What can cause "too high" mileage?
 

Not that I'm complaining :rolleyes: but it was pointed out in another thread that the max mileage for 300Es is about 25mpg (consensus determined). I have hit 30 mpg before, and consistently get between 25-30 mpg highway. I don't normally keep a log (I mainly just do it for S&Gs), but I did do a post a little while ago with results from a trip. The two that I logged on my last trip between Philadelphia and Hartford were (note: distances are all GPS tracked, not odometer):

29 mpg:

209 miles total trip
61mph average, 86.5 max
7.2 gal to fill up (and overflow)

Coming back down, 27.5 mpg:
225 miles total
48mph average, 78.1 max (hit traffic pretty bad in CT and NY...)
8.1 gal to fill up (w/overflow)

I do know that my fuel pumps are pretty well worn and due for replacement. WOuld this contribute to increased mileage if they don't pump as much? I'd imagine the engine would run like crap and be leaned out--but I don't know enough about CIS to know if it would compensate and just have less throttle.

My 91 300E consistently gets 24-25MPG highway @ 80mph. I do wonder what I could pull off cruising at 75/65!!

Don't question good mileage. :D

If the MPGs are being derived by using 10% ethenol fuel...and you're using a high-octane, non-ethenol blend, you're going to get better fuel economy.

First off, see if the posted EPA figures are "ethenol related" and if they are and you happen to be using a high-octane (92 or better) fuel with no ethenol added, you're getting the better mileage because of that and that alone.

I noticed a 2mpg difference in my 450SLC between 10% ethenol-blended and non-ethenol fuel fill-ups.

"Corn is Porn when it comes to the Benzs."

(My saying...use it at your leisure...) :thumbsup:

Good luck finding non-ethanol fuel in a lot of states. Everything has 10% ethanol in Colorado.

Our 300E averages about 22mpg, though it is mostly driven in the city. On a few trips with it I have averaged between 23 and 27mpg.

Are you certain your odometer is accurate? Have you compared it against a measured mile and/or a gps?

Are you running excessively lean?

As mentioned in the original post, distances were all GPS tracked rather than odometer. I do only put in 93 octane in here, but I can't imagine it makes 5 mpg difference, maybe 1-2 max--I'm not sure if the fuel I've been using has added ethanol or not, I would guess no since it doesn't make sense over here on the east coast (corn not as plentiful).

If the air temp is high that can give 10% more mpg (or more). If you drive 60 that helps a lot also (vs going faster). Tailwind can make a big difference. Technically, higher octane does not yield more mpg, even though "everyone" says it does since it is "common knowledge". But octane has nothing to do with energy content, only resistance to preignition.

The higher octane fuel affords a bit better mileage BECAUSE of it's resistance to preignition. not having to compensate (more foot) for lost energy transfer due to a poorly timed mixture ignition = mileage!

My 300E is a manual transmission car and I log EVERY fuel stop in my log book. On a long distance trip several years ago I logged several tanks above 30 MPG. The overall average for the trip, which included some driving in the Guadalupe Mountains, was 29+. I would have to confirm the exact number in the log book, but if memory serves me correctly, it was 29.56.

If everything is right on one of these cars, medium speed cruising can give you this kind of fuel mileage. High speed cruising would be 100 MPH or more. Remember these cars were designed to cruise on the Autobahn well above 100MPH.

You know, that's exactly what I told the state trooper who stopped me sailing along I-10 in west TX. He was not pursuaded. Mileage on that trip (Pensacola to Sacramento) was 22.97.

Alright, got some fresh new numbers. Not trying to get anyone on here to hate me but I broke the "impossible number" yet again. Here is just about every variable I can think of:

207 miles, GPS tracked, not odometer
Average temp 72*F (started at 77, ended at 67)
Distance measured gas station to gas station, volume measured by when the pump tops off.
Gas to fill up: 6.753 gallons
207 miles / 6.753 = 30.65 MPG.

More variables:
Average temp 72*F (started at 77, ended at 67)
Total ascent: 1,538 ft
Total descent: 1,546 ft
Average speed: 61.2 mph (really good since I hit a lot of construction)
Max speed: 78.2 mph
Total time: 3:22 and 36 seconds
Humidity 81% (hit fog on the way so this varied quite a bit)
No A/C, just econ mode on the low fan setting the whole way
lights on the whole time, fogs on most of the time
Fuel was 10% ethanol (noted this)

Well, I know that there's a small variance in accuracy with my GPS, as it states my speed at 50Kmh when I'm actually doing 46~47.

Google says 205 miles but due to construction I was routed around a detour on the NJ turnpike so I think the 207 is spot on. As far as speed I do agree the GPS can be a bit wrong, but I have just found that it is delayed...It is more accurate than my speedometer though :D

High humidity and fog lowers air density believe it or not. Can help MPG quite a bit.