Generally fuel consumption comes down to usage. With my 2 JCWs and one S with instantaneous fuel consumption displayed it is quite enlightening (or depressing...) to see how one's right foot affects gas mileage.

Low tires could have some effect but not lots. Still of course check the tires cold and be sure they are properly inflated. My memory of the owners manuals for my MINIs is the inflation pressures are for loaded vehicles.

There is a decal/tag on the door jam that I think gives inflation pressures for vehicles that are less than fully loaded.

The car is used but new to you. Unless you have documented proof this was done recently change the engine air filter and while you are there the cabin air filter. I have let engine air filters go too long between changes and while I didn't really see any change in gas mileage I did notice a distinct improvement in engine performance with clean engine air filter installed.

Added: In the same vein as above check if the plugs are due to be replaced and what is in the engine now are the right plugs. I will add with a number of cars -- none of my MINIs accumulated enough miles -- new plugs even if due on miles did not effect how the engine ran.

But one time -- at 132K miles -- I had the O2 sensors replaced in another car -- the CEL was on due to a bad sensor heater -- and after the engine ran better. Yet it didn't feel like it was running poorer before the new sensors.

And some several thousands of miles later at around 140K miles on a whim I had the coils replaced. To my surprise the engine ran better.

Check MINI service schedule to see if (and then when) plugs, coils, O2 sensors are to be replaced.

Be sure you use the recommended octane grade of gasoline. Oh, and buy from a busy station to ensure you buy the freshest gasoline.

And pay attention to your right foot. Without too much difficultly I think you'll see an improvement in fuel economy,

 

Just to say great post from @RockCrusher and not attempting to disagree but just a question from my experiences...

I've found more "economical" cars to be more sensitive to how you drive them than "fast" cars are

i.e. my parents had a 1.0L EcoBoost Fiesta and a 1.0L Toyota Aygo, those both could exceed 70mpg if you drove super gently, but if you were in a rush and pushed it, not super hard just like 75mph or something and the mpg would drop to under 50

I always had slightly sportier cars and they're not too economical regardless of how you drive. My JCW does 30-35mpg in normal driving and if I try really hard it only gives me a few more, maybe just over 40

P.S. am using British gallons if anyone thinks these are overly high

 

Economical cars because they are well so economical when driven reasonably do raise an eyebrow or 3 when driven in a less economical manner.

But I owned larger cars with bigger engines -- up to 6.4l -- and more powerful engines -- up to 707hp -- and of course they could never come close to matching the MPG my JCWs delivered.

As an aside, my JCW 2.0l engine produced 228hp. If the engine was scaled up to 6.2l (the same size as the engine in my Hellcat) and if the HP scaled up linearly the 6.2l "JCW" engine would make 706hp. For its size the JCW engine is a bit of a beast.

 

Thx a lot, ill be sure to check out all of this in order to see if nothing is wrong, i should probably change the air filters since i guess that should be due. I took it recently to my mechanic to change the oil and he checked the filters and said they were all right that they could wait 2 months before they were due.

 

I don't know how anyone could tell an engine air filter had 2 months' service life left.

I had a car -- ok my Hellcat -- in for an oil/filter service at 25K miles. On a whim I asked the engine air filter and cabin air filter be changed, too. (Oh, the Hellcat was about 30 months old.) I asked to see the old filters. Not because I distrusted the service department -- as I like to point out if I didn't trust the service department I would not ask to see the filters instead I'd have the car at a service department I trusted. The SA brought out both filters. They were filthy. Clearly was past their change by date/miles. The Hellcat engine ran noticably better after the engine air filter replacement.

After I looked at the owners manual. (Better late then never.) The engine air filter was due to be replaced at 20K miles. Oops.

Which leads me to this: The 3 cylinder engine is small but turbo-charged. It consumes air like a much larger engine. Thus the air filter replacement schedule is probably shorter than it would be with say a naturally aspirated engine.