Dave M

I didn't know where else to go with this question and since I visit this forum almost exclusively I thought I'd post it here. I need some ideas.

My daily driver is a 2003 Honda Civic. When I bought it in 2011 it only had 19,100 miles on it. I track my gas mileage religiously with an iPhone app... record every fill-up. The first year I had the car I got a consistent 36-38 mpg and could even crack 40 on pure highway driving.

After 1 year of ownership the car went in for the sstate emission/safety inspection and I also had the tires replaced because the ones on the car were OE and were now 9+ years old. After that day, there was a very noticeabe step change in gas mileage to 33-35 mpg. At the time I chalked it up to the new tires because the old ones were probably rock hard and could have been low-rolling-resistance tires to begin with. That mileage was consistent for the next year.

This year I took the car in again for inspection (no other work done) and ever since it has come out of the shop I'm now getting a consistent 30-32 mpg. There was another ~3 mpg step change starting with the tank of gas after inspection.

I should note that I've been going to this same garage for over 10+ years and have always trusted them and have never had any problems. I do any work I can myself, but for things beyond my ability (like inspections) I go to this shop. This was not a gradual change... each time it happened in the span of one tank of gas and then was consistent for the next year, and the change happens to coincide with when I took the car in for inspection. The car now has 54,000 miles, no warning or check engine lights, tire pressure is good, I've replaced things like spark plugs and air filters but nothing has made anything but a minor difference. I recently made a trip for Christmas in which I got 33 mpg for the trip. Two years ago when I made the exact same trip I got 41 mpg. The car seems to run fine with no noticeable noises or anything different from 2 years ago.

Can anyone give me any idea what is going one or what might be causing this?

Thanks!

pillarpod

it's a 10 year old car. They don't keep factory spec all through their life. Problem or no problems.

nomoneyz

iTrader: (15)

I would just chalk it up to the car getting old. Do you get on it to clear out the carbon buildup every now and then? Do an injector flush or sea foam to clean up the intake tract.
I had a Kia rio that did the same thing and I messed with it for a while trying to get the mileage back but nothing would change it. Granted I bought it with 164k on it and it only got 29mpg at that time. After a year it dropped to 25mpgs and never could get any better.

Dave M

I understand that and I don't expect it to get 40 mpg it's whole life but the change also shouldn't come one day every year. It should be a gradual lowering of the mileage number, not a step change by 3 mpg in a tank.

SS RRR

iTrader: (2)

Check to see if you have any vacuum leaks as well as any tears in the induction tube between the TB and MAF. I highly doubt the mileage has anything to do with age.

96capricemgr

iTrader: (1)

OEM tires are usually LRR and the first new set of tires you put on a vehicle will often drop mileage 10% if you don't specifically get LRR tires put on.

Maybe age and suspension settling are affecting alignment? Good solid tuneup?

Dave M

I just filled up last night and got 29.5 mpg on the last tank. That's the first time since I've owned this car that I've gotten less than 30 mpg... on a car that used to get 38. That's a more than 20% drop in 2 years, and again it came in 2 sudden increments, not a gradual change. A 1.7L Civic should not be getting 29 mpg.

I'll look for vacuum leaks but wouldn't that cause the car to run rough or show some other sign of the leak? I'll also get the alignment checked, but it doesn't pull to one side when driving.

SS RRR

iTrader: (2)

How old is/are the O2 sensor(s)?

bowtienut

iTrader: (1)

Did the shop mess with the alignment either time?
Most tire shops are clueless about what a real alignment is. You're usually better off not letting them touch it.
Any uneven wear on the tires?

Kevin97ss

Agreed.

Other shop induced issues are fuel system cleaners that slowly kill O2 sensors, and the big killer of MPG...rear drum brake adjustment.

Dave M

The O2 sensors are original, so 11 years and 54,000 miles.

Dave M

I haven't noticed any uneven wear on the tires but I haven't taken a close look either. I'll have the alignment checked.

Dave M

Are you saying the rear drums could be dragging? If so, how do I go about adjusting it?

Kevin97ss

Possibly, was just throwing that out there. I don't know if an 03 Civic has rear drums. If it does, adjusting them may be part of your shops inspection/safety check. The right way is to use a Safe-Set but I don't think too many shops know what that is, let alone have them. Most just crank them open till they feel drag on the wheel. If you have drums out back just jack the back of the car and give the wheel a spin to see if that's the issue.

SS RRR

iTrader: (2)

Get the car scanned and see what they're doing.

leadfoot4

iTrader: (1)

Thing is, just a little more toe-in or toe-out may not affect tire wear, but might add just enough rolling resistance, to drop the fuel mileage.

pillarpod

what about the lil guy most folks forget...the fuel filter Did you ever replace that?

Dave M

I've thought about the fuel filter but haven't replaced it yet. Time to do that too.

It'llrun

Age and use of part of your issue. Part may also depend where you live. Some(most) areas of America have switched in the past few yrs from pure gasoline to ethanol mixed garbage, which in turn robs you of about 15-20% economy(our governments idea of saving fuel). If that happened in your area, this is surely part of your problem. Also, do that FF change.

I was recently able to purchase pure gasoline(for $.60 more per gallon) and decided to give it a go. My economy went up incredibly for that tank. I was stunned because I'd forgotten what that 20% difference really was. When I bought my Explorer, it got 20mpg hwy and when I got rid of it, it was getting 15. The only real change was... ethanol in my fuel.

Btw, why use an i-phone app when you could simply write down the mileage at fill up on your receipt, subtract the mileage on your previous fill up and then divide by the gallons added for a precise reading? Or, if you car has a trip meter, you could use that and save a step of the process.

Dave M

I use an iPhone app because it's easier than carrying around a pen and paper and it does the math for me and shows me graphs of mileage and all kind of stats about cost per mile, etc. I can also track all my maintainence and repairs. And it was free. Why wouldn't I use it?