odd sidekick mpg

[urrlord]

i have an 89 suzuki sidekick 4 cylinder 5 speed,4wd.when i first got it about 5 years ago it got around 20 mpg around town and 25 highway if i kept the speed 65 or lower,if i went faster the mpg dropped to 23-24.this was with an engine full of wal-mart dino 10w30 and regular gas,195/75r15 tires and what looked like spotty maintenance by the second owner.(the first owner used it as a tow behind,so the miles on it are unknown).
since hanging around here and a few other places i did a few oil changes with good oil(semi-synth and synth) and additives(MMO,etc).added MMO ,tcw3 and acetone to the gas at different times.i drained the tranny and rear end of the old lube(the front diff fill looked good ,like honey) refilled with good synthetic lube.upgraded the tires to 215/75r15.
in the last 6 months i have noticed the my city mpg is around 22 mpg and the highway mileage is 24/25 if i keep it around 65.if i go 70 the mpg goes up to 26/27.on a trip to atlanta where i had to go 80-85 mph to keep from being ran over and the sidekick turned in a 31 mpg rating.
this seems rather odd to me.i think the cycles of good o/c's has given the engine easier function and the use of acetone ,MMO and tcw3 may have cleaned the fuel system and lubricated it and allowed it to function better.but i really would not expect it to give me better mpg at higher speeds.
any theories guys?

 

[JHZR2]

the biggest isue with going fast is wind resistance. Your sidekick isnt exactly aerodnamic. While some efficiency advantage may be had by going faster, I cannot see 31 MPG worth. I think your tires have made the speedo go a bit out of calibration, and youre getting phony readings. A 215/75 tire is taller than a 195/75 tire, and thus has more circumference. This means that your odometer is telling you that youre travelling at a different rate than you really are, and your mileage is inaccurate...

 

[Nick R]

Originally Posted By: JHZR2
the biggest isue with going fast is wind resistance. Your sidekick isnt exactly aerodnamic. While some efficiency advantage may be had by going faster, I cannot see 31 MPG worth. I think your tires have made the speedo go a bit out of calibration, and youre getting phony readings. A 215/75 tire is taller than a 195/75 tire, and thus has more circumference. This means that your odometer is telling you that youre travelling at a different rate than you really are, and your mileage is inaccurate...


+1

 

[urrlord]

i'm aware of the tire size difference,in this case there is a 4.5 % difference.my observation is that at higher sustained speeds my mpg goes up.it would seem that for u.s. driving of this era the engine would be tuned for best efficiency at 55-65 mph .unless the suzuki engine is designed to operate at maximum efficiency at hi-rpm's.which is odd for an engine in a 4wd that should be tuned for low end torque.
to wit speeds of 55-60 =22mpg,60-65=24,65-70=26 consistently yield these averages.the 31 mpg trip was speeds of 75-85 on a rainy day.which i know cooler,denser air and moisture laden air will up the mpg .i did not expect that large of a boost.
my mazda tribute's mpg goes down on sustained speed trips around 75 mph(23mpg).around 65-70 mph i'll get 25-26 mpg.this is what i would expect out of the suzuki also.
btw i check gas mileage every time i gas up.trip meter divide by gallons=mpg.i use the same station and pump most(85%) of the time( bp/amoco).in the last two weeks i have started using murphy express about 50% of the time.
any other theories out there?