Compression

[Vinalero]

I went to the garage today to get an oil change. (its effin cold and dirty outside and I dont have access to a garage )

I figured I would ask to check some things while im there. Im wondering why I cant do better mileage with the car (360 km out of 48 litres that makes 13,3L/100km or around 16mpg) with a 4 cyl.

I ask the guy to check my compression, and to check for some codes in the computer.

I found out my spark plugs were offset (55 gap minimum, one had 60 when they say that factory its a 50 gap) So I asked for some new plugs. Also, compression is good in cyl 2-3-4 (around 200 in each) but in cyl. one he got 100-130. First of all he put a small amount of oil in cyl by the spark plug hole to check if it was a ring failure. He got 280 compression when trying it after that so he told me its a ring problem.

After he told me maybe the cylinder was "washed by gasoline" (I didn't really understood that, but basically he explained me that if the spark was bad, then the fuel that was goin in the combustion chamber was "washing" the piston, and that could cause a lost of compression)

Assuming that, he told me to do some milleage with the car this week, and get back to check if with the new spark plugs, the compression was back to normal. Else it would be either a injection module problem or a piston ring problem (which brings me to a rebuild which is a "dont want to/cant afford" kind of job)

There was absolutely no codes in the computer, and we checked the A/F ratio and it is 14.6, which is from what i red here the correct value.

Questions are:

Was he shitting me?
What could be wrong?
Is it really a bad problem?

[Vinalero]

dumbidibump

[Oldsman]

bad plugs can cause bad mileage. i would try fuel injector cleaner in the car and run high octane check your mileage then put in low grade gas check mileage then back to high grade. also try different gas stations. you should get 24-26 in town unless you keep your foot to the floor a lot. then you are the gas mileage problem.

[jayson_waltz]

dunno. i only get about 16-17 around town but my foot is usually on the floor. hence 1 blown within the first week i had it. but yea get good plugs see if ur cylinder is still getting washed.

[Vinalero]

I'm goin back to the garage next monday, i'll recheck the compression on the cylinder.

For the bad mileage, I usually shift around 3000 rpm and I'm not pushing it. (It happens, but not enough to make a big difference)
I generally fill up with high octane once a month. I just hope I get higher mileage with new plugs and correct gap.

Any other plausible situation that can decrease mileage?