I don't know is anyone suffer MPG drop without any reason. My car has 7200 miles now. I used to get around 26-27 mpg, but now i only can get 22-24 mpg. i never change my driving habit and same commuting route, traffic. The only difference is i used to pump 93 gasoline, now i use Sunoco 91. Would this be the cause my MPG drop?

 

Check your tires to see if they might need air. Have you been doing any more idling than usual (warming up the car before you leave home)?

 

mpg drops in the wintertime--cars need longer to warm up and the engines run richer.

 

Plus they use a different blend of gasoline in the winter.

tsxuser, what you are experiencing is completely normal and it happens to me in the winter also. Definitely check your tire pressure, though, and it never hurts to do that and have properly inflated tires.

 

also 91 has less BTU's then 93 so that will lower MPG a little.

 

My tire pressure, even here in FL, dropped due to a two week cold snap. I needed to inflate as much as 3-4 lbs per tire. This effected my MPG by lowering it 1-2 compared to what I was getting. Yes, fuel mixture is a bit different in winter also.

 

now that you mentioned about it..i DID noticed my mpg dropped slightly...

 

my car is in the garage, i never warm up my car before leave. actually my tire pressure is 2-3 lbs over. it reads 35-36 lbs each. is this a factor to make my MPG drop due to over inflated? I do agree winter MPG is drop 2-3 than summer because when i notice drop I compare summer time. (26-28 mpg)

 

Get a diesel ... 47mpg regardless of the weather

 

Over inflation won't cause a drop in fuel mileage, it will actually help fuel mileage.

The main things I see here are weather and fuel.

The octane rating is used to determine how easily the fuel will burn. Higher octane ratings mean the fuel does not ignite as easily as lower octane ratings. On the '09 models, the manual and fuel door recommend premium fuel only (highest octane rating available). This is because the TSX engines are high compression engines. Premium fuel is a requirement because under high pressure, the gas will ignite easier. Think of it as a bike pump. When you're pumping alot, the bike pump gets warm due to the compression of the air.

If premium fuel is not used (lets say regular 87 octane fuel), the gas will ignite before the piston reaches top dead center, TDC. When this happens, you will experience engine knocking. The ECU senses this through the knock sensor and compensates by retarding timing and dumping more fuel in the engine to prevent it from igniting so easily. In the original post, you said you switched from 93 to 91. That maybe one of the thing that cause the drop in mileage. I suggest switching back to 93 for a few tanks and see if that has an effect.

Also, I'm not sure where you're located at but a change in temperature will affect fuel mileage. Cold air is denser which means it will contain more oxygen per volume of air compared to warm air. The engine compensates for the oxygen by increasing the amount of fuel in order to prevent the engine from getting too much air/too little fuel AKA too lean of a fuel mixture. In the summer it does the opposite since warm air is less dense and will contain less oxygen per volume of air compared to cold air. In the summer the car will provide less fuel to prevent too much fuel, too little air AKA too rich of a mixture. This is the same for humidity, more humid:less fuel, less humid:more fuel.

The engine temperature will also affect fuel mileage. Lets say its summer and your car warms up fast. The oil warms up fast along with all the internal parts. All the parts have a running tolerance that it was design to run at and the parts warm up fast, expanding to this tolerance. Now its winter and your engine has been sitting cold. Oil's viscosity will increase causing more drag on internal parts. Internal engine parts take more time to reach full operating temperature which may cause slightly more drag on rotating surfaces due to the thicker oil.

Some engines may also have a cold start enrichment which will dump extra fuel into the engine until the engine temperature reaches a certain level. On cold days, the cold start enrichment will dump more fuel in compared to a warm day due to the fact it takes longer for the engine to warm up.

So in the end, there are many things that affect fuel mileage in just fuel grade and temperature alone.

/end engineering soapbox

 

Thanks thunderbt3. good analysis. I locacted in MD. It is cold out here. (around 20 F in the morning and night). It might be a blame for my low MPG. since there is no thing i can do for a weather. I would try to switch back to 93 to see it make difference. Thx.

 

NICE... Thanks thunderbt3.

 

Actually the opposite is true. The lower the octane the greater the btu.

High octane relates to burning speed, not energy.

 

I was at 31 with mostly highway driving. Now it's hard to keep it at 30. I guess I will see what spring brings but I'm happy.