Mercedes Idle Speed

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).

9,000??? You mean 900, don’t you? It’s not adjustable, it’s controlled by the computer. What is your mpg on the freeway, and in the city? It will be much lower.

Oh yes, it’s 900. Missed that typo, sorry. I only have rough estimates, but based on miles/tankful I get near 25 mpg on the highway, and more like 15 in city driving. I expect city to be lower with a non-hybrid vehicle of course, btu that just seems like a lot relative to other cars I’ve owned.

I did just look up that year and model on, and it lists it as 24 hwy and 17 city, which isn’t far out of line with what I’m seeing, so I guess there’s nothing to worry about. Little disappointing for a car that size I suppose, but other than that, I am happy with it.

Miles per tank full? You know fuel efficiency is measured in Miles Per Gallon for a reason, right? Record odometer reading, fill tank. Drive until you need to fill up again. Note number of gallons to fill. Record odometer reading. Subtract the first odometer reading from the second, divide this by the gallons needed to get Miles per gallon.

What size engine do you have? This isn’t unreasonable mileage for a V8. Also, what octane fuel do you use? If your car requires higher octane fuel, then definitely use it, but otherwise don’t–it will waste your money and typically you get less MPG out of higher octane fuel.


OP has a V6, not a V8

The car definitely requires premium fuel