poor fuel mileage

[norm]

I can use some info on improving my fuel mileage if possible . I am getting around 9 mpg around town with ac on. I have a 55 Bel-Air it has a 1977 305 ci with 350 trans. 4bbl Edelbrock 500cfm. alum intake off Camero. duel exhaust with stock manifolds. The metering rods are set for normal from factory for idle and one step leaner on Cruise. middle hole on acel pump. My area does have a lot of traffic with pretty long lights at most major intersections. My temp gage is usually about 190 deg
Thanks for any input
Norm

[markm]

You are missing the most important information what rear axle gear are you running. Stock 55s had 3.55, 3.70 and very few 4.11s. Non of them are good for economy. A 59 or never 3.08 jug would be a big help. My 55 does much better and I run a 408 BBC with a solid lifter cam and two Holley 600s on a tunnel ram. I run a 3.54 Dana 60 rear with a TKO-600 5 speed with .62 5th. My 56 has a 327 with 3x2s carbs off a 57 Olds J2, 3.42 gears in a 9.3 Olds and a 700R4. Cruise RPM is the key to milage.

[norm]

I do have the stock rear 3:55 if I remember right

[norm]

If I should want to change rear end gearing . Would I change just the gears or swap out the whole rear? If swap what would fit?

[chevynut]

Wow, only 9 MPG with a 305 and 3.55 gears? Something is wrong, most likely that antique carburetor sitting on top of the engine. Or maybe it's your right foot?

If you change your gearing to a numerically lower ratio, you're going to lose street performance especially with that 305. Yes it will help on the highway, but it sounds like you don't drive at highway speeds that much and just cruise town. Remember that if you're sitting there idling a lot, your mileage is zero.

What's your highway mileage?

[norm]

don't know , never checked it and you right hardly do highway driving. Guess best to leave it alone?

[chevynut]

don't know , never checked it and you right hardly do highway driving. Guess best to leave it alone?

I personally think a 3.55 gear is good for street use, but it also depends on your transmission gearing. I looked it up and a typical T350 has a 2.52 first gear. That makes the overall ratio 8.95. I would not change the rear gear ratio myself, based on the way you drive your car. I think you'll be disappointed in the loss of street performance. I don't know what you should expect out of your setup but as for me I'll never own a carb again unless it's a stock collector car.

[norm]

Thanks so much for all your help . I'll leave it as is

[markm]

Wrong a 3.08 gear and a Th350 is a great combo I had that in my 78 Corvette, still have it in my 72 Cheyanne Super, 72 K/5 Blazer and Dads 46 Chevy 1/2 ton which has a 305 Q-jet and 79 Camaro 3.08 8.5 10 bolt posi. This old truck scoots pretty good with a stock 305. In 1959 started building 3.08 jugs for the 55-64 8.2 drop out rear end. If you get one your milage will improve. Alternatively an Overdrive trannie would work very well with your 3.55. Remember your 3.55 was designed for a Powerglide with an 1.82 first and you Th350 has 2.52.

[chevynut]

Wrong a 3.08 gear gives a 7.76 overall first gear ratio which will make the car a sick dog stoplight to stoplight. A stock 55 Chevy with a powerglide is a sick dog. In-town mileage won't really improve with a 3.08 rear because it's all stop and go and idling at stoplights. That's likely why your mileage is so bad, because you're burning fuel not going anywhere with the long lights. For driving around town the 3.55 gears should give great performance. Lots of guys with automatic transmissions use 3.55 gears. For highway cruising the 3.08 gears would be okay. The best of both worlds would be a 700R4 with overdrive and the right gears to keep the overall ratio in first gear around 9 or higher. My 56 Nomad with a fuel injected Ramjet 502, Viper T56 6-speed and aluminum 94 Corvette Dana 44HD has a 10.9 overall first gear ratio and a .5 overdrive 6th gear that gives me a 2.05 overall ratio. I still haven't driven it but I believe it will be amazing to drive .

[markm]

Not buying it, My Dads 46 Chevy 1/2 ton is not a dog, your math fails to take into account the effect of torque converter multiplication. I have seen a 69 Torino with a 466 BBF break a 4.88 gear in its 9 inch, come back to same strip a week later with a 3.50 and run only a tenth of a second slower. Said car had a 5000 stall TCI 8 inch convertor. The 3.55 gear is about perfect with a 700R4.

[busterwivell]

OK, so here's my two cents worth............My 56 has a basically stock 350/700R4 with a 8.5 10 bolt with 3:42 gears. I have 29inch tall rear tires. On the flat Interstate, I can have the AC blowing cold, cruise control set at 80, and get 20mpg. The car accelerates well in town, I'm sure if I drove it exclusively in town I'd probably get 15 or so. I live out in the desert and the nearest place to spend money is 13 miles away, so I get lots of highway driving as a rule. You may give some thought to a rear gear change and/or an overdrive transmission.

[chevynut]

OK, so here's my two cents worth............My 56 has a basically stock 350/700R4 with a 8.5 10 bolt with 3:42 gears. I have 29inch tall rear tires. On the flat Interstate, I can have the AC blowing cold, cruise control set at 80, and get 20mpg. The car accelerates well in town, I'm sure if I drove it exclusively in town I'd probably get 15 or so. I live out in the desert and the nearest place to spend money is 13 miles away, so I get lots of highway driving as a rule. You may give some thought to a rear gear change and/or an overdrive transmission.

So you have a 350, a 3.42 rear gear and a 700R4 with overdrive and get 15 mpg in town. Norm has a 305, 3.55 gear, and a 350 transmission. Norm wasn't complaining about highway mileage because he doesn't even know what it is.

In stop and go traffic in town I think the overdrive is irrelevant because I'd say you vitrually never use it. So you basically proved my point that Norm's 3.55 rear gear choice is fine in town. There is some reason he's only getting 9 mpg in town and it's not his rear gear or transmission. I don't think changing his rear gear or transmission will have much, if any, effect on his mileage in town. Do you?

[busterwivell]

Laszlo, I agree with what you have said. I, too, think he has some other problem, most likely the carb. I'd be inclined to try an Edelbrock 1406 and see how that does.

[chevynut]

Nobody's talking about drag racing here, it's Norm's mileage driving in stop and go traffic. A 3.08 rear gear isn't going to help improve mileage much when driving around town when you're mostly in first and second gear and idling a lot. I understood that a 700R4 was basically a 350 with an overdrive so what's the point of going to a 700R4 to fix an in-town mileage problem? I don't buy it. And performance WILL suffer with the 3.08 gears versus the 3.55s. Trying to bandaid poor mileage with a gear change when there's clearly something else going on is a bad move.