Miata Mailing List: December 1993, Message #221
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   From: Will Zehring 
Subject: gasoleeeeen
   Date: Thu, 23 Dec 1993 08:46:00 -0500

Fellow fiends:

Joining the "I'm no engineer" chorus, I always thought that one of
the principal reasons for lower mileage in the winter is that cold
air mixed with cold fuel just doesn't burn as efficiently as the
warm stuff does.  This goes double for the cold start (i.e. cold
head, cold induction, cold pistons, cold everything!).  So, to get
a certain level of power more fuel has to be shoved in because a
lower % of that fuel actually burns (hence nasty emissions must be
much higher too, and not just because the catalytic converter isn't
hot yet).  As the engine warms up the mixture is leaned down via the
automatic choke.  In warm weather this leaning down occurs many
miles (minutes) sooner.

On the other hand, I am not sure why people are reporting lower mileage
in the summer, unless it is due to increased drag with the top down.
I don't see why power/gallon should be down, but with the top down I
can see why power/mph must be up (so gallon/mph would be up) (or is my
8th grade algebra teacher spinning in his grave?).

For the record, I have a 13 mile commute and seem to wander only +/-
1 gallon from 30mpg all year.  I hazard to confess that during my
commute (a true race to the death on a Detroit freeway with, count them,
26-wheelers (I s--t thee not)), there is just too much noise for top
down motoring.  By the exit ramp though, that baby is down.  During long
highway cruising I get up to 34-35 mpg (during the summer, mostly)

Doesn't driving style have a lot to do with this?  I mean, shift points.
I don't know what the power/rmp curve looks like on the Miata, but I
have a suspicion that peak horsepower occurs well before 6700 rpm.
Does anyone know the facts?  If peak is at 5000 rpm, is there any point
in going beyond?  Shouldn't peak horsepower rpm be the ideal shift point?
This gets complicated because break horse power also has something to do
with the car's drag?  Stop me before I ask another stupid question......