Optimal rpm for fuel economy???

[Browning David]

I think another very important factor is wind resistance.  I think I've
heard arguments about this before.  It seems the wind resistance is more
than just proportional to speed.  (x^2 maybe?)  I dunno.  In my A4 2.8, I've
gotten 27mpg at 70mph, and almost 29mpg traveling around 60mph.  My 4ks on
the other hand seems to be immune to road speed.  I always get around
30-32mpg in city or on the highway.  (I guess it probably does do better on
the highway.)  You would think that newer cars with fancy computerized
ignition and fuel, would get better mileage than the old CIS cars.  But I
think the increased weight of newer cars has neutralized this advantage.

My 2 cents.
dB


>-----Original Message-----
>From: Mihnea Cotet [mailto:mihnea_cotet at hotmail.com]
>Sent: Thursday, September 20, 2001 12:14 PM
>To: quattro at audifans.com
>Subject: Optimal rpm for fuel economy??? 
>
>
>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 
>ones)?
>Thanks to all,
>Regards,
>
>
>Mihnea Cotet
>
>85 Coupe quattro
>
>http://www.audifans.com/registry/view.php?action=viewCar&carid=350
>
>
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