Are MPG ratings much less accurate then they used to be?

When I bought my old Concorde in 94 the MPG ratings were pretty much spot
on, the car got 22 in daily driving and 29 on long highway trips which is
what the window sticker claimed. The MPG ratings for the new 300C is 17
city/25 highway which isn't even close to the real values, I'm getting
15-16 in daily driving, just did a 220 mile round trip yesterday and it
peaked at 20 MPG. The article on hybrids (quoted in another thread) also
mentions that the real mileage is nowhere near the sticker values. So my
questions are these, has the method for determining the MPG ratings for
cars changed in the last 10 years? Have manufacturer's gotten better at
gaming the system? How do they determine the MPG ratings? It's hard to see
how Chrysler managed to come up with a 25 MPG highway rating on the 300C
unless they did the test on a road that was down hill all the way.

 

AC is off during tests. That is why hybrids results are ridiculous. As for
the Hemi, perhaps it was optimized to turn off extra cylinders during the
test and your driving habits do not mirror those test circumstances.

 

The government determines the fuel consumption ratings not the Manufacturer.
The tests are conducted on a flat surface without using air conditioning,
and using average temperatures 70 - 80 degrees. And does not factor in,
stop and go driving ,or different road grades.

 

Hard to believe that AC makes a 25% difference in a 4000lb car. The flat
surface for the government tests could be the reason for the huge
difference. 300Cs are very heavy and New England (where I live) is very
hilly even on highways. Lugging a 300C up a hill probably puts enough
strain on the system that all 8 cylinders are engaged which means that you
are going to use much more energy going up a hill then you get back going
down the other side because the engine is operating in it's less efficient
mode on the upside of the hill. I wonder how hybrids behave on hills. On
the one hand a true hybrid like the Toyotas will have to engage it's
gas engine more on the other hand they recover more energy on the down
side of hills because of the regenerative breaking.

 

Actually MPG is determined by unused energy that is left in the emissions it
is a chemical analysis with factors of weight and other physics that overall
determines the MPG all this is done on a dyno. in a very controlled lab.
I used to know the formula (sorry be awhile since I worked in the lab)
the only way to truly achieve the listed MPG is with a lot of down hill
driving.

 

In the hybrid, as soon as AC goes on, gas engine goes on. That is why EPA
test results are a joke.

 

They never were accuate. Those tests are done by starting the vehicle up
and driving with out a load on a lift and run untill the tank is empty,
There are no cold starts, (just one start up) no a/c or stop and go
traffic.
Glenn Beasley
Chrysler Tech

 

I don't recall what the EPA MPG ratings were for our '02 300M, but
driving back from Chicago to W. Michigan yesterday w. A/C on (outside
temp. in the 90s) the EVIC showed an average of a little over 27mpg.

BTW, does the grade (or brand) of gasoline affect mpg? I sometimes use
89, but mostly 87.

Perce

 

No, except if the motor is into serious knocking. Too much octane is a waste
of money and resources.

Richard.

 

Outdated info, no longer correct. All modern engines use knock
sensors. Depending upon how the knock sensor and ignition timing control
is implemented and calibrated, using fuel of higher octane *can* improve
mileage by permitting more spark advance. Whether the increase in mileage
balances the increased cost of higher-octane fuel is situational.

 

The Owner's Manual for our '02 300M says 87 is acceptable, but 89 is
preferable -- but doesn't say why.

Perce

 

Yes, but too much octane is a waste of money and resources. If a vender
recommends something higher than 87 but allows 87 as an alternative, then
your comments are on point. One benefits from the higher octane the most on
a hot day pulling a load, but your point is well taken.

Richard

 

Depending on how the knock sensor and ignition timing control is
implemented and calibrated, using fuel of higher octane can improve
mileage by permitting more spark advance. Whether the increase in mileage
balances the increased cost of higher-octane fuel is situational. Which
part of that is hard for you to understand, Richard? If using
higher-octane fuel nets sufficiently higher mileage to more than offset
the increased cost per unit volume of the higher-octane fuel, then it is
neither too much octane nor a waste of money.

 

If that's what the owners manual says then do not use a higher octane rated
fuel. It will cause more drivability problems and leave more deposits in the
combustion chamber possibly causing a pinging problem
It will not improve MPG . If anything it will cause a lower MPG over a
period of time.
Glenn Beasley
Chrysler Tech

 

Don't you mean a lower octane? Since when does higher octane cause more
deposits?

 

No

Since when does higher octane cause more
deposits?

Since certain vehicles were designed to use it depending on the
compression ratio. The octane rating of gasoline tells you how much the
fuel can be compressed before it spontaneously ignites. When gas ignites by
compression rather than because of the spark from the spark plug, it causes
knocking in the engine. If a vehicle that is designed to use a low octane
fuel 87-89 uses high octane fuel consistently 93+(slow burning fuel) it
leaves deposits on the pistons, valves and so on.
This will eventualy cause compression problems.
Im no chemist and im sure there are people on here that may be able to
explain it better, but i will say this, I have pulled down many heads from
vehicles that have excessive amounts of carbon deposits on the valves ( caus
ing lack of compression, check engine lite on) because the owners insist on
using the wrong octane rated fuel in their vehicles, And Chrysler does not
warranty fuel related problems either

 

I just checked the owners manual for my 300C and it says that use of
premium fuel is not recommended. If an engine like the 5.7L Hemi doesn't
use premium fuel what does? Are there any modern engines that need premium
fuel or are the oil companies just counting on consumer ignorance to get
an extra dime a gallon?

 

What do you/they mean by "premium"? 93 Octane, 91, or only 89? ISTR that
DC recommended 93 Octane (or perhaps it was 91) for the '02 300M Special
*if one wanted the maximum power*, otherwise 89 Octane would do. As I
wrote earlier, for the regular '02 300M they said that 87 is OK but 89
is better -- or perhaps they used the phrase "for optimum performance."

AFAIK, all kinds of factors could affect the required Octane rating,
e.g., size and shape of the combustion chamber, as well as the
compression ratio.

Perce

 

Premium is defined as 91 or higher. The 300 owners manual says use 87 for
the 2.7L engine, 89 for the 3.5L and 5.7L. It also says that use of
premium fuel is not recommended. Don't know if that's because it's a waste
of money or because it damaging as maxpower suggested.

 

Interesting. On a recent 280 mile trip my 2000 3ooM achieved 27.8 mpg on
the interstates, full vacation load, wife, kids, dog, full trunk, etc.

I usually get a max of 26.5 mpg. Only changes were a fresh load of Mobil
1 10-w30 synthetic oil , try to keep it down to 65 mph, and 32 psi in
the tires (my old tire gauge was giving false readings, about 3 pounds
low)