So why's my mileage decreasing?
 

I know, I know, mpg's the big buzzword these days. Seriously, though, I'm seeing a decrease in mine. Help me figure this out. We went from a bing, bing, bing consistent 33 and a fraction down to about 31, plus or minus a fraction.

The way I see it, there aren't that many variables in this equation--gas, driving habits, air filtration, the air itself, and tires. I haven't changed the type of gas (usually premium Texaco or Shell), or even the stations. My driving habits are the same, as our my husband's. We haven't moved. MINI says the air filter is good for 50,000 miles. In any event Hubby vacuumed out the filter when we noticed the change.

So that leaves air and tires. Yup, it's pretty warm, and obviously the engine isn't as happy, BUT, we didn't have this problem last summer (or the rest of the year--it's pretty hot much of the year here). As for the air IN the tires, it's been checked--nothing new there. I'm running about 37 in front, and 34 in the rear, plus or minus a pound between checks.

The tires ARE new, though. We went from stock 16" runflats to 195 x 55R 16" Bridgesport Potenzas. So no change, right??? Or is there that much more rolling resistance with the Potenzas?

http://i155.photobucket.com/albums/s...sign0134-1.gif for your thoughts.

I would try boosting the tire pressure to see if it makes a difference. It's a cheap and easy diagnostic.

I run 38 psi front and back with Dunlop runflats, and have been getting 34-35 mpg on the highway at about 70+ mph.

You running the AC or have it on auto? That will do it.

What air pressure are you running in the tires?

Perhaps you are running the air conditioning more due to the summer heat. I've heard stories that an AC compressor can suck up 20 hp. That might reduce your fuel economy. I run mine 100% of the time at this time of year in Las Vegas, NV and I notice a reduction in fuel economy around 1-2 mpg. Maybe a possible explanation.

A/C is on, as usual--same as last year. I don't want to increase air pressures as I don't like the way it handles when they're boosted. In any event, there's been no change. Robin, as I posted, "I'm running about 37 in front, and 34 in the rear, plus or minus a pound between checks."

Nope--this is a CHANGE. Not different pressures, same gas, also a/c the same, temps the same (mid 90's), etc. The only thing I can think of is more rolling resistance with the Potenzas.

Unless, of course, there is also a change in the gas itself. Did they change the formulation over that of last year's? Pumps in this area (north of Houston) are posted "up to 10% ethanol." Was there a change in that? Hmmm, maybe the station is shorting us? Then again, I'm stopping at about the same point each time, and filling with about the same amount of gas, given the usual variability.

Oh, one other thing--I'm still working at home, and still driving the same roads, so it's not traffic patterns.

I'm really stumped. Anyone else had a change when they went to non-run flats?

Maybe someone is sneaking out of the house in your Cooper in the middle of the night and driving it like a madman or madwoman... :grin:

The heat should help your efficiency while reducing the power output. The ethanol content can have a fairly noticeable change in mileage. You should probably figure out if there was a change there. Just less energy per volume in ethanol so you need more. Tire rolling resistance should make that much of a difference either unless you have gone significantly wider or were previous on low rolling resistant tires, which are not standard. 31 sounds pretty bad for these cars, i still am averaging above 32 with an autocross or two on the tank.

how long since your last oil change? I noticed my MCS getting about 2-3mpg less on the freeway (35mpg was my average) around 3800miles. I changed the oil myself at 4200miles. After the oil change my MPG went up to 36mpg on the freeway (hand calculated both times). I'm sure there might have been some outside factor that might account for 1 or 2 mpg increase but not 4mpg difference.


just something to consider.

Interesting. I had the oil changed about 2000 miles ago.

Maybe I'm sleepwalking? Hmmm--I HAVE been wondering why I've been finding my keys in the freezer. Oh wait--those are just senior moments...

What oil are you using? Same as before?

It wouldn't surprise me if the ethanol content has increased with the price of oil, although it shouldn't go over 10% without a message on the gas pump.

31 is bad? Huh? The 2007 Cooper S is rated 25-32 by the EPA. Driving it like a grandma I've never seen better than 33. Driving normal (WOT a few times per drive, shifts at 3-4k, AC most of the time) I get 30 mpg like clockwork. Edmunds is averaging 26 with their longterm 2007 MCS MT6. http://blogs.edmunds.com/roadtests/2...my-update.html

I'd be happy with 31 if it meant I didn't have to keep the AC off and coast constantly.

Robin--oil is whatever MINI is doing, since I had an annual oil change, then another one when the counter did its thing, which was about 3000 miles later (go figure!).

I'll double check the ethanol, but the last time I looked it was "no more than 10%" or whatever the wording is in Texas.

bdgc--you mentioned coasting. I've heard two arguments about coasting. One says coasting is bad because the injectors are working constantly vs. engine braking when the injectors are off. The other, more intuitive one, says that you go farther while coasting compared to the distance covered when you are off throttle and it is slowing down, but in gear.

I HAVE been coasting more--again, to me it's the more intuitive way to increase mpg, but maybe that's it? Thoughts?

Now you guys got me thinking about the air in my tires...I only have 350 miles on the car and checked the air a few days ago (cold check of course after it sat all night) and I got 33psi on all 4 tires....think I should bump it up? If I remember correctly I've got 16" continental runflats that came with the sport package.

If all things have basically stayed the same then maybe it is the change of the tires themselves causing the slight decrease in MPG's.Keep in mind I am no tire expert so this is just a thought.

As the SUV owners trade in their gas guzzlers for small cars your opportunity to draft behind a SUV decreases. So the mpg goes down. It is not you or your MINI but it is the environmental change that caused it IMO.

You could try, its supposed to get better mileage, though I've never noticed that when I came to fill up before. I'm trying that again now as an experiment and I think the ride is a bit harsher and bouncyer, also it could compromise grip so I'd do it gradually and see if you notice a difference. (In either MPG, ride or handling.)

http://i155.photobucket.com/albums/s.../happy0193.gif

Did you see the Mythbusters program on drafting? They compared various distances behind a semi, and of course, the best was pretty darn close--just a few feet. There was a surprising percentage gained in a reasonable distance behind the truck, but of course the trailer prevents you from seeing ahead, or the truck driver from seing you. So, ya takes your chances if mpg is number uno on your preference list.

I have a similar situation to yours. I've noticed that Blue Belle's mileage has gone down in the past few weeks from 37 to about 34. I also just got new non-run flat Bridgestone tires (3 weeks ago) and just had MINI change my oil (2 weeks ago). :) I've assumed, however, that my mileage is being affected by our lovely Central Texas heat. My AC seems to always be running at full blast.

The only other car that I've had and paid as much attention to on mpg was my TDI Beetle. In the summer my Beetle's mpg would go from 45 down to about 42.

Are ALL of the spark plugs in order? Probably are.

Since the Houston area is now hotter than hell, giving up a couple MPG due to blasting the AC is acceptable.
I'm in DFW and it's HOT.

Still, I rarely need to take the AC over half speed.
The Mini AC is far better than my old BMW.

Velly interesting... I think it's time for a chat with Tire Rack, don't you? Other than the a/c, heat, and possible gas change, I don't know what else to check.

What about my question on coasting vs. in-gear deceleration?

In all seriousness, a couple of miles/gallon shift really is noise, could be the roads, the type of travel you have be doing, the length of time at traffic lights. Perhaps due to the increase of the cost of raw material the fuel companies may be cheating on the quality of the product to get better margin. I know in upstate NY we went from 3 to 6% ethanol in summer to a constant 10%, now my mileage in all my vehicles is poor. Byproduct of our current economy and leadership.

Very true that it's not much of a change, but after a consistent 33+ over more than a year, and now a definite change to 31, it IS worth wondering about. After all, that's about 8% less.

My prevailing theory is the change in tires, since that perfectly coincided with the decrease. I'm going to call Tire Rack tomorrow.

8% really is out of the range of a the tire impact. I don't notice much of a change between the stock all seasons i run in the winter and my 215 aggressive summer tires. usually do a decent amount better on average in the summer just due to the temperature changes, but right when i shift over I don't see anything noticeable. Should get better with grippy tires, no reason to slow down as much for corners :thumbsup:

Chili, you've mentioned a couple of times that there's an advantage with summer heat. If this is true, why all the cold air intake mods? Or am I missing their purpose?

cold intake air is good, but cold engine block and cold exhaust is bad. Winter with the turbo will give you more power with the colder intake and more effective intercooler, but in the hot summer you wont have nearly the mpg hit due to warm up and keeping the exhaust temp up helps out with the efficiency. Perhaps if you are just always running the AC in Texas you never see the benefit. Here in Ohio its just not that hot and even running the AC is mostly just to get the humidity out. I see a significant difference, solid 1-2 mpg. Ive never gotten through a summer tank without an autox on it so its hard to tell exactly the impact with the mini.