Question: Fuel economy change with last fill-up

[TheLandYacht]

Question:

So with my last fill-up, I noted an approximate 20% difference in my MPG fuel economy.

A bit of background.

• I use this vehicle almost exclusively for commuting, like 98%. And that's a 40-mile commute that's almost entirely highway driving. And the route (except for the first appx 6 miles) is almost all straight & flat.
• I drive at night, so I rarely use the AC and there's no heat-load on the vehicle or motor.
• I've taken to setting my cruise to 60mph.
• I've had tanks with an average of 54mpg for the last 3 fill-ups PRIOR to this one.
• Now with the most recent tank, I've dropped to 46mpg.

I'm within just a few thousand miles of my end-of-warranty so...I'm obviously concerned if this is something I should be worried about taking care of.
But otherwise I've been fairly diligent about maintaining the vehicle, regular-cycle oil changes, actually...more often than the manufacturer recommends. As a former taxi driver I was diligent about changing that oil at the 5k mark.

Perhaps a simple plugs&wires tune-up?
What else am I looking at or for?

The vehicle doesn't seem to noticeably run or start rough.

[mohammad]

my best guess is head wind, more stop and go traffic than usual or a seasonal tire swap(doubtful considering where you are located).

[7milesout]

I think you'll want to obtain more datapoints. There are just too many variables to even begin to pay much merit to this latest mpg. Here's something not thought about much. The feature that shuts off fuel fuel (off-hand, I forget what it's called), can have a lot of variation. Maybe you used one that either just happen to let your tank fill more, or maybe something happened to that particular pump that has caused it some variation.

Did you happen to just change tires? Probably not, or you would have probably mentioned that. So many variables. Another one, did you have and drive through a lot of rain during that tank? Down here in the dirty-dirty, when it rains, it usually does it with gusto, and then just stops. When there is any amount of water on the pavement, it absolutely requires more energy to push that water away. So, if you see "tracks" from the car in front of you, and your car is doing the same (leaving tracks), it will absolutely use more fuel and your mpg will be lower, verus all the same conditions but dry.

I would imagine it rains the same way in Texas. When I lived in Michigan, we would have these long, quiet, pleasant light rain events. No thunder, no lightening. It seems to thunder and lightening rarely in Michigan, compared to the southeast. Rain in Michigan was pleasant most of the time. Down here, its more dramatic. And a lot of water gets pushed out of the way.

[dspace9]

My Mirage's MPGs vary a bit, seemingly without rhyme or reason.

Like yesterday, I was avg'ing 6.1 L/100 kms. Then the day before, I'm doing the same drive, same conditions, and I'm getting 5.3 L/100 km's. Then the next day I'll be getting 4.2 L/100 km's. I don't get it.

Summer definitely gets better numbers than the middle of winter, with winter tires.

But my Big Average over 200,000 km's driving my car now is approx 5.5 L/100 kms. Almost always over 40+ miles a gallon, and sometimes over 50+ MPGs. Pretty good for my little piggy bank.