Optimal rpm for fuel econmy???

[Phil Payne]

> Been wondering for the past month while watching how my milage varies
> depending on how fast I drive to work, how is the optimal rpm figured for
a
> car?

Optimum rpm for any speed is always the lowest - you should always be in the
highest gear possible.  In the UK, manufacturers are required to publish
figures for economy at 56 mph, since this was once considered the optimum
speed for high mpg - advances in streamlining have raised this figure. It's
determined entirely by air resistance - proportional to speed cubed.  Most
other factors are speed squared at most, some are linear.  Some actually go
down slightly with speed - I think tyre rolling resistance is one. On the
Big Red Bus, things change dramatically at about 75 mph, though the ideal
figure is probabl lower.

When Piech first announced Audi's plans for the Type 44, one of the claims
was that it would get from Hamburg to Munich on a single tankful. The idea
that Audi was going to have the temerity to build a luxury car at all caused
a lot of mirth, but the concept of Hamburg-Munich without filling up caused
ridicule.  Henri Nannen's Stern (Star) magazine (since better known for the
Hitler's Diaries episode) had a field day with the idea.  I remember them
speculating on how big the tank would have to be,  and a cartoon of an Audi
80 with lots of bolt-on goodies towing a fuel tank on a trailer.

So much so that one morning a car transporter turned up at Stern's offices
in Hamburg.  On board were three pre-production Type 44s - a CIS car, a
carburretor I5 and a diesel. Sealed tank fillers.

Stern reporters and a photographer took them out, starting late at night
when there was no traffic and driving as close to 'Richtgeschwindigkeit'
(130kph) as possible.  Crawling up hills in top gear, etc.

All of the cars got way past Munich.  I think they all made Austria and the
diesel car got to Budapest.

Stern printed the whole story and a retraction on the front cover.  "Herr
Piech hat Recht".  The Type 44 changed the prestige car market in Germany
and gave both Daimler-Benz and BMW a lot of catching up to do - the
up-market versions of the Type 44 were status symbols. I think one of the
Daimler-Benz people said the car "exploded onto the market like a bomb".

I lived in Germany at the time and had a subscription to Stern.  I kept the
copy with the retraction in it for many years but it got lost in one of the
moves following my divorce - I'd LOVE to find another copy. I wanted a 200
turbo (FWD - wasn't into quattro at the time and the feature was expen$ive)
but my boss had one - he put in my request but the company turned it down
and offered me a Mercedes!

In practical terms - the ignition system (good plugs and spark) plays a
major role, as do the OXS sensor and the various overrun shut-off devices.
I not uncommonly hear reports that a car I've worked on is suddenly showing
10% or even 20% better fuel economy just because new plugs have been fitted,
a few contacts cleaned, and some vacuum hoses replaced.  The '30,000 miles
on three-prong spark plugs' idea is for the convenience of servicing
outfits - if you're self-maintaining, change them more frequently for better
economy.  You can easily get 2-3 mpg by spending half an hour in the engine
bay every month making sure everything's clean and tight.