A few months ago I bought a used 2007 Mercedes C280. Overall, I’ve been very happy with the car. The first month or so was heavy on longer highway driving, and it seemed to be getting decent gas mileage. The last couple of months have been mostly short range city driving, and the mileage seems significantly less. I wonder about the idle speed, on the assumption that it may be wasting a lot of gas sitting at stop lights and in traffic. The tachometer reading isn’t always consistent at idle, but fairly often it’s up close to 9,000 rpm. What should I be seeing, and does this need attention?
Thanks in advance for any advice (assuming it turns out to be good advice).