Fuel consumption suddenly gone higher

[ravpickle]

I have a 2009 I have only owned since March. I was getting average MPG at about 24 mixed highway/city driving. Can't possibly complain. I took it on a road trip and got 29MPG on a 7 hour drive. Thrilled. I had a pathfinder before this, LOVED the car, hated the fuel economy.

Suddenly, I am getting 19mpg driving the same route to work/home. The car doesn't sound strange, feel different. I am at 39,700 miles. The only things I can think of are the drop in outside temperature (I live in upstate NY) or maybe the tires? I had the tire pressure monitor go off a few weeks ago. One was at 23 psi, I was shocked, it didn't look flat. I re-inflated, initially it dropped again then has been holding air. I was going to go have them look for a slow leak, but since it hasn't dropped again I never bothered.
Could it be the tires? Is there another cause I should take it to the stealership for? I got the extended Jerry Warranty...because at that price who could refuse? So if it is something covered, that would be great.

It has the original Yokohama tires that came with it. There is still ok tread on them but know I'll need to replace them. Been trying to figure out best tire, if I should keep these as a "summer" tire and get a more aggressive for winter now. So, I've been procrastinating replacing them.

What could be causing my sudden drop in fuel economy?

[avs]

The air density of the outside air?

[RTexasF]

Change to a different brand of fuel for at least two tanks and see if it makes any difference. It certainly did for me - Valero Gasoline

Make absolutely sure all tires are properly inflated. Speaking of tires, at just under 40K miles those have got to be low on tread, have them checked with a tread gauge at a tire store. If they are 4/32" or less it's time for new.

[vanib]

Winter gas.

[autotechazo]

all of the above, the mix for winter fuel will cause a drop, the computer is also mixing richer because of the temp and it takes longer to get the engine up to proper temp. tire wear and inflation cause the tires to be smaller in diameter, so you travle less per revolution. differant brands of gas will have differant additives so try other brands.
watch as spring gets here and you will see the snake oil salesman come out to sell you products to help your milage. that was coming back on its own, they never sell in the fall

[MrPulldown]

Do you have a short commute. That mixed with longer start up times can really kill your MPGs. How many tanks of gas is the 19mpgs calculated off of? Have you done any work on the car recently. I'd say just keep an eye on it. 24 to 19 sounds a bit much for jsut winter/summer varianace; 2 mpgs is typical. Might just have a weird fill up (the autoshut off clicked sooner on last tank giving you a high 29mpg, and you are feeling the ffect this tank).

[ravenuer]

Originally Posted by ravpickle

I have a 2009 I have only owned since March. I was getting average MPG at about 24 mixed highway/city driving. Can't possibly complain. I took it on a road trip and got 29MPG on a 7 hour drive. Thrilled. I had a pathfinder before this, LOVED the car, hated the fuel economy.

Suddenly, I am getting 19mpg driving the same route to work/home. The car doesn't sound strange, feel different. I am at 39,700 miles. The only things I can think of are the drop in outside temperature (I live in upstate NY) or maybe the tires? I had the tire pressure monitor go off a few weeks ago. One was at 23 psi, I was shocked, it didn't look flat. I re-inflated, initially it dropped again then has been holding air. I was going to go have them look for a slow leak, but since it hasn't dropped again I never bothered.
Could it be the tires? Is there another cause I should take it to the stealership for? I got the extended Jerry Warranty...because at that price who could refuse? So if it is something covered, that would be great.

It has the original Yokohama tires that came with it. There is still ok tread on them but know I'll need to replace them. Been trying to figure out best tire, if I should keep these as a "summer" tire and get a more aggressive for winter now. So, I've been procrastinating replacing them.

What could be causing my sudden drop in fuel economy?

I've never experienced a 5mpg drop from summer to winter, 2 maybe. Couple of questions, are you checking mileage by hand or on the dash mpg readout(if you have one)? Regarding tires, are you checking them yourself with an accurate gauge? How much are you putting in? Don't forget, cold temps can lower tire pressure more than you think. I like 34 all around on mine. Do you warm it up in the driveway for more than a minute or two since it got cold?
By the way, with 40K on the tires, they're "coming down the homestretch".

[JTRAV]

Winter gas.
Colder air is denser and needs more fuel to keep a proper air to fuel ratio.
When colder, parts generally have more friction in them.
Warm up idle time.

In dead of winter, -35 C I can get 17-18 US MPG, in summer +35 C I can get 21+ US MPG.

[ravpickle]

And the culprit...
I didn't have a nail in 1 tire, but 2! And now with city driving only so far I am getting 21.9mpg. Phew. All better.

Now I can obsess over what to do about the tires for winter. I asked about tire tread and they said I could get another 8,000-10,000 more miles on them and not to replace (when was the last time they said NOT to replace?) I am concerned about slipping around in the snow. Keeping my fingers crossed for mild winter so I don't have to figure out whether or not to invest in better all seasons or get winter tires and save these for fairer weather.
Thank you for all the help!

[pepepeter]

Originally Posted by ravpickle

And the culprit...
I didn't have a nail in 1 tire, but 2! And now with city driving only so far I am getting 21.9mpg. Phew. All better.

Now I can obsess over what to do about the tires for winter. I asked about tire tread and they said I could get another 8,000-10,000 more miles on them and not to replace (when was the last time they said NOT to replace?) I am concerned about slipping around in the snow. Keeping my fingers crossed for mild winter so I don't have to figure out whether or not to invest in better all seasons or get winter tires and save these for fairer weather.
Thank you for all the help!

How many xx/32's will help out. The shop must have written down somewhere these numbers (at least three per tire). The mileage estimate is really not what u want; u can burn off a lot of rubber by simply peeling out and cut the 8k mileage into half easily. In my experience with the OEM yoko i was slipping and sliding all over the place with 5/32 and 6/32, lasted for about 36k miles. If i didn live in PA at the time but instead AZ like i do now, the yokos would have stayed on till 3/32 or 4/32 (~45-50k miles).