Ever since I installed my tuning kit, I’m burning a lot of gas. Like 13L per 100km (18mpg). My driving style hasn’t changed. Maybe the occasional manual shifting to hear the burbles but that’s it. I have about 24k km on a 2017. The dealer recently did some recall update to the CPU that did nothing. I can’t figure out what it could be. I run 91 or 94 gas, and the only other performance upgrades is a dinan intake. I’m a novice at this but from what I read about potential causes, maybe the JCW has different spark plugs than the standard S?

 

As far as I know the JCW uses the same plugs as the S. The tune will decrease your gas mileage it did on mine as well.

 

Is your car an All4 or FWD? The FWD is rated at ~25 mpg? The All4 S is only rated at ~23 mpg. Dropping to 18 after a tune does not seem too unreasonable. Especially if you're goosing it more often.

Are you still running OEM wheels and tires, or are you aftermarket? Wheel weights can have a big effect on mileage, also. As can summer vs winter gas formulations.

 

I have an All4 S...stock wheels for winter are 18” on continental runflats, for summer are 19” Michelin a/s3. I’m having fun with it, so no real complaints but the gas mileage is nothing to brag about.