MPG help for a C3 please. THX
Thread Starter
Intermediate
Joined: Jun 2008
Posts: 30
Likes: 1
MPG help for a C3 please. THX
Hello all,
I am now realizing that my new C3 gets about 6 miles to a gallon. Is this right? She passed her emissions test beautifully.
Here are the basics-
1970 L46 convertible. Original block with some performance upgrades to mimic the 70 LT1. It use to register around 370HP in the late 90s.
The tank gets about 18 gallons and right about 100miles it looks pretty bare in there. I can’t really trust the gage since it fluctuates so much at the end. I do warm her up about 20 mins. before every drive and use only 91 octane. (93 is not available in CO). I do mostly easy crusing around the town.
I live in Denver, so she is getting her Holey carb re-jetted for altitude at a corvette shop here shortly. Will this help? Is there anything else I can do?
Thanks
Ardy
I am now realizing that my new C3 gets about 6 miles to a gallon. Is this right? She passed her emissions test beautifully.
Here are the basics-
1970 L46 convertible. Original block with some performance upgrades to mimic the 70 LT1. It use to register around 370HP in the late 90s.
The tank gets about 18 gallons and right about 100miles it looks pretty bare in there. I can’t really trust the gage since it fluctuates so much at the end. I do warm her up about 20 mins. before every drive and use only 91 octane. (93 is not available in CO). I do mostly easy crusing around the town.
I live in Denver, so she is getting her Holey carb re-jetted for altitude at a corvette shop here shortly. Will this help? Is there anything else I can do?
Thanks
Ardy
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
You are on the right track w/ tuning. A std. part of a perf. tune is advancing the initial timing & recurving the dist.
After that is exhaust (since you should have duals) like high flow mufflers, crossover pipe & headers.
After that is exhaust (since you should have duals) like high flow mufflers, crossover pipe & headers.
Instructor
Joined: May 2008
Posts: 154
Likes: 3
From: Indiana
Ardi, 6 MGP sounds a little low. Good news is you can pass emission so you're not blowing too much gas through the exhaust. I have a 327/350HP and think I am around 10 to 12 MPG average. If your transmission is stock, you don't have overdrive so your highway mileage will be almost as bad as city. Your best MPG highway will be at about 50 MPH plus or minus a few. I only checked mine once because I am either cruisin around town (bad for mileage) or going from one light to the next in a big hurry (even worse on gas). Let's see if anyone else has info for you...
Safety Car
Joined: Feb 2006
Posts: 3,557
Likes: 0
From: Ridgeway VA
This may sound silly - especially since you don't trust your gas gauge, as I have a 1970 also, the next time you fill up take a flashlite and look in the tank - if you are filling it up to the seam line in the back of the tank that's only about halfway. Bet you get better mileage now. Also - Hollys love gas. I have one on my 330hp and I only get 10-12mpg - compare that to a 1975 that gets 18-20 mpg with a good tune up.
Team Owner
Joined: Jul 2008
Posts: 23,419
Likes: 22
From: In limbo
St. Jude Donor '13-'15, '17 thru '22
6 miles to the gallon is pretty low, especially for a small block. My last one was a 350/350 and I would get about 12-14 driving around and a few times got close to 20 on the highway. My first thought would be that somehow, you are dumping way too much fuel down your carb, but you said you passed emissions, so that probably isn't the case. Are you the type that is on it ALL THE TIME? Even still, I would think it would be hard to only get 6 mpg if the engine is running right.
Race Director
Joined: Nov 2002
Posts: 10,489
Likes: 576
From: Pace Florida
St. Jude Donor '05-'06-'07-'08-'09-'10-'11
National Champs Gators 4 St. Jude '09
The only time I've checked my '70 L46 was on the highway and it got around 15 on a 450 mile trip. Probably 12 or so around town depending how heavy my foot is.
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Michael S. Palmer
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
Besides the engine being in tune, what rear gears are in there. Standard for L-46 was 3.36, but I think you could get up to a 4.11 rear option. Also gears could have been swapped some where in it's history. What are your revs at 60? Also make sure your tire pressure is correct. That alone could add a mile or two per gallon.
I have a 3.08 rear and at 60 mph, engine revs are 2300 rpm. I guess I get around 15-17 on the hiway at 60 mph if I go easy on the pedal, which we all know can sometimes be hard to resist.
I have a 3.08 rear and at 60 mph, engine revs are 2300 rpm. I guess I get around 15-17 on the hiway at 60 mph if I go easy on the pedal, which we all know can sometimes be hard to resist.
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
My 406 gets between 10-12 mpg which I'm not really happy about. The plugs indicate an only slightly rich burn,(darkish tan) I run a 3000 stall th400 and 3.70 gears, and to be honest, it sees 7000 rpms quite often, so I'm sure if I were a good boy, mileage would be better.
I'd pull the spark plugs and look to see just how black and nasty they are.................You have some tuning to do for sure.
I'd pull the spark plugs and look to see just how black and nasty they are.................You have some tuning to do for sure.
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
You need to stop wasting gas like that before you start putting out cash for improvements. Especially if you are taking short trips after your warmup. During the winter when I start mine periodically to keep the battery charged I get 0 MPG.
Instructor
Joined: Jan 2009
Posts: 145
Likes: 1
From: Cudjoe Key Florida
ide say also tune it up do the whole deal and if you dont know how have someone who does tune the fuel air mix on the cab and it will get better
i was gettting 8 mpg on the highway with a stock 71 350 and after i had it tuned up and carb adjusted its now getting 18 mpg on the highway
what a difference
and im still useing 100 low lead avation fuel
(no lead in car gasoline it depleates the ozone but theres lead in avaition fuel ......ummm arent airplanes closer to the ozone?)
i was gettting 8 mpg on the highway with a stock 71 350 and after i had it tuned up and carb adjusted its now getting 18 mpg on the highway
what a difference
and im still useing 100 low lead avation fuel
(no lead in car gasoline it depleates the ozone but theres lead in avaition fuel ......ummm arent airplanes closer to the ozone?)
Drifting
Joined: Feb 2005
Posts: 1,738
Likes: 2
From: San diego ca
Also, Not mentioned in any post above is how one drives.
My 78 with the TKO can get stellar gas mileage, if I drive in a conservative manner. Even with the TKO my city mileage is dismal- in the 12-14mpg range.. why is it dismal? because I simply cannot restrain myself when I drive the car in the city, stop and go I get to listen to the car run up through the gears and generally I just dont and cant consistantly drive it in the city for MPG.
constant state cruising on the freeway and 20mpg can be achieved fairly easy.
8mpg sounds pretty low to me, but alot depends on how you drive.
My 78 with the TKO can get stellar gas mileage, if I drive in a conservative manner. Even with the TKO my city mileage is dismal- in the 12-14mpg range.. why is it dismal? because I simply cannot restrain myself when I drive the car in the city, stop and go I get to listen to the car run up through the gears and generally I just dont and cant consistantly drive it in the city for MPG.
constant state cruising on the freeway and 20mpg can be achieved fairly easy.
8mpg sounds pretty low to me, but alot depends on how you drive.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
When my Corvette was fairly new (I had already modified), I was at a store like a 7-11 & a guy driving an economy car, may have been a VW bug (who cares!) said to me something like "nice car-it must get terrible gas mileage". I told him w/ a straight face "It gets great gas mileage maybe the best I've ever got."
He looks at me stunned "How much do you get?"
Well I can almost get double digits.
He is stunned.
I was running a BB hot rod 10 sec. street legal race car slicks gears in the 5s. Work car SB solid lifter hot rod 4.56 ...
He looks at me stunned "How much do you get?"
Well I can almost get double digits.
He is stunned.
I was running a BB hot rod 10 sec. street legal race car slicks gears in the 5s. Work car SB solid lifter hot rod 4.56 ...
Burning Brakes
Joined: Nov 2006
Posts: 1,125
Likes: 4
From: Flower Mound TX.
I have a 72 with a 350 and 400 auto trans. I run a edelbrock performer intake, a edelbrock 650 carb, and a mild cam. I get 10 to 15 MPG city driving ( depending on how I'm driving that day ). With a 3.55 rear end.
Melting Slicks
Joined: Feb 2009
Posts: 3,438
Likes: 30
From: Fairfax Virginia
Although C3s do not get great mileage, at 6 MPG you are definitely abnormally low. Somewhere near 10-12 can usually be expected. Your warm-up procedure seem excessive. On the coldest days 3-5 minutes should be plenty before driving IMO.
