Optimal rpm for fuel economy???

Mihnea Cotet mihnea_cotet at hotmail.com
Thu Sep 20 20:13:51 EDT 2001

Rob Beatty wrote:

>Let the arguing commence :)

Well, I'd say that it is pretty weird as fuel consumption depends on fuel 
mixture and RPM, (the gear you're in). HP also depends on RPM and on torque 
and I think that the most economic RPM is the one that burns fuel the most 
efficiently. So, you couldn't get good mileage at WOT as the mixture is 
slightly richer at those engine loads (on turbo engines or EFI ones). As for 
my car that isn't turboed nor EFI-ed (yet), it has a 2226 cc engine, 10:1 
CR, runs on 95 octane fuel, spark is set at 19 degs BTDC and the mixture is 
set at 0.8% CO as the car has no OXS nor cat...

The point with that is that I've achieved several times fuel consumption of 
50.4 mpg, driving the car with the full tank, 2 people in it and at speeds 
between 50 and 70 mph. No hard accelerations (except one to overtake an A4 
TDI and went to 110mph), really cool driving...I thought this one was a 
mistake and that I had worked it out bad, but it was ok as the next similar 
trip was 50.2 mpg...all this trip was done at about 2500-2700 RPM in 5-th...
Michael Gough even asked me if my gallons were bigger than his!!!
Actually, no because I worked it in liters per 100 kms and then converted 
this into US gallons, not UK (UK ones are indeed bigger!).

So what's the point? Maybe it has to do with the compression ratio...maybe 
with spark timing...the car has no knock sensors, so maybe the timing isn't 
even the right and the best one for fuel economy.
The car's owner manual states a fuel consumption of 7.1l/100km at cruising 
speeds of 90km/h, so this makes 33.35 mpg...weird, isn't it?
At cruising speeds of 120 km/h, the manual states 9.1l/100kms...this would 
make 26 mpg...at 75 mph, right? Well my car burns 8.2l/100kms at speeds 
between 125 and 140 km/h, about 29 mpg...at about 3500-4000 RPM Where's the 
mistake? Are those figures measured with the A/C on?

As for me, I'm pretty happy with the actual figures, these are among 
"normal" figures for today's gas cars, but very unusual for 16 years-old 
cars with CIS and a 2.2 engine...
Could anyone enlighten me with this? What could make my car use less fuel 
than the factory's figures (that are usually more optimistic than the real 
Thanks to all,

Mihnea Cotet

85 Coupe quattro


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