow, do you think I would see a boost in mpg's from it? (I'm hoping I will).

 

yea its horrible...good thing gas prices are coming down lol...I usually get around 16mpg..75% city driving.

 

Yeah! It was 1.36 a litre here for regular a while ago and 1.48 for premium. So it was like 5.60 a gallon , that hurt haha. Now it's 1.11 a litre for premium or 4.16 a gallon. No complaints from me!

 

keep in mind the idiot gas light comes on with 3.5-4 gallons of gas in the tank still. if you fill as soon as the light comes on change your formula to :

(miles traveled) / 13.5 gallons
(miles traveled) / 14.0 gallons

you will see a better data set this way.

my guestimate is your dividing 350-375 /20 and getting the 19mpg

 

Why would you burn through tires? I would recommend running full sidewall, but people only like to listen to the internet dogma when it comes to inflating their tires, so..yeah.

Been running full sidewall psi ever since I could drive and have never had anything but long lasting, fully even tire wear.

 

I don't actually run my gas tank full but I've just been adding an amount (16-17 litres) after the gas tight goes on and then seeing how many km's im getting and then converting it to mpg's. Okay awesome! I'll up my psi and hopefully that will help a bit! But I still don't get why I'm getting 19mpg on the highway with like 95% highway driving? An that's an improvement from before I did the plugs ect? Is there something that can mess up gas mileage that bad that's not any of the issues I've already checked for?

 

Bad compression.... REALLY bad plugs... about it.

 

on that note.. does you oil smell like gas ?

 

I checked my oil and it smells like oil, not gas. I haven't noticed any loss in power so I would assume my compression is the same? Also I replaced my spark plugs with denso iridium sand got a boost in gas mileage and the old NGK double platinums still looked good minus a little carbon. Would it help if I took a picture of the old plugs? Also would a clogged injector hurt my gas mileage? I haven't had any cel lights or anything and do put sea foam in my gas tank 2-3x a year and actually used a bit of it the other day but I don't know what else it could be so I'm just guessing at this point? Also an evap issue would trigger a cel?

 

You're not calculating it right. The gas gauge in our car is not that precise, and the light may come on at different fuel level. To really find out your mpg you need to fill up the tank to full, reset the trip meter, do your daily driving until the light comes on, top off your gas tank again (at the same station/pump you filled up last time) and get the mileage you've driven, then use the mileage and the gallons (litres) you fill to get your mpg. If you do this way every time, you should be getting more consistent results.

And on a side note, I would strongly suggest against inflate normal passenger tires to 50 (max) psi under normal driving condition, because everyone's tires, sizes and ratings, max load, curb weight, ambient temperature, driving and road condition are all different, while others claim it's running fine for them you might not get the same results. I certainly would not sacrifice my ride comfort and safety for a tiny bit of mpg. Just my .02.