Have you lost weight? Have you stopped carrying something heavy in the car? Have your habitual routes changed? (e.g have you stopped doing a school run?) Are you driving more slowly?
The coolant is circulated by the water pump, which is running regardless of the heater settings. The coolant must constantly flow in order to prevent overheating of the engine, whatever the ambient temperature.
The air conditioner uses a compressor to circulate the refrigerant gas, and that is declutched from the engine if the air conditioning is off. Running the A/C requires the engine to devote some of its power to drive the compressor, but only when the A/C is on. Note: I believe on some cars the front windshield defroster uses conditioned air, which is dehumidified.
P.S. My bet is on your gas provider switching off of oxygenated fuel, as mentioned upstream. I would suspect a decrease in ethanol level, but no states to my knowledge have decreased those recently.
I lost 40 lbs. but I think that is pretty minor compared to a 2500 lb. car. But my weight loss started before this mileage change.
All my habits are the same , no change in school runs or anything like that.
The engine is circulating coolant whether the heat is on or not. If the heat is on, coolant is being circulated through the engine head/block, heater core, and radiator; if it’s off, coolant is being circulated through only the engine head/block and radiator. The heater core is typically routed in parallel with the radiator, so having the heat on (and fluid flowing through the heater core) actually reduces the work the pump has to do, though I suspect the amount is not really significant to fuel economy.
As was noted, cabin heat utilizes waste heat that would otherwise be dumped via the radiator. So using heat will not really affect fuel consumption.
Re: engine break-in (some people call it “wear-in,” which seems more appropriate), on a new engine the cylinder walls and piston rings have not yet worn themselves into agreement; there is slightly elevated friction, and more importantly, an imperfect seal, resulting in loss of efficiency as hot combustion gases blow by this interface instead of helping to push the piston down during the power stroke. That friction is caused by microscopic metal-to-metal contact between the ring and the bore, which slowly wears away the sharp/new edges of the cross-hatching on the bore. This is why new vehicles typically come with a warning not to exceed a certain RPM for the first 500-1000 miles, until the sharp edges are broken/worn down enough so that things won’t overheat/gall when you go to high RPM. Break-in still is not finished at this point, but you can begin to rev the engine higher and put your foot in it harder and finish the process. Once break-in is done, the rings are sealing properly, and the engine is running more efficiently since there’s less leakage past the rings.
Newer engines that are designed to endure more miles than the vehicles of yore seem to take longer to fully break in. BMW motorcycles (and some other vehicles) use Nikasil linings on the cylinders and are notorious for having a very long break-in period, on the order of 20K miles.
27K miles seems like a very long time for this to happen on a Honda Accord, but if you’ve got very conservative driving habits, then break-in could indeed have taken a very long time.
I am pretty sure Honda no longer tells people to be careful with their engine during the break in period. I can check my owner’s manual about that.
In many states, there is a winter formulation of fuel. I regularly notice my gas mileage drop by a few mpg in the late fall, and increase in late spring. If I see a station in the far north of NH when I’m up there that says they have “regular formulation” (rather than winter, and yes, it’s no-lead) I go out of my way to buy that station’s gas. It will give me a few more mpg for the same cost.
I know someone who drove in “Drive” all the time, never selecting Overdrive (OD).
They didn’t understand what it was for. When they finally were told to use it, their mileage went up significantly (duh) and the engine stopped revving at 3500 rpms on the highway and dropped below 3000.
But this person swore they did everything right in trying to figure out why their car was noisey and got poor mileage.
The break-in period referenced in the manual and this break-in period are slightly different. The first is just to make sure rings and seals and stuff get conditioned properly before you start redlining it. The second is what happens when certain components like cylinder linings wear to the proper degree for maximum efficiency.
Thanks, by the way, Joe F F and Raven.
My Civic isn’t quite 2 years old and I was not told to “take it easy” on the new engine. I’m not a hard driver in the first place, but it was nice to not have to baby it. I’m at 13K km now. I don’t drive it much.
I had a highway trip last week and I got 40 mpg on the way up. Best gas mileage yet! Keeping the cruise control set for the speed limit helped, I think. I’d rather get good gas mileage than get to my destination 20 minutes faster.
My Toyota Camry gets better mileage in the spring, between winter driving (using the defrost a lot) and summer (using the AC a lot). I finally figured out that when I turn on the defroster, it actually turns on the air conditioner to dehumidify the air, then heats it before blowing it out the defrost vents. If I turn the defrost temp all the way down to the coolest setting, it is cold, air conditioned air blowing out! I didn’t notice this because the AC light doesn’t not come on when it is operating in defrost mode, but it is running the AC compressor and using extra gas, the same as when I run the Ac in summer.
Do other makes do this?
Hondas use the AC for defrost too and the AC light comes on for me.
Absolutely, other makers do that. Back in the 60’s when I was learning to drive, when you turned on the defroster and blew all that damp air onto the cold windshield, it immediately fogged up the windshield. After the car warmed up, it would slowly start to clear it. Having the car turn on the A/C with the defroster dries out the air and prevents that. Huge improvement.
You can, at least on all the cars I’ve driven, manually turn off the A/C while the defroster is on. In my experience, once the interior of the car is warm, you don’t need the A/C to prevent fogging.
I know you’re joking, but when my wife’s old car blew a cylinder her gas mileage went to pot. An engine that’s damaged enough to not have all its cylinders working is probably not going to be a model of efficiency.
I haven’t figured out how to do that on my '99 Camry. The AC button doesn’t do anything when defrost is on. Any ideas? I think I could save some gas in winter.
A blown cylinder means that to produce sufficient power, you have to press the throttle farther to extract even more from the remaining cylinders, increasing the load and the revs (and fuel consumption) in the working cylinders that are making power, all while some extra fuel is being dumped into the blown cylinder.
If you wanted better mileage at cruising speeds or at speeds which place a very light ‘load’ on the engine, it can make sense to shut down a cylinder by turning off the injector, thereby cutting fuel consumption.
The OP went from 360/16 (22.5 mpg) to 25/16 (25 mpg). That’s a 10% difference or 2.5 mpg, with no change in driving habits.
The change from winter blend to summer blend gasoline would account for some of that, the car warming up faster would account for a tiny part, and the fact that the OP says he lost 40 pounds would account for a tiny portion more.
Maybe traffic is a tad lighter – maybe just enough so that you make one light in the summer that you would have had to stop for during the earlier months.
I’m going to vote for a combination of little things – maybe each one improves mileage by 1-2%. Not enough by itself to make a noticeable difference, but they all came together at the right time.
My 13 year old Saturn is getting about 15% better mileage over the past few months, but I’m pretty sure this is directly related to the economy. More unemployment, fewer cars on 880, less stop and go driving during my commute, better mileage.
My son-in-law just got rid of his '92 manual Saturn - only because it got flooded in the parking lot, not because it was defective. Except for changing the oil he neglected it for years due to lack of money. It had about 225K miles on it. My '93 died to save my wife, who was driving it and got hit right in the driver’s side door by a fool running a red light. She spent a few hours in the emergency room being checked out, and walked away. Great cars back then.