Gas Milage Question
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Gas Milage Question
This '73 has the 190hp SB with a TH400 and the standard (and freshly rebuilt) QuadraJet carburetor.
I checked the speedometer accuracy today with a GPS receiver: It's 1 MPH off at 60. I can easily live with that.
Then I filled the tank and calculated the bitter truth: 6.2 MPG.
I don't drive with a lead foot, by any strech. And the milage was accrued by way of city driving.
Is this normal?
Thanks in advance.
Steve
I checked the speedometer accuracy today with a GPS receiver: It's 1 MPH off at 60. I can easily live with that.
Then I filled the tank and calculated the bitter truth: 6.2 MPG.
I don't drive with a lead foot, by any strech. And the milage was accrued by way of city driving.
Is this normal?
Thanks in advance.
Steve
Pro
Joined: Aug 2013
Posts: 591
Likes: 3
From: Monroe Washington
Just calculated mine with a 383 (450-500hp) and 700r4 trans... 13.82 average... And I'm not shy about the skinny pedal either... That was trying to use highways when applicable..
If I hot rod a LOT, and stick to town and back roads... Solid 12 mpg...
Going on a road trip Friday... A few hundred miles of freeway... Gonna try to keep my foot OUT of it and see what I can average...
And I just swapped this trans in, with the th350 I was averaging 11-13 still...
If I hot rod a LOT, and stick to town and back roads... Solid 12 mpg...
Going on a road trip Friday... A few hundred miles of freeway... Gonna try to keep my foot OUT of it and see what I can average...
And I just swapped this trans in, with the th350 I was averaging 11-13 still...
Le Mans Master
Joined: Aug 2006
Posts: 7,426
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Who cares about milage?
12 around town in stop and go traffic and under 35 miles per hour.
16 if I'm on the highway.
16 if I'm on the highway.
Team Owner
Joined: Aug 1999
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From: Ontario
Pro
Joined: Aug 2013
Posts: 591
Likes: 3
From: Monroe Washington
Stuck in Cali traffic jams often? My work commute is roughly 20-25 miles each way... Can take back roads or highway but still have town to deal with at each end... 6.2 mpg seems pretty low... Does she run good and no fuel leaks?
My dad had a 69 with a 427 manual and a bunch of upgrades straight from the factory, and always boasts that he got 7 mpg everywhere... Lol. Dunno the full legitimacy of that as he hasn't had that car in a looooong time... But the stories are sure fun when the topic comes up... ;-)
My dad had a 69 with a 427 manual and a bunch of upgrades straight from the factory, and always boasts that he got 7 mpg everywhere... Lol. Dunno the full legitimacy of that as he hasn't had that car in a looooong time... But the stories are sure fun when the topic comes up... ;-)
Last edited by FireballXL5; May 14, 2014 at 08:31 PM.
Racer
Joined: Nov 2011
Posts: 374
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From: Charleston SC
From the guy with the nicest '74 on the forum, that is a much appreciated comment. Thanks. The seats are '99 Vette and really need recovering. I have the originals in the attic but like the '99s because of the adjustable back. (no power on either one) Thanks again for the comment and have a wonnerful week, Bill
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2026 Loser of the Year
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From: New Or-leens Loo-z-anna
350CI stock/250hp/600Holley/700R4/2.9? Diff. Gears/BFG 235-15/3100lb/'34 street truck...(for reference)
13mpg in town..............
20 "lbs." vacuum at idle in neutral
12 "lbs" vacuum with A/C on at 60 mph
22mpg on Interstate.......VeryLightFoot.........
Subtract 5mpg from each number when Foot gets HEAVY!!!!
13mpg in town..............
20 "lbs." vacuum at idle in neutral
12 "lbs" vacuum with A/C on at 60 mph
22mpg on Interstate.......VeryLightFoot.........
Subtract 5mpg from each number when Foot gets HEAVY!!!!
Last edited by doorgunner; May 17, 2014 at 08:54 AM.
Race Director
Joined: Jan 2007
Posts: 10,682
Likes: 696
From: Danville Illinois
2025 Corvette of the Year Finalist - Modified
2024 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
2019 C3 of Year Finalist (appearance mods)
My 75 sb L48 with top end mods, tb400 and 308, i can get 16 on the h/y with the air on. City maybe around 10/12
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Jet size?
Of note: This car's exhaust area (square pipe tips & lower tail panel) were extremely carboned up. And the shop guys who tuned the motor for me said it seemed to run rich. Now I haven't done a search yet on this particular carburetor's jet size, but perhaps if I go smaller, the milage will increase without much loss in performance. Anyone here do that?
I don't speed. I'm a rather dull car driver I'd say. Last speeding ticket I got was in '84. Was the last of ANY ticket I ever got. Now that I just jinks'ed meself.....
Great mileage Phil! A helluva difference from mine.
Anyway, is a rejetting the way to go with a low MPG car or not?
BTW, bought gas today for the first time this Spring for under $4/gal.
OK, $3.99.9
Steve
I don't speed. I'm a rather dull car driver I'd say. Last speeding ticket I got was in '84. Was the last of ANY ticket I ever got. Now that I just jinks'ed meself.....
Great mileage Phil! A helluva difference from mine.
Anyway, is a rejetting the way to go with a low MPG car or not?
BTW, bought gas today for the first time this Spring for under $4/gal.
OK, $3.99.9
Steve
Last edited by Cavu2u; May 14, 2014 at 09:48 PM.
Racer
Joined: Jan 2014
Posts: 334
Likes: 6
These cars were never known for great gas mileage but a (mild build) small block that gets less than 10 mpg is definitely not right, sound like you're on the right track thinking it's rich.
Le Mans Master
Joined: Feb 2001
Posts: 6,317
Likes: 291
From: Ocala FL
Of note: This car's exhaust area (square pipe tips & lower tail panel) were extremely carboned up. And the shop guys who tuned the motor for me said it seemed to run rich. Now I haven't done a search yet on this particular carburetor's jet size, but perhaps if I go smaller, the milage will increase without much loss in performance. Anyone here do that?
I don't speed. I'm a rather dull car driver I'd say. Last speeding ticket I got was in '84. Was the last of ANY ticket I ever got. Now that I just jinks'ed meself.....
Great mileage Phil! A helluva difference from mine.
Anyway, is a rejetting the way to go with a low MPG car or not?
BTW, bought gas today for the first time this Spring for under $4/gal.
OK, $3.99.9
Steve
I don't speed. I'm a rather dull car driver I'd say. Last speeding ticket I got was in '84. Was the last of ANY ticket I ever got. Now that I just jinks'ed meself.....
Great mileage Phil! A helluva difference from mine.
Anyway, is a rejetting the way to go with a low MPG car or not?
BTW, bought gas today for the first time this Spring for under $4/gal.
OK, $3.99.9
Steve
http://forums.corvetteforum.com/c3-t...ell-plugs.html
http://forums.corvetteforum.com/c3-g...-on-q-jet.html
Another possibility is the composite float in a Q-jet goes bad and sinks. Gas then flows over the bowl and down the throttle bores. The car starts easily on a cold day, because it is running very rich. Top end and throttle response aren't all that great once warmed up, though.
I usually put a brass float in to eliminate that, since the composite floats often break down from the junk in the gas anymore.
Once you get these fixed, then you are set.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
No, that would not be normal mileage for your car...unless it has a 4.11 or steeper rear gear and that is "in town" mileage.
My SB '71 coupe with THM-400 and 3.36 rear gear would get about 16 mpg on the highway and about 13-14 mpg overall. In-town mileage was likely around 10-12 mpg.
You have some "economy" problems. Most likely, there are some issues with the carb. But, for the mileage to be that low, you likely have some timing issues, too. Your vacuum advance can might be 'toast'; the mechanical advance system in the dizzy could be locked up; your timing setup could be off, as well.
Setting up everything related to timing would be where I would start. Once those issues are put to bed, check again.
'73 cars did not yet have a CAT system, so that isn't an issue. Points, condenser, plugs, wires can have effects on mileage, but they would have to be in pretty bad shape for those kinds of numbers. Still good to be sure the ignition system is in good condition.
My SB '71 coupe with THM-400 and 3.36 rear gear would get about 16 mpg on the highway and about 13-14 mpg overall. In-town mileage was likely around 10-12 mpg.
You have some "economy" problems. Most likely, there are some issues with the carb. But, for the mileage to be that low, you likely have some timing issues, too. Your vacuum advance can might be 'toast'; the mechanical advance system in the dizzy could be locked up; your timing setup could be off, as well.
Setting up everything related to timing would be where I would start. Once those issues are put to bed, check again.
'73 cars did not yet have a CAT system, so that isn't an issue. Points, condenser, plugs, wires can have effects on mileage, but they would have to be in pretty bad shape for those kinds of numbers. Still good to be sure the ignition system is in good condition.
Drifting
Joined: Apr 2007
Posts: 1,970
Likes: 45
From: Cameron Park CA
No, that would not be normal mileage for your car...unless it has a 4.11 or steeper rear gear and that is "in town" mileage.
My SB '71 coupe with THM-400 and 3.36 rear gear would get about 16 mpg on the highway and about 13-14 mpg overall. In-town mileage was likely around 10-12 mpg.
You have some "economy" problems. Most likely, there are some issues with the carb. But, for the mileage to be that low, you likely have some timing issues, too. Your vacuum advance can might be 'toast'; the mechanical advance system in the dizzy could be locked up; your timing setup could be off, as well.
Setting up everything related to timing would be where I would start. Once those issues are put to bed, check again.
'73 cars did not yet have a CAT system, so that isn't an issue. Points, condenser, plugs, wires can have effects on mileage, but they would have to be in pretty bad shape for those kinds of numbers. Still good to be sure the ignition system is in good condition.
My SB '71 coupe with THM-400 and 3.36 rear gear would get about 16 mpg on the highway and about 13-14 mpg overall. In-town mileage was likely around 10-12 mpg.
You have some "economy" problems. Most likely, there are some issues with the carb. But, for the mileage to be that low, you likely have some timing issues, too. Your vacuum advance can might be 'toast'; the mechanical advance system in the dizzy could be locked up; your timing setup could be off, as well.
Setting up everything related to timing would be where I would start. Once those issues are put to bed, check again.
'73 cars did not yet have a CAT system, so that isn't an issue. Points, condenser, plugs, wires can have effects on mileage, but they would have to be in pretty bad shape for those kinds of numbers. Still good to be sure the ignition system is in good condition.
With my timing set correctly (mechanical and vacuum advance) and running a Holley double pumper I get way better mileage. ~10 around town (into the secondary pump a lot) and ~15 highway. You have work to do. 
Pete
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Thank you men. All great input for me.
Although the distributor is a new HEI and the carb had been rebuilt by a pro, things can still go wrong with them. Never did a compression check either. 66K on the odometer.
Interesting point on timing: The harmonic balancer shifted it's mark and the shop had to go by "ear" to time it. I have the replacement balancer here, along with the tools to pull & install it, as well as all the gaskets, and even a replacement timing chain/gear set. That timing may very well be the culprit. Just waiting for cooler weather to do it. This town is burnin' up right now. Really!
Thanks again.
Steve
Although the distributor is a new HEI and the carb had been rebuilt by a pro, things can still go wrong with them. Never did a compression check either. 66K on the odometer.
Interesting point on timing: The harmonic balancer shifted it's mark and the shop had to go by "ear" to time it. I have the replacement balancer here, along with the tools to pull & install it, as well as all the gaskets, and even a replacement timing chain/gear set. That timing may very well be the culprit. Just waiting for cooler weather to do it. This town is burnin' up right now. Really!
Thanks again.
Steve
Instructor
Joined: Feb 2009
Posts: 159
Likes: 0
From: asheville nc
My 327-350hp gets over 20 mpg on the parkway...45 mph speed limit...keep in mind our mtns here are the highest east of the mississippi...a lot of going up but also a lot of going down on the return trip.
Once in the 80's, running 70-80 plus on the interstate heading to Atlanta...got 14 mpg but every truck was saying listen to that vette that is getting ready to pass you...and I would get on it passing them....little did they know I had a cb also in my vette at that time....probably why only 14 mpg.
Once in the 80's, running 70-80 plus on the interstate heading to Atlanta...got 14 mpg but every truck was saying listen to that vette that is getting ready to pass you...and I would get on it passing them....little did they know I had a cb also in my vette at that time....probably why only 14 mpg.
