All you RX8 owners that live in states that allow
you to improve your engine I think you can get better
miles to the gallon by using MSD coils.

If so measure the gas mileage before and after.

The most accurate way is get a OBD II software and
USB cable for a notebook computer.

That will give you the mass airflow every second and record
it on the hard disks.

Then set the cruise control to 60, 65, 70, 75, and 80

When you are getting better MPG the mass airflow will
go down for steady speeds. You will know this right
away.

My theory is these MSD coils will help greatly in
those speed ranges.

Paul Lamar
I've kept track of every tankful of gas that I've put in the car
since I bought it in 2003, and have a spreadsheet of all the
mileage data.  About 6 months ago, my average went from 17-20
mpg, to a minimum of 22 miles per gallon for every tank! And was
climbing slightly every tankful until I changed the coils. My
highest was the week before I changed the coils...24.7 mpg. Same
gas, same station, almost always the same pump. Only the price
keeps changing!
Since I put the new coils in, the last three tank fulls have been
18.5, 19.1 and 18.2! I calculate it by using the amount of gas
reported on the pump when I fill it (I fill it up every time I
get gas) and the mileage on the trip indicator, which I then
reset.  After 65,000 miles odometer indicated, the total in my
spreadsheet is within 100 miles.
Harley

Very interesting.
If this proves to be true in the long term my guess is Mazda
changed the primary winding to cool the new 100 B coils. The
effect of this is less spark energy I suspect. I can check this
perhaps on the test bench as I have both the early coils and the
new 100 B coils. If that proves to be true that is a bit of
evidences that more spark energy over a longer period of
time  will improve the BSFC. Something I have suspected
about the rotary moving combustion chamber for a long time.
The MSD coils come closer to this ideal than anything I
have seen so far.
http://www.rotaryeng.net/Why-CDI.html

When you fill up Harley you should make
a note of the OAT temperature. Fuel expands and contracts
with temperature and I don't think the gas station pump can
compensate for that. I think the gas station flow meter just
knows the gallons and not the pounds of fuel.  Another factor
is the percentage of alcohol in the fuel. Since alcohol gives
less energy per gallon that too affects the MPG. Since it has
been steadily getting colder up there I think you were getting
more energy in your tank up until the time you changed the coils
to the 100 B.

When I first got the RX8 in 2004 I did the OBD II trick and saw
25 mpg
at a steady 65 MPH. If you know the mass airflow and the mixture
ratio you can calculated the pounds of fuel going into the engine.
The on board computer is very good keeping this ratio at 14.7
pounds
of air for every pound of fuel. It is called the equivalence
ratio or
Lambda in this spread sheet of OBD II data. 1 Lambda is 14.7:1.

If I am using 3.6 pounds of air per minute I am using .245 pounds
of fuel per minute. That is 14.69 pounds per hour. So I am going
65 miles on 14.69 pounds of fuel. At a fuel weight of 6.25 pounds
per gallon that is 2.35 gallons an hour or 27.66 MPG!

The excessive fuel consumption of the rotary is an urban
myth perpetrated by lead foots. If you use all the 250 HP
you are going to burn the fuel.
Notice the weight of a gallon of fuel varies according
to the temperature. One must also pick a flat road on
a windless day near sea level.

This is a very accurate way of measuring the steady speed
fuel consumption. It is the way some cars are able to display
the MPG in real time on the instrument panel.
Just about any OBD II software that runs in a note book
computer will work.
Paul Lamar

I never noticed that much of an increase (I'm talking about the
increase in mileage per gallon) in the previous 7 winters,
though. Has always been fairly constant, with a MPG or two drop
during the winters (except for the one 17 mpg average when I
found the chipmunk nest in my air filter!).

Although there may be another reason, I can only relate that
recent increase (which was the highest MPG I ever got with one
exception, where an all highway trip got me up to 26) to the two
trailing coils going bad.

I'm mostly interested in the mileage I get per tankful, and not
instantaneous, or under ideal conditions, as that reflects my
actual driving under all conditions, and determines what my
overall cost for gas is (as if I really care...I just bought a
1952 Dodge M37...the army 3/4 ton pickup...MIGHT get 10 mpg with
it!).

I regularly drove a friend's Grand Caravan (before she totaled
it) with that instantaneous mpg feature, kinda neat, but rather
useless IMO...nothing like seeing it drop to 4 as I accelerated
away from a light, or peak at 99 as I coasted down a long hill!

Here's another anecdote. I've added another cell to my
spreadsheet to calculate the separate average since the gas I use
started adding 10% ethanol in August of 2009.  Haven't really
noticed much of a difference in the averages (19.28 before the
ethanol date, 19.01 after that date, and 19.25 overall including
all tankfuls since I bought the car). Those numbers pretty much
reflect my overall driving habits and destinations.

Harley

Well I am fond of the OBD II method  because it indirectly reveals
the BSFC. Drag and therefore HP required is constant. It would not
surprise me to find out the trailing plugs hurt the mileage as they
are there mostly for smog reasons.

Lets see 19.01/19.28 = 9.8% hmmmm 10% ethanol you say :)

Paul Lamar

Oops, math class.

(19.01/19.28)100 = 98.599%.  100 - 98.6 = 1.4%. Now 10% ethanol will not
affect milage by 10%. It will reduce it due to the lower BTU content.

Dale Davies

John beat you to it :)

Paul Lamar