gas millage went down as engine oil getting dirtier?

[gekko668]

Did anyone notice that their millage drop by a few 2-3 gallons as their engine oil is getting dirtier? On average I get about 28-30 MPG on both highway and city driving with AC on most of the time. Now i'm getting around 23-25 MPG as the oil is approaching 4000 mile. I'm using synthetic oil BTW.

[Promise Land]

I don't run my oil past 3500 (but I use normal 5w30) and I've never noticed my mileage going down.

[gekko668]

I wonder if the hot floridian summer is causing this. My car is only about 20,000 miles and the air filter only have several dirty spot and it not like i have a jack rabit foot.

[Promise Land]

the heat is definitely a factor. My trans fluid is about shot at 25,000 miles. It's supposed to be changed at 30k, but the heat is definitely wearing it faster than other climates.

[eeshazor]

Hmm.. I'm in between Gekko and Promise lol. I also don't usually run my oil past 3500miles and I also use regular 5w30, however.. I do allllll freeway miles on my car, daily, and I actually do notice my mileage slightly getting worse as I go over my miles. I think I'm just gonna start using better oil and hopefully problem solved

[nmaiorin]

It is probably your air filter getting dirtier, rather than your oil - 4,000miles is nothing for oil, especially a syn oil.
Check your air filter and give it a few taps and vacum it - Also check the air pressure in your tires, keep them above 35psi.

[nmaiorin]

And yes - change that ATF ASAP - if the fluid is getting beat up by the Florida heat, the anti friction agents, and clutch pack additives in the fluid are probably lower/gone,and as a result the Trans is doing more slipping between shifts causing the engine to use more fuel.

[nmaiorin]

4 cylinder engine are extremely hard on ATF fluid because of their nature, very high stall speeds on the converters and very high RPMS - stressing fluid in almost any climate zone.
Every one on this board with the Auto 4 speed should change their fluid between 30-60,000kms depending on driving style and inspection.
I usually change my cars at 40-50,000kms - My versa's fluid was black and nasty at 50,000kms and after 4 drain and refills the trans hooks up better and is getting better mileage.
FYI - I have other threads on this board discussing the virtues of fluid changes.
You change your engine and often forget about the most complicated component in your car - the transmission.

Manual transmission are usually much easier on fluid compared to conventional automatics.
Cheers