Reg versus Premium Fuel experiament in 09 PT Cruiser

I have pretty much gotten a decent feel for the gas mileage to expect
from my PT. The last three tanks averaged about 23 mpg. So when I
filled up yesterday I put premium in it instead of regular. It's too
early for a definitive answer but so far it looks like it's down 2 mpg
over what I'd been getting. About what I expected but I thought I'd
test it out.

Ashton Crusher wrote:
> I have pretty much gotten a decent feel for the gas mileage to expect
> from my PT. The last three tanks averaged about 23 mpg. So when I
> filled up yesterday I put premium in it instead of regular. It's too
> early for a definitive answer but so far it looks like it's down 2 mpg
> over what I'd been getting. About what I expected but I thought I'd
> test it out.

I don't know for sure, but there are those that would say that you have
to give your PCM time to adjust to the different fuel to make a
meaningful mileage measurement.

--
Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

On 2009-11-02, Ashton Crusher > wrote:
> I have pretty much gotten a decent feel for the gas mileage to expect
> from my PT. The last three tanks averaged about 23 mpg. So when I
> filled up yesterday I put premium in it instead of regular. It's too
> early for a definitive answer but so far it looks like it's down 2 mpg
> over what I'd been getting. About what I expected but I thought I'd
> test it out.

It should be down a little. Premium has less energy per unit volume.

> > that is why running hi-test in a car designed to use regular is a
> > waste of money.
>
> It may or may not be a waste of money (the only way to find
> out for sure is to try it). The EPA specifies higher octane fuel for
> its fuel economy tests - so it would stand to reason that some cars
> designed for regular fuel would get slightly better mileage with
> increased octane.
>
> -jim

You've got that exactly backwards. Octane is a measure of the fuel's
resistance to pre-ignition (knock). This means higher octane fuel
doesn't burn as easily. Thus, if you put high octane fuel in a car
designed to run on regular (like U.S. 87) then your fuel economy is
likely to DECREASE slightly.

While this isn't technically correct, you could think of high octane
fuel as having less potential energy. The reason high octane fuel does
OK (mileage wise) in a car designed to used high octane fuel is that
high octane engines tend to be high compression. Thus, the engine gets
more energy out of the fuel. -Dave

Bill Putney > wrote in
:

> Ashton Crusher wrote:
>> I have pretty much gotten a decent feel for the gas mileage to expect
>> from my PT. The last three tanks averaged about 23 mpg. So when I
>> filled up yesterday I put premium in it instead of regular. It's too
>> early for a definitive answer but so far it looks like it's down 2 mpg
>> over what I'd been getting. About what I expected but I thought I'd
>> test it out.
>
> I don't know for sure, but there are those that would say that you have
> to give your PCM time to adjust to the different fuel to make a
> meaningful mileage measurement.
>

if the motor is not knocking with regular fuel,then changing to hi-test
isn't going to make any difference.ECUs don't have any way of discerning
octane levels,and the O2 sensor will not read different for hi-test.

that is why running hi-test in a car designed to use regular is a waste of
money.

--
Jim Yanik
jyanik
at
localnet
dot com

"Brent" > wrote in message
...
> On 2009-11-02, Ashton Crusher > wrote:
>> I have pretty much gotten a decent feel for the gas mileage to
>> expect
>> from my PT. The last three tanks averaged about 23 mpg. So when I
>> filled up yesterday I put premium in it instead of regular. It's
>> too
>> early for a definitive answer but so far it looks like it's down 2
>> mpg
>> over what I'd been getting. About what I expected but I thought
>> I'd
>> test it out.
>
> It should be down a little. Premium has less energy per unit volume.

That used to be true (say 30 years ago), but these days it is not
ture.

Ed

Ashton Crusher wrote:
> I have pretty much gotten a decent feel for the gas mileage to expect
> from my PT. The last three tanks averaged about 23 mpg. So when I
> filled up yesterday I put premium in it instead of regular. It's too
> early for a definitive answer but so far it looks like it's down 2 mpg
> over what I'd been getting. About what I expected but I thought I'd
> test it out.


I had a Neon RT. I did an extensive milage test early on. I did ten
tankfuls of regular, then ten of premium, figuring the variance of each
set. The milage with premium was down a little, but less than one mpg.
However, the variance in each set of runs was over 1.5 mpg, so I had
to conclude it made no difference.

I think the Neon engine was very similar to that in the PT (though mine
had the DOHC heads).

On 2009-11-02, C. E. White > wrote:
>
> "Brent" > wrote in message
> ...
>> On 2009-11-02, Ashton Crusher > wrote:
>>> I have pretty much gotten a decent feel for the gas mileage to
>>> expect
>>> from my PT. The last three tanks averaged about 23 mpg. So when I
>>> filled up yesterday I put premium in it instead of regular. It's
>>> too
>>> early for a definitive answer but so far it looks like it's down 2
>>> mpg
>>> over what I'd been getting. About what I expected but I thought
>>> I'd
>>> test it out.
>>
>> It should be down a little. Premium has less energy per unit volume.
>
> That used to be true (say 30 years ago), but these days it is not
> ture.

If higher octane ratings are achieved through oxygenates it certainly
will be lower because those high octane oxygenates have less
energy/volume. I think it is highly unlikely that higher octane ratings
would be achieved through aromatics these days for fuels one can buy at
regular gas station.

http://www.epa.gov/oms/rfgecon.htm
http://books.google.com/books?id=J_A...olines&f=false

Jim Yanik wrote:
>
> Bill Putney > wrote in
> :
>
> > Ashton Crusher wrote:
> >> I have pretty much gotten a decent feel for the gas mileage to expect
> >> from my PT. The last three tanks averaged about 23 mpg. So when I
> >> filled up yesterday I put premium in it instead of regular. It's too
> >> early for a definitive answer but so far it looks like it's down 2 mpg
> >> over what I'd been getting. About what I expected but I thought I'd
> >> test it out.
> >
> > I don't know for sure, but there are those that would say that you have
> > to give your PCM time to adjust to the different fuel to make a
> > meaningful mileage measurement.
> >
>
> if the motor is not knocking with regular fuel,then changing to hi-test
> isn't going to make any difference.ECUs don't have any way of discerning
> octane levels,and the O2 sensor will not read different for hi-test.

Actually the computer does have a way of detecting octane - it is called
the knock sensor. And the O2 sensor may read differently if the fuel
oxygen content is different, which may or may not be the case depending
on where you get the fuel.


>
> that is why running hi-test in a car designed to use regular is a waste of
> money.

It may or may not be a waste of money (the only way to find out for
sure is to try it). The EPA specifies higher octane fuel for its fuel
economy tests - so it would stand to reason that some cars designed for
regular fuel would get slightly better mileage with increased octane.

-jim

Brent wrote:
> On 2009-11-02, Ashton Crusher > wrote:
>> I have pretty much gotten a decent feel for the gas mileage to expect
>> from my PT. The last three tanks averaged about 23 mpg. So when I
>> filled up yesterday I put premium in it instead of regular. It's too
>> early for a definitive answer but so far it looks like it's down 2 mpg
>> over what I'd been getting. About what I expected but I thought I'd
>> test it out.
>
> It should be down a little. Premium has less energy per unit volume.
>
High Octane has the same energy. It has a higher OCTANE and is wasted or
may not be burned as completly in a low compression motor or with
retarded or less advance in the timing of ignition. It burns slower and
does not detonate under heat of compression as easily as regular.
Fuel that uses more ethanol to increase octane has less energy. Regular
fuel with ethanol has less energy.
A 12 to 1 compression or even 14 to 1 compression motor burning 105
octane or higher will get better mileage and torque if the ignition
curve etc are right.
Just like diesel the motor has to be built for the stress.
What we have now and for a long time is junk engines designed to be
built as cheaply as possible and to run on junk fuel as per EPA or
California really. The electronics are good at getting the most from
junk. Just imagine what great engine structure and electronics would do
with great fuel.