Lots has to do with tires and rims....heavy rims are a contributing factor to poor mpg....tire choice and size is too...
With about a 45 mph avg speed, on the trip computer, I often do 30+ if I drive sanely with lower rpms....on a modded 2005 s....

 

My car has the Dinan badge, and from what I can tell, the only thing that has been upgraded is the CAI and exhaust. I had to put a trans in it and wheel bearings, when that was done i upgraded the ignition to MSD, wires, and new plugs, haven't seen any mpg improvement, then again, with the new clutch I haven't been keeping my foot out of it either. I drive very spirited, I run with the cruise on going to work and have it set at 70 ish. I get almost 300 to a tank, and that is combo highway/city.

 

I agree, I thought mine would get better mileage too. I have an '05 MCS, I normally get around 27 miles a gallon. I have found that the key is keeping it out of the supercharger lol. When I can cruise under 3k RPMs, between 58-63 MPH, I get some really good results on trips.

 

Ok this doesn't make sense to me. You choose to keep it out of boost? I have to keep it out of boost until my oil warms up, it's annoying to keep it out of boost. That's the whole reason I bought the r53 instead of the r50, for the blower.

 

I have an '03 MCS with 99kmiles. I just did a "Rhode" trip to Western Maine, averaging (by computer estimation) 33.6 MPG. By comparison, when I bought the car four years ago, I got 36 MPG on the return trip from Portland, Maine to Providence, Rhode Island. Since I am in Rhode Island, it is hard to put on any length of highway miles "without leaving the state".

So, given the mileage was worse than when I purchased it and that MINI suggests a 100kmile change interval, I replaced the original spark plugs (marked BMW NGK BKR6EQUP) with new NGK BKR6EQUP plugs. Afterwards, I took the car on a little highway trip all the way out of state into Massachusetts and back! The result? 33.1 MPG.

Wondering if it is O2 sensor related...

 

Remember your motor now has nearly 100k miles and has worn over all those miles. So the mileage may not be what it once was. I averaged 24 mpg before I had a stronger motor built. I think i may have averaged closer to 26mpg way back in 06 when it was new.

The replacement interval for nearly everything on the r53 is less than what mini says. 10-15k oil changes. Nope 5k. Serpentine belt 70k, nope 15k. Plugs 100k, nope 15k-30k. Upper motor mount, lifetime... Nope 30k. Trans fluid, lifetime... Again nope, 50k probably less. Whatever the o2 sensor interval is... Nope, less. Way way less if you use Shell gas. I used shell exclusively for a couple years and it killed my main o2 sensor. Additives in the gas.

 

O2 senors dont last forever, it could be, you could also have drafted less or had a head wind, heavier foot, driving style makes a big difference as you can see by this thread. i normally see 28 people normally see 25, some 22, some 30

i have gotton 34 for a bit going at 65 mph on a highway relativly flat, i dont normally get that, most of the time i drive to RI ill get 31 but i ussually travel at 70-75 cause us i-95 folk are cray cray!

 

One extra tip....
for those of you that KNOW you are driving gentle enough and still get poor MPG,,,say under 25...
Look at the rear brake caliper....
on the MINI they tend to seize up and drag a bit....for me it was the CALIPER, NOT the cables....
I just replaced one....and looks like I am gaining another 1.5 MPG!! Will know in a couple days..have only driven 100 miles or so.....

 

I can see how spark plugs and O2 sensors could degrade mileage. In your estimation, what other components subject to engine wear degrade mileage?

One remarkable take away from the spark plug change, for me, was that even after 99kmiles, the sparkplug electrodes looked to be serviceable, like they could have gone even longer than 100k. There was some normal discoloration on the ceramic insulator. So it really should not have been too surprising to me that my mileage essentially stayed the same.

I think I am going to change the O2 sensors and see if that improves the mileage.

 

I am gravitating toward old O2 sensors more than driving style. I deliberately drove in a similar style for the comparison. Environmental conditions such as wind could contribute of course...