My car has steadily lost about 0.25 liters/ 100 km of fuel economy steadily every year since I bought it. I just figured this was the car aging normally, as I don't exactly drive it a tiny amount. When I parked it in December before I went to travel, it was getting 6-6.5 L/100 km, which isn't great, but it's colder and I do put on winter tires. It usually goes back into the 4s in the summer. However when I got back and drove it after my trip, the economy has just been unacceptable. It's getting like 8+ L/100 km.
I've done some research. There have been signs that the spark plugs may have needed a changing for maybe 70000 km. The car sometimes does rev on idle which apparently indicates misfire. Apparently that itself can cause a 30% decrease in economy. I'm deciding on the plugs. I can buy the OEM (8 CAD), cheaper iridium ones from bosch (6 CAD), or upgrade (?) to ruthenium (11 CAD). I'm deciding if it's worth it or if there's a big difference. The prices are so low that if there is even a marginal benefit, I'll do the ruthenium, but there's not a lot of consensus online.
I will also change the air filter, it's probably just time anyways.
It's so bizarre that the mileage just tanked after sitting there. I've gone through like 4 tanks of gas since I've been back and it hasn't really corrected.
Does anyone have any other strong suggestions of things I should do?
Thanks!