mitsumotors
NAXJA Forum User
- Location
- Winnipeg
1991 4.0l, AW4, 4.5"lift, 33x12.5" mfg m/t's, regeared to 4.56.
Getting really poor mileage(like 13mpg) and i have tried almost everything, i've done all maintenance and tune-up stuff as well as changed my driving habits.
I hooked up a vacuum guage to the line that goes between the manifold and MAP sensor. i ran it into the truck and obsereved it while driving. I have been told numerous times that higher vacuum means better mileage. here are the results:
3rd gear - 2250rpm/15in.-Hg
3rd gear - 2500rpm/15in.-Hg
3rd gear - 2750rpm/14in.-Hg
OD(4rth)- 1500rpm/14in.-Hg
OD(4rth)- 1750rpm/12in.-Hg
OD(4rth)- 2000rpm/12in.-Hg
OD(4rth)- 2250rpm/10in.-Hg
OD(4rth)- 2500rpm/9in.-Hg
OD(4rth)- 2750rpm/7in.-Hg
I really don't understand what this all means but maybe you do?
the only thing that I am assuming is that 4.56 gears are a bit undergeared with33" tires at highway speeds and that maybe if i was using a bit bigger tires(35") i would be running less rpm at highway speeds in OD and get better mileage? although i can't seem to understand why there was more vacuum in 3rd gear(that would lead me to believe that i would get better mileage in 3rd gear running 2750rpm on the highway(getting 14in.-Hg) than being in OD running 2250rpm on the highway(getting 10in.-Hg). but maybe i have no idea? let me know.
Getting really poor mileage(like 13mpg) and i have tried almost everything, i've done all maintenance and tune-up stuff as well as changed my driving habits.
I hooked up a vacuum guage to the line that goes between the manifold and MAP sensor. i ran it into the truck and obsereved it while driving. I have been told numerous times that higher vacuum means better mileage. here are the results:
3rd gear - 2250rpm/15in.-Hg
3rd gear - 2500rpm/15in.-Hg
3rd gear - 2750rpm/14in.-Hg
OD(4rth)- 1500rpm/14in.-Hg
OD(4rth)- 1750rpm/12in.-Hg
OD(4rth)- 2000rpm/12in.-Hg
OD(4rth)- 2250rpm/10in.-Hg
OD(4rth)- 2500rpm/9in.-Hg
OD(4rth)- 2750rpm/7in.-Hg
I really don't understand what this all means but maybe you do?
the only thing that I am assuming is that 4.56 gears are a bit undergeared with33" tires at highway speeds and that maybe if i was using a bit bigger tires(35") i would be running less rpm at highway speeds in OD and get better mileage? although i can't seem to understand why there was more vacuum in 3rd gear(that would lead me to believe that i would get better mileage in 3rd gear running 2750rpm on the highway(getting 14in.-Hg) than being in OD running 2250rpm on the highway(getting 10in.-Hg). but maybe i have no idea? let me know.