rolodoc

I took my 2008 C300 Sport in for service A last week and was given a loaner car for 3 days. The loaner car was a 2009 C300 Sport which I drove for about 200 miles. What is interesting is that the 2009 had much better mileage on the trip computer than my 2008. Both are automatic transmission.

I noticed the main difference in the trip computer is when I press reset mileage "From Start", the 2009 resets at 25 MPG, while my 2008 resets at 21.5 MPG. This is on local road and on freeway. Freeway MPG ends up about the same after 10 minutes drive, averaging 29-30 MPG on both cars. However, local driving was big difference, with 2009 getting 22-24 MPG while my 2008 gets 18-20 MPG.

I don't understand how this could be, since both cars have the same engine, same weight, same transmission, and same EPA mileage estimate.

Can anyone verify if the 2009 trip computer always reset at 25 MPG, while the 2008 resets at 21.5 MPG and explain why the difference?

jstaneff

The trip computer, when reset, will reflect your activity. There is no set point where it reaches 21.5 or 25, or whatever.

If a newer car gets better mileage with the same equipment and driver, then it's likely that you've trained your car, over time, to perform in a specific way. Try to reset the computer (procedure listed elsewhere, involving a 2 minute press of the accelerator, etc.) to see if that makes a difference.

If it doesn't, then maybe something is wonky in your car?

rolodoc

These trip computer MPG reset numbers are constant on freeway or local driving, so it seems independent of the current activity. I was aggressively driving the loaner 2009 C300, flooring the gas and still seeing 25 MPG everytime I reset the trip computer. I drive my own 2008 C300 very conservatively, so I would expect my 2008 to show better mileage numbers.

What MPG are people seeing immediately after reset of the trip computer?

jstaneff

If you're resetting while driving, then the mileage will always drop to some lower point than you expect, and it will take some miles of driving to "catch up" with the real world.

Even when I'm seeing my trip mileage climb through 30 mpg for the whole trip, if I reset the indicator, it will drop several mpg lower. Why? To me that smacks of bad programming, but what do I know? I do know that it will quite possibly not catch up to where it was before I finish my trip.

So: I rarely reset the trip mpg indicator while driving. Only for the whole trip. I find that more satisfying. When I hit 32 mpg, it just feels good!

rolodoc

I agree the MPG first drops when the trip computer is reset. I'm used to seeing that on my 2008. What got my attention was when driving the 2009 C300, the trip computer resets HIGHER, to 25 MPG even when driving aggressively on local road.

At least for that particular 2009 C300, its trip computer has 25 MPG as default and it seems to have much better mileage than my 2008, which I drive very conservatively. I'm tempted to ask the dealer for a trade.

ljn21

my wifes C300 is the same - regardless of how we have been driving and what the average or since resent mpg is showing, push reset and it always goes to 21.5 as a starting point and then goes up or down from there. No kidding, it goes to 21.5 every since time. We have even tried reseting while sitting still at the gas pump right after filling up and still sitting in the gas station lot so reseting it should read low as there are zero miles going on and the engine is running - goes to 21.5. We spent a few months in Florida for my work and on the drive back home, we averaged 31mpg - filled up with gas, reset, and voila, 21.5. Strange if you ask me.

drpasteur

for service A --> takes 3 days?

rolodoc

Dropped my car off on Wednesday for Service A and got loaner car, should have finished same day but they were backed up so it wasn't ready until next day. Since I live 50 miles away from dealer, they let me come back on Saturday to return the loaner car. During those 3 days, I had fun playing with the 2009 C300.

rolodoc

This confirms my observation about the 2008 C300 trip computer always resetting at 21.5 MPG. Can anyone report what MPG your 2009 C300 trip computer resets at?

Diesel Benz

It always starts from a figure close to the average mileage figure, at least in Europe we have official testing for highway, city and then a combined or average figure is derived from the two. That is the figure the cluster meter starts from. And then when you start driving, it gets gradually lower (on my CDIs at least). Oh, I should have clarified, lower in litres per 100 km, higher in mpg.

RickP330

4matic vs non 4matic????
RP

rolodoc

Both 2008 and 2009 C300s were non 4matic.

FrankW

mileage / gallon = mpg. do your own math instead of just looking at the trip computer.

also you'll avg better mpg if you don't have the optional 18" or aftermarket wheels. the stock touring tires have much less rolling resistance.

Will38

I know this has nothing to do with the topic, but you got a loaner car for service A? OMG I wish they gave me a loaner car.. dam those jackasses... Plus doesnt it only take 1 day to get a service A? Btw does it cost anything to get a loaner car or is it free?

jstaneff

Okay, I reset my trip meter last night to test this in my 2008 C300.

Turning on the car, after it had set for more than 5 hours yielded an mpg of __.__, followed within seconds by 21.5. Letting the car idle caused the numbers to visibly and quickly drop by about .2 mph in every second. Then hitting the reset got me 21.5 all over again.

So, I'm thinking that this is a programmer's heuristic. The basic assumption is that you start your car when you're not moving (0 mph), and you reset it when you're not moving. That means that the 21.5 is fairly close to the 19 mpg we are supposed to get in city driving (and 21.5 might be more accurate in Germany?). Why did they move it up to 25? Dunno, maybe more of the diesel cars, and 4-bangers are getting better mileage and the new 25 is designed to reflect how poor our 6-cyl cars do?

misterng

I got a loaner everytime I drop my car off (doesn't matter if it takes a few hours or couple days). If you live in Bay Area, try Autobahn Motors in Belmont next time and ask for my SA, Karl Kreitner (very good guy).

rolodoc

Thanks for the confirmation on 2008 C300. Can someone do the same test on a 2009 C300?