Worse MPGs after oil change?

[RatLabGuy]

I track my mileage religiously. At every tank (~10-12 days) I fill it and divide my mileage w/ adjustment for tires.
Looking back through records (13 months)... after last 3-4 oil changes (since bought it), my mileage drops ~ 1.5-2mpg, (to ~27.5) then slowly creeps back up to about 29-29.5ish.
What's up with this? Anybody else experience this? Never seen it in a car, if anything, the opposite.
Using 10w30, always new Motocraft filter, rarely gets below freezing here.

 

[Erci]

I bet you it has nothing to do with your oil changes (though I'm not sure why you're running 10w30 if you're looking for best fuel eco.. 5w30 will do better for that). Winter gas will drop your MPG by about 2 / gallon. Then there are other factors.. tire pressure, driving habits.. etc

 

[shane02pro5]

Only factors I could think would be tire pressure or softer tires, alignment off, plugs, wires, air filter??

 

[geomatics_tech]

Fresh oil creates more resistance and will cause a slight drop in mileage.

 

[BOOSTR]

I 2nd the change in gas blend for such a slight MPG difference.

 

[RatLabGuy]

I 2nd the change in gas blend for such a slight MPG difference.

I would agree w/ this except that
1 - they don't change gas in the late spring, nor in early October, and
2 - since I get gas every 10 days, the chances of it coinciding with it perfectly (I mean the very next tank of gas) every time I change oil is very slim.

Re: driving habits, yes, I know and it's possble. However since I log it every time I know exactly how much fluctution is normal - and it isn't much, within 1-1.5 mpgs at most. I have a pretty consistent routine, 30 mi round trip daily and drive the same route most days. In general my mpgs are pretty consistent.
Remember I'm not talking about changing 1 time here, but several. I've been through a few tanks since the last one again watching the trend (e.g., not just popping off to conclusions).

I use 10w30 b/c that's what it says in the manual, and my other cars use the same. I'm open to other views on thst vs 5w though.

 

[Erci]

I use 10w30 b/c that's what it says in the manual, and my other cars use the same. I'm open to other views on thst vs 5w though.

Perhaps new oil does create more friction *shrug*

As for 5 vs 10. Manual says to use 10 where it never goes below freezing, but it says that it's safe to use 5 in any temperature and 5 is definitely thinner = better fuel eco.

 

[i12drivemyMP5]

That new oil is so thick it takes awhile to beat down the viscosity to the point where the mpg recovers. Then it's time to change it. That's the only thing it could be. (yes) .....................NOT (chair)

 

[westerboyz]

Yup i agree with the oil theory. When you change the oil it goes through a "Break-in period" in which you would experience a slight drop in mpg.

 

[Typrus]

........

I always see fuel economy go up then taper down.... In - 84 Toyota Tercel 4wd, 81 Tercel Coupe, 96 Geo Metro, 96 Passat TDI, 99.5 F-250 7.3L Powerstroke, 2002 Excursion 7.3L Powerstroke, 02 Honda Magna V-4 Motorcycle.... Need I go on? I think "oil break in" is kinda out...

What kind of oil do you use? Synthetic? Conventional? Blend? Quaker? Pennzoil?

I've never heard of this... Now I'm curious.

 

[westerboyz]

I use mobil1 full synthetic. And from what i have researched, the oil goes through a period when first put in which there is more engine wear while the oil is "broken in"........i could be wrong......

 

[RatLabGuy]

........

I always see fuel economy go up then taper down.... In - 84 Toyota Tercel 4wd, 81 Tercel Coupe, 96 Geo Metro, 96 Passat TDI, 99.5 F-250 7.3L Powerstroke, 2002 Excursion 7.3L Powerstroke, 02 Honda Magna V-4 Motorcycle.... Need I go on? I think "oil break in" is kinda out...

What kind of oil do you use? Synthetic? Conventional? Blend? Quaker? Pennzoil?

I've never heard of this... Now I'm curious.

I use Catrol 10w30, non-synch. Next time I'll give the 5w a shot.
I've never encountered this before either, all other vehciles I've had ('64 Malibu, '90 Cherokee, 89 4Runner, 97 Saturn) have been better right after change, then slow decline. Which makes more sense.

 

[nanook223]

the best oil i have used and maintain my gas mileage after actually improving with this oil surprisingly is royal purple 530 synth. it runs smoother,idles smoother and i actually got an extra 1.5 mpg better than mobil 1. weird but it worked.and dont buy grocery store gas i go to ma and pop stores for some reason the gas is more pure. literally makes me lightheaded every time i fill up.haha

 

[redpr5]

im wondering about my fuel economy as well. im running mobil1 full syn 10-30 but have never had a mpg change due to an oil change, but recently my mpg is the suck. im talkin 93miles and im like 3 lines above half a tank (and i stay below 4000rpm 95% of the time)
winter gas is an issue here (which is stupid because nc doesnt get that cold) so that i can understand, but not this bad. im about to run some fuel system cleaner through my car at my next fill up and see how that affects things.
and next, my car is getting a dyno tune for the a/f ratio and a few other things.
any insight as to what could be goin wrong with my car would be appreciated
btw, i have new ngk g-power plugs in, but i havent changed the coils or wires yet since ive owned the car, so that might be one cause.