MPG or GPM?
Thread Starter
Racer
Joined: Nov 2011
Posts: 374
Likes: 21
From: Charleston SC
MPG or GPM?
'74 convertible, L48, 4spd, 3.36 gears. Car runs great. Just not long enuf. 11 MPG is the best I've done and that's on the the highway. Has anyone on this forum had close to this rotten mileage and actually increased by 4/5 mpg without spending a bunch? If so, I'd sure like to hear how you did it! (I consider $5,000 for a 5spd, a bunch.)
Thanks, Bill
Thanks, Bill
Pro
Joined: Feb 2012
Posts: 577
Likes: 6
'74 convertible, L48, 4spd, 3.36 gears. Car runs great. Just not long enuf. 11 MPG is the best I've done and that's on the the highway. Has anyone on this forum had close to this rotten mileage and actually increased by 4/5 mpg without spending a bunch? If so, I'd sure like to hear how you did it! (I consider $5,000 for a 5spd, a bunch.)
Thanks, Bill
Thanks, Bill
Give us a benchmark. At 70 MPH, what are your RPMs?
Drifting
Joined: May 2011
Posts: 1,536
Likes: 1
From: Boca Raton FL
I have a '69 L46, 4 spd, 3.73 and get around your MPG. When I am on the highway, I am at 3200 @70MPH so highway doesn't provide any help for MPG. I am in the process of trying to get a volume discount for people from this forum to buy a TKO 5 speed transmission. I am not buying it to improve my MPG, even though it certainly will on the highway, but more for drivability/fun on the highway. I have a thread related to trying to get a volume discount - post if you would possibly be interested.
Melting Slicks
Joined: Mar 2007
Posts: 2,100
Likes: 12
From: Frederick Maryland
I have a '69 L71 427/435 with 4.11 rear and an L88 camshaft the previous owner installed. If I drive it fast...6mpg...if I drive it carefully...6mpg.
I wouldn't complain with 11mpg. About all I could do to improve that would be change the rear or add a 5-speed and/or change the cam back to the appropriate grind. None of those options is really cost-effective for maybe 400-500 miles it gets driven per year.
I just live with it.
I wouldn't complain with 11mpg. About all I could do to improve that would be change the rear or add a 5-speed and/or change the cam back to the appropriate grind. None of those options is really cost-effective for maybe 400-500 miles it gets driven per year.
I just live with it.
Administrator
Joined: Jul 2000
Posts: 53,972
Likes: 6,196
From: About 1100 miles from where I call home.
I can honestly say in over 30 years and many, many Corvettes owned, I have never once thought about gas mileage. That includes having to buy Sunoco 270 in college for my 396.
It's like asking for a calorie count in a chocolate milk shake.
It's like asking for a calorie count in a chocolate milk shake.
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
They're certainly not cars for people concerned with gas mileage. I swear I can almost see my gas gauge moving down as I drive. A couple hours of driving will polish off a half tank easily if not more... It's been a while since I've checked but 10 -12 MPG would be doing well for many.
Thread Starter
Racer
Joined: Nov 2011
Posts: 374
Likes: 21
From: Charleston SC
I'm not really concerned as much about the mpg as I am about the distance between fillups. The guy I bought it from (I have documents showing he owned it from '93. I am the 5th owner.) swears he didn't change the tank, but the other day my gauge was bouncing on empty and visually I saw hardly nothing in the tank - took 12.1 gallons to fill up to the neck. Somebody changed something. I'm thinking my cheapest out is to change tanks. Won't help mileage but at least I won't get nervous after 100 miles. BTW, being a convertible wouln't be a reason for a smaller tank, other than the 17 gal it is supposed to have?
Thanks, Bill
Thanks, Bill
Pro
Joined: Feb 2012
Posts: 577
Likes: 6
I'm not really concerned as much about the mpg as I am about the distance between fillups. The guy I bought it from (I have documents showing he owned it from '93. I am the 5th owner.) swears he didn't change the tank, but the other day my gauge was bouncing on empty and visually I saw hardly nothing in the tank - took 12.1 gallons to fill up to the neck. Somebody changed something. I'm thinking my cheapest out is to change tanks. Won't help mileage but at least I won't get nervous after 100 miles. BTW, being a convertible wouln't be a reason for a smaller tank, other than the 17 gal it is supposed to have?
Thanks, Bill
Thanks, Bill
If your tank has the rubber bladder inside it may have collapsed and fallen in. Once that happens the overall tank capacity drops by quite a bit and the tank must be dropped and have the bladder replaced or repaired. I don't recall what years had what exactly but I remember that some had the rubber bladder and some had a hard plastic liner.
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Pro
Joined: Feb 2008
Posts: 556
Likes: 13
From: C3 Forum, Colorado
Now this is a thread I can relate to. Recently there was another thread going on with people getting 15-18 MPG and all I could think to myself is how in the "H" "E" double hockey stick are these guys doing it????
Maybe carburetors are to lean? Maybe no warm up time just start the engine and go? Maybe they are just tuned that well. Who knows??
I've got two 350 small block Chevy's getting about 9-10 MPG, A 454 big block Chevy getting about 9-10MPG. (Can't explain that one but don't really care other than I wish the small blocks were better).
Got a big block 460 Ford in my Bronco getting 7-8 MPG.
And a 428 Ford in my truck getting about 6-7 MPG.
This thing is just a pig. 
I haven't messed with them too much in a long time to try and get any better mileage so I guess that's my own fault.
I do know that there not running overly rich because they've all passed the emission standards that they're required to meet.
Sometimes I wonder if it's just the junky gas these days or maybe the 6200' elevation I live at.
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
There's so many different engine/trans/diff combos in C3's that you could ask 100 different owners their mileage and get 100 different answers. Not to mention different driving styles.
I get 16mpgs in my stock '81 with TH350 and 2.87 rear end.
I get 16mpgs in my stock '81 with TH350 and 2.87 rear end.
Melting Slicks
Joined: Jul 1999
Posts: 3,030
Likes: 17
From: White Pine / Dandridge TN
I'm guess-estimating that I get around 10-11 mpg highway as well. 350 with vortec heads and carter 625 cfm carb. Carb was tuned on a dyno with tailpipe sniffer. Don't know what cam is in the car. 4 speed manual tranny and 3.55 rear end. I purchased an aftermarket T5 made for mustang cobras, same specs as ford racing T5, that I plan to install when I have the time. I paid $450.00 for it with about 3,000 miles on it. Hoping the overdrive improves my mileage a little.
Pro
Joined: Nov 2007
Posts: 710
Likes: 0
From: Northern Florida
'74 convertible, L48, 4spd, 3.36 gears. Car runs great. Just not long enuf. 11 MPG is the best I've done and that's on the the highway. Has anyone on this forum had close to this rotten mileage and actually increased by 4/5 mpg without spending a bunch? If so, I'd sure like to hear how you did it! (I consider $5,000 for a 5spd, a bunch.)
Thanks, Bill
Thanks, Bill
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
I have a '71 L48, 3.36 axle and THM-400 auto tranny. With that combination, I routinely would get 15-16 mpg on the highway. I didn't do that much 'in-city' driving to check for that alone, but I'd guess it would be in the 12 mpg neighborhood.
After swapping the THM-400 tranny for a 700R4, I get a little over 20 mpg on the highway. And, it's a lot easier on the engine...and my ears! (2050 rpm @ 70mph instead of 2900 rpm)
After swapping the THM-400 tranny for a 700R4, I get a little over 20 mpg on the highway. And, it's a lot easier on the engine...and my ears! (2050 rpm @ 70mph instead of 2900 rpm)
Burning Brakes
Joined: Mar 2010
Posts: 987
Likes: 7
From: Surrey British Columbia
Converting my 4-speed to a 5-speed is more about driving comfort on the highway than gas milege. But I will be honest, gas milege is on my mind, especially when you consider that the Vette has a 92 liter gas tank and gas in $1.29 per liter on the coast right now.
Advanced
Joined: Aug 2010
Posts: 83
Likes: 0
From: Monterey CA
My 72' was getting 12 City/15 Highway with a zz430 clone/TH400 600cfm Edelbrock. I swapped carbs to a 770 street Avenger and now I'm at 10/11. She runs much better though and plugs still look good. Might try and bring it down a jet...
. She runs much better though and plugs still look good. Might try and bring it down a jet...
Drifting
Joined: May 2011
Posts: 1,536
Likes: 1
From: Boca Raton FL
80 - not sure if you are aware but we are doing a discounted group buy of 5 speed TKO transmissions. Let me know if you are interested in participating.
Instructor
Joined: Oct 2011
Posts: 131
Likes: 0
From: Asuncion
I was getting about 6-7 mpg between city and highway 60/40% with my 350 and th350 80 vette with an 650 holley.... now I swap the intake and carb for a street avenger 770. Couldn't test again. But I was having a leak in the vacuum that runs to the tranny so it was changing at much higher rpms. So my guess is that maybe with the new carb and the leak solve it will be about 8... I will have some test as soon I can and let you know...
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
3.36 axle's actually a pretty good choice for performance street/highway driving, and should be doing better than 11 mpg highway. I'd have a look at the carb and dizzy, to make sure they're working properly. Carb floats can sink, and dizzy advance mechanism's gum up. Vacuum hose leak somewhere, could be a problem too.
Le Mans Master
Joined: Oct 2004
Posts: 6,314
Likes: 508
From: South-central Missouri
I'm a technology junkie...
My wife's L46/M-21/ 4:11s gets about 11-12 mpg tops - 3200 rpm in 4th, at 70, makes north of 350 hp at the crank w/ long tube headers and mild performance cam. It has an Edlebrock 600 cfm carb. Runs great and reminds me of my wasted youth!
If I wasn't laid off at the moment, I wouldn't be typing this, and I'd have my hand up for that TKO group buy!!
Next to it sits a computer controlled, sequetially injected, 350 cid, 508 hp LT5 with stock cams and idles like a stocker that gets 26 mpg in cruise at 70mph/6th gear.
Not dissing, but one has to appreciate the technilogical evolution. (Just sayin'
)
P.
My wife's L46/M-21/ 4:11s gets about 11-12 mpg tops - 3200 rpm in 4th, at 70, makes north of 350 hp at the crank w/ long tube headers and mild performance cam. It has an Edlebrock 600 cfm carb. Runs great and reminds me of my wasted youth!
If I wasn't laid off at the moment, I wouldn't be typing this, and I'd have my hand up for that TKO group buy!!Next to it sits a computer controlled, sequetially injected, 350 cid, 508 hp LT5 with stock cams and idles like a stocker that gets 26 mpg in cruise at 70mph/6th gear.
Not dissing, but one has to appreciate the technilogical evolution. (Just sayin'
)P.
Last edited by Paul Workman; Feb 14, 2012 at 10:19 AM.