My FE is suffering

[Jasen]

My previous tank of fuel I switched from Chevron @ $3.95 a gal. to Costco @ $3.54
The 1st. tank of Costco my UG showed 15.8 avg. mpg the same as the previous Chevron tanks. I had an exhaust leak that I fixed at my 2nd. tank of Costco fuel. My 1/2 tank avg. is 15.1 and no matter what I do I can't get over that, driving the same route on my commute. The avg temps here have dropped about 10* as we head into Autumn.
My cost per mile was .25 cent's, now it's .21 cent's, which is better on the wallet.
I'm wondering, is it

A. weather change

B. fuel brand

C. the exhaust leak was just right to increase FE

D. all of the above

I had hit 15.9 mpg a couple times on the previous tank and 16 was within reach <sigh>

[Frank Lee]

You think you can stay within 4% consistently?!?

And don't use apostrophes where they don't belong.

[RedDevil]

A temperature drop will definitely cause higher fuel consumption, due to:
- The engine needing more time to reach normal operating temperature,
- The air being more dense, hence higher air resistance,
- The tire pressure sagging due to lower temp, causing higher rolling resistance;
- The tire flexibility lessens with cold, again higher rolling resistance;
- More friction from relatively cold transmission oil and bearing grease etc.;
- The fuel mixture will be adapted to weather conditions, so winter gas is more volatile and less dense so less energetic than summer gas;
- Heavier traffic as the schools have begun again;
- The school run itself (it has an effect on my FE for sure);
- Helping out Santa

How in hell did you keep your FE within just a 4% drop?

[Jasen]

4% ? no way, 98% of my driving is rush hr. I leave the house and leave work the same time everyday. If today was a rainy commute and tomorrow was sunny or windy or dry and dark over cast, tomorrows commute would be more congested.

If I didn't put apostrophes where they didn't belong, that would mean I was better at spelling and written grammar then I am, but sorry to say, I'm not.

[Jasen]

Originally Posted by RedDevil

A temperature drop will definitely cause higher fuel consumption, due to:
- The engine needing more time to reach normal operating temperature,
- The air being more dense, hence higher air resistance,
- The tire pressure sagging due to lower temp, causing higher rolling resistance;
- The tire flexibility lessens with cold, again higher rolling resistance;
- More friction from relatively cold transmission oil and bearing grease etc.;
- The fuel mixture will be adapted to weather conditions, so winter gas is more volatile and less dense so less energetic than summer gas;
- Heavier traffic as the schools have begun again;
- The school run itself (it has an effect on my FE for sure);
- Helping out Santa

How in hell did you keep your FE within just a 4% drop?

Maybe next tank I'll fill up with Chevron and see if my EF improves.

[user removed]

I wouldn't pay 41 cents more a gallon for any gas. Last fill in my bike was $3.169 @ gal. Today I saw gas at $3.159. I'll buy the cheapest gas I can find. Bike 5 cents a mile, Fiesta 7 cents, truck a little under 10 cents with gas prices dropping.

I missed the Syria spike by buying 25 gallons a month ago at $3.199 in Hopewell (August 24th). Still got more than half a tank in the truck, 215 miles in a month. The Fiesta is pushing 500 on this tank, might refill Saturday, 1 day shy of a month, still have 1/4 tank left. The two bikes are pushing 2800 miles since early June at about 61 MPG average about 45 gallons for the bikes total for 4 months.

regards
Mech

[Jasen]

OM, that's my thinking, I'm getting better FE for the dollar.
Just would like to determine why the loss, then maybe I can do something to gain some FE back, or maybe this is just how it is for 4 months until the giant orb in the sky returns to warm things up again.

[cbaber]

Some tanks are just explainable. A few weeks ago I changed my oil and rotated my tires. The tank before that was about 46 MPG. It suddenly dropped to 42. Then 43. Then 44. Then 46. I can't really pinpoint a reason why my fuel economy fluctuated, but I've just come to expect it will fluctuate.

[Jasen]

CB, could be, but I've had 3 significant changes within a 2 day's of my last fill up.
Still think I'm going to fill up at chevron next tank, then back to Costco and see what happens. I had 16 mpg just in reach with my V6 powered billboard, and then it slipped away...lol

[riva2model64]

Testing out the Chevron again sounds like a good idea. I'm curious to see if the additives in the Chevron really give you a bit better MPG (4%), though I'm guessing most of it is due to the 10 degree drop in weather. But you can never be too sure without testing data.

Either way, the best way to find out what is causing the change in FE is to try to single out the variables. This is not always feasible or easy to recreate, especially since some things are out of our control. But good luck either way.