awhile back i posted (july 6) a request for advise re poor fuel
well, i figured out why i have been getting poor fuel economy....
in a previous post i indicated that there were no drivability
problems, but that i was getting really poor mpg.
this past weekend i decided to verify once and for all my highway fuel
economy so i filled up my tank, and went on a nonstop drive for 82
miles-speed was ffor the most part between 55-65 mph. i had an
autoxray hooked up and was monitoring maf (lb/min). put on the cruise
control at 55,60,65 mph and recorded maf as 1.9, 2.6, and 2.9 lb/min,
respectively. after the drive, filled up again. used 2.6 gallons of
fuel= 31.5 mpg!!!!
i then monitored the vacuum gauge i installed, the problem was my lead
foot. i had gotten use to putting the pedal down atleast 1/2 and
usually more when i wanted to accelerate therefore consuming much more
fuel (especially when in the W.O.T. mode which provides for fuel
now i am back to driving like a little old lady, just putting down the
pedal so that the vacuum gauge does not fall below 5" hg, and
preferably 10" hg. -this feels like depressing the pedal less than 1"
!!! which also results for the most part of keeping the rpms below
back to the maf results-notice that the maf increases dramatically
from 55mph to 60mph
using 1.9 lb/min of air at 55 mph, and assuming gasoline weighs 6 lb
per gallon, stochiometric ratio or air to fuel is 14.7, but my long
term fuel trim seemed to average around +3 % so.... 1.9(lb/min)x
60(min/hr)/(14.7x.97x6lb/gal.)=1.33 gallons of fuel per hour so
55/1.33Ampg!!! this number has to be truly verified.
this seems incredible, the condiitions are no ac, perfectly flat, no
headwind, cruise control. however, in reality the maf readings at 55
varied substantially =when going up even the slightest grade maf went
up to 2.5 and when going down even a slight grade it decreased to 1.3.
i did the test on I66 in virginia and it was difficult to determine
when the grade was perfectly flat, and even drafting behing another
car provided measureable benefit.
bottomline to save mpg:
1) when accelerating from a standstill, keep the rpms around 2000
(therefore avoiding fuel enrichment and downshift)
2) on the highway, 55 mph (at least for an olds alero 3400 v6) saves
substantially on gas.