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MPG Question.
Thread Starter
Cruising
Joined: Aug 2017
Posts: 13
Likes: 1
MPG Question.
My fairly new to me 92 bone stock LT1 is averaging 11.5 around town. On the highway the best I see is around 16-17mpg cruising along at 70. My rough calculations through two tanks agrees with the cars computer. I know it's not a C6, but my LS2 is head and shoulders better and from what I read, my 92 should do fairly similar on the hwy. Just had new O2 sensors put in thinking thst might fix it, no change. Now I do run the AC because it's hot here in Central Florida but I know that can't kill mileage like what I'm seeing. Mechanic says injectors are good as well. What else can I or should I look at? As always, thanks for the help!
Pro
Joined: Sep 2014
Posts: 588
Likes: 35
From: Smithfield VA
Definitely something wrong. I'm getting a 20.6 mpg average since buying my '93 in Jan, and my right foot weighs about 20 lbs more than my left. People without heavy foot syndrome are known to get +25 mpg with LT1 powered Vettes. Is the car stock? Injectors good, O2 sensors new, what about the spark plugs and wires? I'd also be wondering if it had a tune done that may be causing the bad mileage for an extra 5-10 hp.
Last edited by 1stVetteFinally; Aug 23, 2018 at 07:58 AM.
Le Mans Master
Joined: Oct 2004
Posts: 6,314
Likes: 508
From: South-central Missouri
My 95/M6 Vette got between 29 and 31 mpg on level ground in cruise control mode. I used the actual gallons/mileage calculation (not trusting the computer...which was pretty close, actually). When not in cruise mode, mileage was closer to yours - but I have a lead foot and love to play!
Working O2s are critical to fuel economy, as well as fresh plugs and a good tune.
Working O2s are critical to fuel economy, as well as fresh plugs and a good tune.
Team Owner
Joined: Sep 2001
Posts: 62,141
Likes: 1,729
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Yes there is a problem somewhere, when O2s fail they show this symptom but rarely set codes. Since you replaced them I'd go ahead and change plugs and wires, and I'd check to see if the injectors are leaking, but a scanner might show something else to point towards the problem. I assume you have no other codes set.
Thread Starter
Cruising
Joined: Aug 2017
Posts: 13
Likes: 1
Yes there is a problem somewhere, when O2s fail they show this symptom but rarely set codes. Since you replaced them I'd go ahead and change plugs and wires, and I'd check to see if the injectors are leaking, but a scanner might show something else to point towards the problem. I assume you have no other codes set.
Melting Slicks
Joined: May 2013
Posts: 2,692
Likes: 349
From: Sacramento California
Make sure you clean the mass airflow sensor (maybe your 1992 has the speed density thing?) and reset the IAC and TPS, just to be sure everything is working properly.
Also remember that lots of short trips (open loop operation before engine warmup) will really kill MPGs. I usually drive at least 10 miles each trip.
My average is 22 MPG with stock 1995 LT1 6-speed, mostly normal driving with occasional spirited run
Also remember that lots of short trips (open loop operation before engine warmup) will really kill MPGs. I usually drive at least 10 miles each trip.
My average is 22 MPG with stock 1995 LT1 6-speed, mostly normal driving with occasional spirited run
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Thread Starter
Cruising
Joined: Aug 2017
Posts: 13
Likes: 1
Yes there is a problem somewhere, when O2s fail they show this symptom but rarely set codes. Since you replaced them I'd go ahead and change plugs and wires, and I'd check to see if the injectors are leaking, but a scanner might show something else to point towards the problem. I assume you have no other codes set.
Team Owner
Joined: Sep 2001
Posts: 62,141
Likes: 1,729
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Hook up a fuel pressure gauge at the fuel rail, notice starting pressure at cranking. Let car idle and notice pressures. Then shut the car off and watch the pressure. It should hold steady a while.
Intermediate
Joined: Jul 2018
Posts: 48
Likes: 5
From: Central Maine
I got my '92 a month ago and have driven it about 500 miles.
So far my average is just shy of 26mpg,easy on the gas pedal and around here its almost all highway driving,no traffic lights and few stop signs.
So far my average is just shy of 26mpg,easy on the gas pedal and around here its almost all highway driving,no traffic lights and few stop signs.
Team Owner
Joined: Sep 2001
Posts: 62,141
Likes: 1,729
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Safety Car
Joined: Apr 2003
Posts: 3,793
Likes: 9
From: VA
My fairly new to me 92 bone stock LT1 is averaging 11.5 around town. On the highway the best I see is around 16-17mpg cruising along at 70. My rough calculations through two tanks agrees with the cars computer. I know it's not a C6, but my LS2 is head and shoulders better and from what I read, my 92 should do fairly similar on the hwy. Just had new O2 sensors put in thinking thst might fix it, no change. Now I do run the AC because it's hot here in Central Florida but I know that can't kill mileage like what I'm seeing. Mechanic says injectors are good as well. What else can I or should I look at? As always, thanks for the help!
Don't forget that the distributor cap is part of a tuneup - when I first got my 94 it had gas mileage like you're seeing and lots of carbon on the tailpipes. Everybody on this forum said "normal". Several months later the optispark bearing started squealing. With a new optispark and no other changes mileage got about 5mpg better and the carbon was gone.