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Mysterious gas mileage drop

Right, so I have a 2005 Honda Civic LX, I bought it back in 08 and it had ~36,000 miles on it at the time.

Since I?ve had the car I?ve been monitoring the MPG pretty religiously, and like clockwork, I seemed to always be getting between 37-39 MPG. I would always fill up my tank at around 380 miles, and it would always take about ~9.7 to ~10.2 gallons of gas to fill the tank ? and the fuel light would never come on (probably because it?s about an 11 gallon tank).

Anywho, back in November of 2010, I had about 78,000 on the car and the tires were looking pretty worn so I splurged on a new set of Michelin tires from my trusted Mechanic, whom I?ve used for years.

Since getting these new tires my MPG has dropped dramatically (at least it?s dramatic to me), to around 31-33 MPG. I actually had the fuel light come on at about 330 on my first full tank of gas after getting the tires!

So I?ve tried two things to get my MPG back.

First, apparently I was over inflating the tires for the first 5,000 miles. The mechanic recommended 30-33 PSI, whereas my father recommended I do 38-39 PSI, I was taking my father?s advice. Putting the tires back to 30 PSI has given me about another 30 miles before the fuel light turns on, which came out to about between 35.5 and 36.2 MPG over the last 3-4 tanks of gas.

Secondly, it had been about a year since I replaced the air filter, I checked it, it was kind of dirty but not too bad, I replaced it anyway ? no change in performance.

The car now has about 87,000 miles on it, about 10,000 on the new tires.

So any ideas as to what caused the loss in MGP? Is it the tires? Any other recommendations on how to get the 2-3 MPG back?

From what I?ve read the car has about another 10,000 before I need to replace the timing belt and water pump, could that have something to do with it?

No one here accepts the miles per tankful method. You must do the miles traveled divided by the amount of fuel used method.Using this method you must know both,how many miles you traveled and how much fuel you used. Like, “traveled 400 miles used 14 gallons”. 400 divided by 14 = 28.57 mpg. Do this at least 3 times. Winter weather and if your area uses a winter fuel blend it could account for a mpg drop if any does exist.

Thanks for the quick reply. All the MPG’s I mentioned in the post were calculated using that your method, miles traveled / amount of gas it took to fill the tank. The ranges posted for the original mileage were from almost 2 years worth of calculations, whereas the 33 and 36 MPG figures were from about 2 months each.

The rolling resistance of your new tires could be higher than the old ones, leading to worse fuel economy. That much of a drop seems a little high for that alone, though. However, there are other factors, such as entering the coldest part of the year and using winter blends of gas, which can also affect fuel economy.

Overinflated tires usually have better mileage, not worse, so your results are surprising. Regardless, you should be more concerned with safety than a few dollars, so you should use the manufacturer’s instructions for the tire pressure, not guesses from your mechanic or your dad.

As for the timing belt, don’t forget to consider time as well as mileage, although it looks like you’ll probably hit the mileage limit first.

EPA rates your Civic at 29 city, 38 highway. So assuming your driving is slightly mixed, you’re right “in the zone”.

New tires can make a 2-3 mpg difference. And since you were overinflating the other tires, you’d definitely experience a drop in mileage once adjusting the pressure properly. Overinflated tire get better mileage…although they wear faster, handle and ride more poorly, and can be prone to failure.

Just a tip: your correct tire prssure will always be posted on a sticker in the doorjam.

As lion9car noted, tires do vary in their rolling resistance so different tires can be very different for MPG. And you do also have to take into account that colder weather means lower mpgs both because of the winter blends and because cars take longer to warm up and run richer (more fuel) when doing so.

I too find the whole thing about tire inflation to be backwards. More tire pressure means less rolling resistance so you should get better mpg with higher pressures.

That said, anytime you are certain about a drop in mpg you should have the brakes checked out for dragging (easily happens without you noticing) and have the cooling system checked out - especially the thermostat and coolant temp sensor.

A new fuel filter can never hurt anything either.

It’s most likely the tires. They have 10K miles on them and the mileage is probably as good as it’s going to get.

When these tires wear out and you replace them the fuel mileage will probably change again, for better or worse.

Just out of curiosity, what were the original tires, and which Michelin model did you purchase?

Were the replacement tires the same size as the original? If you put on tires that were a larger size than the previous tires, the diameter might be greater. This would throw off the odometer. If this is the case, with the larger tire you actually have a higher gear and would be getting better mileage. However, the odometer registgers fewer miles, so what you calculate is wrong.

Just a little off topic. You should be using the car manufacturer’s recommended tyre pressure, at least for a starting point. The tyre manufactures and car manufacturers all agree that the car’s owner’s manual is the place to start when talking about tyre pressure.
This is sponsored by a tyre manufacturer Bridgstone - Firestone and instructs the reader to check the owner’s manual for the correct tyre pressure.

Here is another from the Goodyear site.

Tire Inflation
Proper tire inflation is a key ingredient in driving safety and long tire life. It is wise to check your tire’s inflation at least once a month with an accurate tire pressure gauge. Continuous loss of inflation pressure is an indication of a possible tire/wheel assembly problem; consult your tire professional immediately if you encounter this situation. Be sure to check the pressure while the tires are cold, and have not been used recently. If you drive even a mile this will cause your tire pressure to increase and give you an inaccurate reading.

Check Your Air Pressure
Keep your tires properly inflated and you could improve gas mileage by more than $1.50 every time you fill your tank. The recommended tire pressure for your vehicle is located on a sticker inside your driver-side door or noted in your owner’s manual.
You might also try the US Department of transportation.
They say:
You can find the correct tire pressure for your tow vehicle in the owner?s manual or on the tire information placard.

The model is Primacy MXV4, the size is 195/60/R15. I do not recall the manufacturer of the old tires, but I’m pretty sure they were OEM.

I don’t think this has been mentioned, but merely replacing worn out tires with new tires will adversely affect your fuel economy. This is because the affect the amount of tread rubber has on rolling resistance.

But there are HUGE differences between tire for rolling resistance. If you removed OE tires - which give good RR, but not so good wear and traction - and replace them with good wearing tires, you will be chosing tires with higher RR values.

I go into more detail here:

And even more detail here:

The Primacy MXV4 has a fairly low rolling resistance so I am surprised that your mileage went down. We use them on our Accord. We also went slightly oversized as the older Honda’s, ours is a 97, bought new in 97, tended to have undersized tires.

First, check to see if your mechanic used the OEM size tires, if he went a little larger, he probably did you a favor, but as pointed out above, it can make your mileage look worse even though it’s not. For this tire, we have found that 35 psi works the best. Lower than that will wear out the edges, more will wear out the center.

You said “from what I’ve read…”, I hope this includes the owners manual. You may find that it doesn’t refer to a timing belt change. Somewhere around this era, Honda switched to timing chains. You need to verify this by reading the maintenance section of the owners manual. Also, you don’t need to replace the water pump if you have done the regular cooling system maintenance with the proper coolants. That is just a recommendation based on fear because the timing belt/chain drives the water pump. While it could damage the belt/chain if it were to seize or leak, it is highly unlikely it would do either before the second belt change is due (if it is a belt).

The timing belt/chain has nothing to do with your drop in mileage. The recent drop in temperatures would be to blame.

Yes the rolling resistance of the tires is a factor but this seems like big drop so it is less likely. If you do suspect the rolling resistance then you can experiment with more pressure in the new tires and then test mileage.

I would then go directly to a worn out O2 sensor. When you have your car running and at operating temp…go outside your car and smell the tailpipe emissions while running…it should NOT smell like RAW gasoline…if it does then it is your o2 sensor OR A CLOGGED AIR FILTER…HELL check that Air filter first…sorry. Then go to the O2 sensor as a possibility.

SO…experiment with higher tire pressures and check mileage… Then check or just swap out your Air FIlter…test… Then sniff that tailpipe when at Op temp…you do not want it to smell like or close to like how raw gasoline smells. If it does and you changed your Filter…it is O2 sensor time.

I think when it comes to tires the best medicine is to look in the owners manual and use the recommended tires and air pressure indicated for the type of vehicle you own

“The mechanic recommended 30-33 PSI, whereas my father recommended I do 38-39”

How about looking in the glove box where they should have an owner’s manual that will give you the recommended pressure for the car you have and the size of those tyres.

At this age and mileage a new thermostat wouldn’t hurt.