Absurdly bad gas mileage (L98 TPI)
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Absurdly bad gas mileage (L98 TPI)
Hello. I've got an '87 Trans Am with an L98 TPI engine. Engine/transmission are rebuilt, less than 10k miles each. I picked this car up a few months ago, and replaced everything from fuel pump to spark plug wires to the distributor etc. The car runs AMAZINGLY. It is SO much fun to drive.
Heres the problem. Based on my trip odometer, I am getting about 7 MPG. This is strange, because anything that bad should be really obvious. If it were a fuel leak, it would be losing more than half the tank, and if the engine were eating the gas it would run like crap (plus it wouldn't pass emissions, but it passed with flying colors). I tried driving like a Prius, instead of flooring it, and the MPG got EVEN WORSE!
I am suspecting the odometer is somehow inaccurate, but I don't really know as I am not an expert. I was hoping someone here might know what could cause this. Any ideas?
Thanks.
Heres the problem. Based on my trip odometer, I am getting about 7 MPG. This is strange, because anything that bad should be really obvious. If it were a fuel leak, it would be losing more than half the tank, and if the engine were eating the gas it would run like crap (plus it wouldn't pass emissions, but it passed with flying colors). I tried driving like a Prius, instead of flooring it, and the MPG got EVEN WORSE!
I am suspecting the odometer is somehow inaccurate, but I don't really know as I am not an expert. I was hoping someone here might know what could cause this. Any ideas?
Thanks.
Joined: Mar 2017
Posts: 4,255
Likes: 427
From: Portland, OR
Car: 86 Imponte Ruiner 450GT, 91 Formula
Engine: 350 Vortec, FIRST TPI, 325 RWHP
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
Re: Absurdly bad gas mileage (L98 TPI)
Get a wideband and check the AFR. And use your phone with a GPS dashboard app to track MPH and distance traveled.
Do you feel like you are filling it with 13 gallons of fuel every 90 miles? Because that's what the numbers you give would equate to.
GD
Do you feel like you are filling it with 13 gallons of fuel every 90 miles? Because that's what the numbers you give would equate to.
GD
Joined: Aug 1999
Posts: 5,261
Likes: 461
From: RI
Car: 1984 Camaro Berlinetta
Engine: LT1
Transmission: T56 6-speed
Axle/Gears: 4.11 LS1 Rear End
Re: Absurdly bad gas mileage (L98 TPI)
Thirdgens a gas-hogs,... but I was getting better than 7MPG with a 355ci and a pig-rich 750CFM carb - your mileage SHOULD be better !
First and foremost; Be sure that your gauges are accurate, Then verify the rear gear........
If Auto; put the shifter in 3rd and accelerate - briefly - to 70 MPH
If Stick; put the shifter in 4th and accelerate - briefly - to 70MPH
Make note of the RPMs; then up-shift.
Your RPM reading in this 'test' should = your rear gear. ( Example: 3400RPM -or so- equals 3:45 gearing, 2700 RPM equals 2:73 gearing. )
* OBVIOUSLY: Your Tach will need to be accurate,...... ( are you sure it's a V8 tachometer and not a V6 Tach ?? ) You can test your Tach reading by connecting/splicing a dwell meter to the TACH circuit @ the distributor.
First and foremost; Be sure that your gauges are accurate, Then verify the rear gear........
If Auto; put the shifter in 3rd and accelerate - briefly - to 70 MPH
If Stick; put the shifter in 4th and accelerate - briefly - to 70MPH
Make note of the RPMs; then up-shift.
Your RPM reading in this 'test' should = your rear gear. ( Example: 3400RPM -or so- equals 3:45 gearing, 2700 RPM equals 2:73 gearing. )
* OBVIOUSLY: Your Tach will need to be accurate,...... ( are you sure it's a V8 tachometer and not a V6 Tach ?? ) You can test your Tach reading by connecting/splicing a dwell meter to the TACH circuit @ the distributor.
Member
Joined: Sep 2013
Posts: 122
Likes: 12
From: Sweden
Car: 1988 Trans Am GTA
Engine: 350 TPI (L98)
Transmission: TH700R4
Axle/Gears: 3.27 9-bolt w/ Torsen LSD
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