<VV> Fuel Economy

[Padgett]

Fuel economy has been something I have been playing with since my teenage 
daze when economy rallys were common and measured on a ton/mpg ratio (and a 
cement truck went off scale).

Do know that for any engine there is a sweet spot - highest load in highest 
gear that does not trigger extra enrichment. Frankly I have never seen this 
sweet spot anywhere over about 2500 rpm and under 2000 is better.

Case in point: when I hear that a 3.55 gear gives the same mpg as a 3.27 
then my first thought is that either there is something wrong with the 3.27 
or it is being driven into the PE zone more often.

Similarly a PG engine at steady state should not give as good MPG as a four 
speed because even with a tight converter like the corvair (mine appears to 
have a 1600 rpm stall which is pretty tight), there is still gong to be 
3-5% slip at cruise.

IMNSHO even the 3.08 with stock tires is too low a gear, about 2.56 with a 
switch-pitch torque converter for better off-the line or 2.73 with a lockup 
would sound better.

The 1964 "Stock Engine" test corroborates this in that   "road load" mpg is 
pretty flat up to 2000 rpm (about 40 mph with a 3.55) and then drops off 
significantly to 2/3 that value at 3200 rpm (about 64 mph) - numbers are 
not that important, the  ratios are.

So if you want max mpg for a given drivetrain, drive 200 miles at 2000 rpm. 
Want more speed, gear for 2000-2400 at your cruise speed. MPG will be less 
than for the lower speed but higher than the same speed at a higher RPM.

Now drop the revs far enough and the load will get high enough to put you 
into PE so you need to juggle things a bit. Lets just say that my 92 
TranSport van with 3800 (231 cid) turns 2k at 70 in lockup o/d and gives 
24-25 mpg of 87 octane at left lane speeds. With the dual a/c on. Keep in 
mind that while it does have feedback FI, it also is 1000 lbs heavier and 
has a lot more frontal area (and a roof rack). It is also 1990 technology 
and a 50% larger engine.

So 30 mpg at Interstate speed from a Corvair on pump gas should be possible 
if we could get the revs down. Unfortunately, with a 2 speed non-lock-up 
trans, it ain't gonna happen. Geared for even 2400 at 70  would require a 
2.42 rear gear (though it would be able to get to 70 in low, take off would 
be a tad slow) .

Are options, since I have some 215x65x15s, going from a 24" to a 26" rear 
tire would be equivalent to swapping from a 3.55 to a 3.23. so is the first 
thing to try but first I need everything running right and am not there 
yet. Hopefully next year I'll get instrumentation fitted and try some 
things. And maybe the horse will learn to sing.

Padgett