mileage...

[Tom Jennings <tomj@xxxxxxx>]

If you have a stock-ish V8 with steep axle you could could
generate actual, hard test data by doing long-distance,
constant-speed testing. I've actually done a bit of this in my
Rambler to determine optimum highway cruise speed on long trips.
(It's 60 mph -- so that's what I drive on trips.)

I drove at least two full-tank fillups at one speed (easy to do on
Route I40 east or west through AZ/NM). There's some margin of
error but it equals out. I did a few 500+ mile one-way runs at
different speeds.


Given a known axle/cam combo, you can drive at various engine RPMs
and work out which engine rpm is best. Yes, there's many other
variables (friction and all that) but in a narrow rpm range I
think the biggest variable will be the cam/carb combo. (For
example, below, I don't believe there's much variation in carb
behavior from 2300rpm/126cfm to 2500rpm/143cfm; I believe the
difference is the engine hydrodynamic junk. I know my spark is all
in by 2000.)

Then look at curves for that cam grind; (in this case, what's it
doing at 2300 rpm) and then get a cam grinder to tweak one for say
2000 rpm, assuming you have enough grunt at that speed to be
drivable -- my little LP six doesnt!) and pick an axle to match.


Tire is 215-75/R15. Flush-O-Manic trans, 3.15 (torque-tube) axle.

These are propane MPGs; 14 mpg/ LP --> 18.93 gasoline; 15 mpg -->
20.28 gasoline. I've got every single mile and fillup recorded on
this car back to 1994 (OK so I'm fanatical).


63 Classic                              1st     2nd     3rd
                        RPM     CFM     MPH     MPH     MPH
                        1000    57      11.7    14.8    26.2 
Axle ratio n:1  3.15    1100    63      12.9    16.3    28.8
Tire diameter   27.7    1200    68      14.1    17.7    31.4
  3.78=9/37             1300    74      15.2    19.2    34.0
  3.31=13/43            1400    80      16.4    20.7    36.6
  3.54=11/39            1500    86      17.6    22.2    39.2
                        1600    91      18.8    23.6    41.9
RPM interval    100     1700    97      19.9    25.1    44.5 
M11 auto                1800    103     21.1    26.6    47.1 
First           2.23    1900    108     22.3    28.1    49.7 
Second          1.77    2000    114     23.5    29.6    52.3 
Top             1       2100    120     24.6    31.0    54.9
                        2200    126     25.8    32.5    57.5 
OD              0.7     2300    131     27.0    34.0    60.2    15 mpg
                        2400    137     28.2    35.5    62.8 
Engine CID      232     2500    143     29.3    36.9    65.4    14mpg 
VE              0.85    2600    148     30.5    38.4    68.0
                        2700    154     31.7    39.9    70.6
                        2800    160     32.8    41.4    73.2
                        2900    165     34.0    42.9    75.9
                        3000    171     35.2    44.3    78.5