I averaged the totals of 3 trips (80% highway, making a 70mile commute everyday ) 29.1 on first tank 30.4 on second 29.9 on third. I drive at 65mph with the cruise control on... what gives X_X

 

I've had exactly the same experience. Granted it has been hotter than heck here, but I never gotten anything this low MPG even with A/C before.

Cleaned MAF. Spark plugs are iridium and are only one year old. O2 sensors are one year old. Cat is not glowing red hot.

Starts up in the morning with no problem (so I wouldn't expect a leaky fuel injector) and I just got a clean used oil analysis (no fuel in oil). Did run Redline SI-1 fuel system cleaner recently - just for maintenance.

Smooth idle. Smooth acceleration. Smooth engine deceleration. Same Shell gasoline.

I'm befuddled.

 

I'll ask the obligatory question before the others do.....automatic or manual?

A word about cruise control from my experience: (sorry if I am pointing out the obvious.) It is a WONDERFUL tool for long, flat, straight stretches of road. Remember that cruise control is taking a constant sampling rate(multiple times a second) of your speed and modulating the throttle as needed to maintain the exact speed selected. All of this constant modulation is what takes a toll on your gas mileage. Even though we have highly sophisticated fuel metering and ignition capabilities in our engine control units designed to squeeze every last possible efficiency available, they cannot deliver the same efficiency under the constantly changing fuel/air/ignition metering conditions of cruise control as they can under more constant "steady foot" conditions.

When driving an automatic, these issues are only exacerbated. For example: While on cruise control, your transmission suddenly shifts down a gear on a slight grade to get back that 1/2 mph you lost and then immediately up shifts again because you're 2 mph too fast after the downshift etc.....

Most people that are primarily going for good mileage are willing to accept minor speed variations (traffic and road conditions permitting) in the spirit of keeping a "steady foot" and keeping fuel metering conditions as consistent as possible thereby reducing unnecessary fuel consumption and increasing mileage.

There are many other variables of course including tire pressures, weight of the car, speed, rate of acceleration from stops, tire setup, wheel alignment etc....some of the simple things like tire pressure and speed are where most folks find their biggest gains in mileage. I would suggest of course checking tire pressures early in the morning before you begin your daily drive, make sure they're ~33psi (if running factory stock). Try going a tank with no cruise control and let the car accelerate/decelerate 5mph or so on climbs/descents and see if you get anywhere at all. If you don't get anywhere, maybe clean up your wheels/hubcaps and see if there's an abnormal amount of brake dust buildup on one of them after your commute (assuming you aren't stomping on your brakes a lot) that might suggest a very minor brake pad stick.

Maybe some of our hypermiling guys (if we have any) could chime in on this too.

 

Are these your first three tanks, or has your fuel economy gone down from what you normally get in your xD? The reason I ask is that it would be helpful to know what you normally get. Some people on this forum normally get 28 MPG. Some people normally get 40 MPG.

Heat is a big factor. I've been getting lousy fuel economy in this heat wave, even when the AC is off. The tank I am on now is showing 36.6 MPG (probably about 34-35 MPG at the pump). I have been taking it pretty easy, but have used the AC almost 100% of the time. I have not towed anything or hauled around anything more than me, the wife, and the baby. I normally would get about 39-41 MPG on the display, which equates to about 37-38 MPG at the pump.

 

My fuel economy has taken a dump too with the heat. I just made a 130 mile trip to drop off my oldest at church camp, going over a lot of gently rolling terrain and the cruise control was horrible, constantly gunning the gas to maintain speed. If I lay off the cruise (except on long flat roads) the economy gets much better. On average in cooler (70-80 deg) weather I've been getting 39-40MPG with my automatic.

BTW gratz on the little one Mcbrew! Mine are 13 and (almost) 6!

 

I've held off on replying since I am the second poster and did not want to threadjack from the OP.

For clarity's sake, This dip in mileage was recent. I have had Scangauge for the last year and in general was averaging 38mpg or 39mph highway. But the last three tanks were down to 32mpg highway. That was my concern.

As mentioned above,everything seemed to be checking out fine mechanically, so I was baffled. Since then I have done a compression test and it came out perfectly (210psi to 213psi for each cylinder). Considering my car has 130,00 miles on it I am supremely happy. Oh, and since the sparkplugs were out I got a chance to check them - they, too, are perfect.

The main difference is what a few of you have pointed out - the heat! This was the first time my car saw 100 degree heat since I've had it and I had no idea it would have such a big effect. The weather has cooled down a bit and my mpg's seem back to normal. I was used to the dip in cold weather but I wasn't prepared for the hit in the high heat.

Thank you all for your help. If I do discover a mechanical problem, I'll get back to posting here, but I think the problem is solved - for me at least. But I am anxious to hear from the the OP about his/her experience.

 

There are two questions not asked yet - how many miles since your last oil change with what oil weight and when was the last time you checked your tire pressure? Reason I ask is because we just got hit with a heat wave here and my mileage went up with mostly highway driving. The fuel mixture will get a little richer if your engine is running hot - so frbl check the coolent temp on your SG when you are on the highway pulling up a hill.

 

There are two questions not asked yet - how many miles since your last oil change with what oil weight

- 1000 miles since my last oil change (5w30 as always for me)

and when was the last time you checked your tire pressure?

- just checked it today and it is still at the 37psi I have the tires set at for mpg purposes (just a little higher than Scion recommends)

so frbl check the coolent temp on your SG when you are on the highway pulling up a hill.

- engine temp is one of my constant displays. During the winter it gets up to 181 degrees and stays there. In the summer it occasionally gets up to 203/204 degrees before the electric cooling fan kicks in and knocks it down a cooler range. During regular driving - especially on the highway - it usally stays between 181 and 190 degrees. Can't say I have paid particular attention to how high it rises while going up a hill. But I will now.

I really am beginning now to think the problem has been the exceptionally higher temperatures and perhaps the higher humidity, too. Just to confirm what I have said about the other mechanical aspects, the car just passed through Pennsylvania State Inspection today with flying colors. I know, those inspections don't catch everything, but they would usually catch the bigger stuff.

 

Higher temps so the air is less dense, engine itself is physically hotter so that is another factor. Fuel switch from winter to summer blend will also cause a change in fuel economy. I myself average around 26mpg per tank but that is almost 100% city driving.

 

frbl and how do you check temps. do you have special mod guage that tells you??

on my 2010 it either cold ( blue ) , hot ( red ) , nothing ( normal ) , no numbers

 

Scangauge II

Have it set to show instant MPG, avg MPG, voltage, and coolant/engine temperature

sometimes I'll replace voltage with throttle position or RPMs

simply plugs into OBD II port

 

thanks frbl, is it reliable? break easy?

 

I, like many others on these forums, love the Scangauge. Do a search on this site and you'll find plenty of comments regarding Scangauge and its use.

 

And you can also get a great deal price wise from here on SL too. I am on my second one because the newer model had more features and also paid for the upgrade of the software too. Yeah I would watch the engine temps when they get over 190 at speed that could be a factor. Too bad if doesn't display A/F ratio. It could be that the lubes are getting too hot but more likely the tires rubber is also getting softer from the hot roads and increasing their drag . . . too many things can be happening here. The fuel should not be much of a factor since it should be the summer blend with higher energy content (more BTUs). I did a quick trip from Tiverton leave 4:55am to Logan in Boston arrive at 6am and got 36.6mpg cranking 65-75mph and took it easy coming back and brought the round trip average up to 39.7mpg 128.4 miles but now my typical driving speeds has my average about 46-47mpg for this tank even with A/C use.
Check your tail pipe for soot is the only other thing I can think of.