Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site cornell.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!cornell!prins
From: prins@cornell.UUCP (Jan Prins)
Newsgroups: net.auto,net.legal
Subject: Re: 70 mph fuel economy
Message-ID: <188@cornell.UUCP>
Date: Thu, 29-Aug-85 01:58:27 EDT
Article-I.D.: cornell.188
Posted: Thu Aug 29 01:58:27 1985
Date-Received: Fri, 30-Aug-85 11:34:31 EDT
References: <1081@homxa.UUCP> <4891@allegra.UUCP> <155@iitcs.UUCP> <553@unisoft.UUCP> <492@lasspvax.UUCP>
Reply-To: prins@cornell.UUCP (Jan Prins)
Organization: Cornell Univ. CS Dept.
Lines: 26
Xref: watmath net.auto:7931 net.legal:2208

In article <492@lasspvax.UUCP> chu@lasspvax.UUCP (Clare Chu) writes:
>
>    Correct me if I'm wrong but I heard that some of today's cars
>    get better mileage at 70 mph.  (16-valve 4 cyl engine???)
>
>                                     Clare
Attractive as this thought is for the anti-55 crusade, I think it's unlikely 
to be correct.
                                                3
It's the tyranny of air drag:  P = 1/2 rho CdA V  .  Rear wheel power at 70 mph
is about twice that at 55 mph.  If specific fuel consumption were constant with
engine rpm, twice as much fuel would be consumed for 55/70 of the time.  Thus
fuel economy would be worse.  Actually, below peak torque rpm, specific fuel
consumption decreases with increasing rpm, but only by a few percent (esp. with
fuel injection).  That difference is insufficient to make up the original
disadvantage.

The induction efficiency (4 valve vs. 2 valve) shouldn't have much effect
in this case, since the engine will be operating at part load (induction
restricted).

Although higher speeds use more fuel, the improvements in car aerodynamics and
engine efficiency over the last decade completely dominate the fuel saved by
the lower speeds.  The speed limit is vestigial: a fool economy.

jan   vax135!cornell!prins    prins@cornell (arpa, uucp, csnet)