So basically stock 2008 jku, only mods are a smittybilt rear bumper and a body armor roof rack base. I was still at at least 19mpg. Now that it got cold BAM! I am lucky to be getting 17mpg. Any ideas why? Is this normal? I put new plugs and air filter in it and no change. Thanx for any suggestions...

 

I experience an mpg drop every winter, I'm pretty sure it's the "Winter Blend" fuel that does it.

 

Not sure where you are, but here in Cali we have winter blend gas. Costs my Jeep about 1mpg, my wife's Subaru loses about 2mpg. High ethanol fuels have less energy per pound, so less mpg.

Cold weather can have detrimental effects on mileage for other reasons as well. Cold weather will reduce tire pressure, so check that.

Cold weather alone makes the air more dense. Dense air has more oxygen. To compensate, your vehicle will burn more fuel to maintain the proper AFR. It'll also make more power, but probably not enough for you to notice.

 

Your problem may not be related to anything but the weather. Take a look at this: 9 reasons why your winter fuel economy bites! - MetroMPG.com

It is normal to drop gas mileage in the winter. Typically the further north you are, the worse the mileage will get. I even get effected here in central Texas. I keep track of my mileage, mostly because I am an engineer and like to see how the things I do to my rig affect how it operates. I even make note of the modifications I make so that I know when to expect changes.

 

Mine too has dropped about 1mpg. Just as it did this time last year. I track every single fill up. I now have over 39,000 miles on my 2012. When I get time I plan to plot the data by month to see affect of colder months.

 

I agree 100 percent! Plus warming up the jeep doesn't help.

 

I noticed it as well. First I thought what was wrong but it's the dreaded winter blend! That and I'm sure my tire pressure is low.

 

I figured it was that crap gas! Plotting the data is a good idea! Spreadsheet that stuff, unless one of those fancy tuning computers can do it for you...

 

As temperature drops, it takes time for all the moving parts, oïl, grease, ..., to get to their operating temperature. I'm used to it here up north.

If it's minus 20 and go on the highway for 2 hours to drive 120 miles, mpg will be close to the same, may be 1 mpg less, but if I make a lot of short trips, mpg will be reduce by near 50% compare with summer time. So it can get as low as 10 mpg.

Nothing to worry about, just my wallet...

 

'08 Only change was 33's. Saw 19 when I first bought it, haven't seen better than 14 since about 40,000 miles. When I went to 33's I was still getting 17, once I hit 40,000 I'm lucky to 14, hell even 12 has been the norm. 6spd.