Gas Mileage - 66 coupe

[John F.]

I've had a 66 327 300 coupe for a number of years and have always had lousy gas mileage - approx 10 mpg. I've always understood that I should be getting 15-18 mpg. I have had a new carb installed and adjusted, a full tune-up, the distributor checked, compression tests with no improvement at all. Any ideas what my next step should be?

Second question: I've working on the exhaust for this car and have noticed that the assembly manual advises a sealing compound to be used when assembling the exhaust pipes and attaching the muffler. Have not been able to find this stuff anywhere. Is it still made, or are there any substitute materials that can be used?

Thanks for your advice

John Farren

[Donald O.]

Re: Gas Mileage - 66 coupe

I've had a 66 327 300 coupe for a number of years and have always had lousy gas mileage - approx 10 mpg. I've always understood that I should be getting 15-18 mpg. I have had a new carb installed and adjusted, a full tune-up, the distributor checked, compression tests with no improvement at all. Any ideas what my next step should be?

Second question: I've working on the exhaust for this car and have noticed that the assembly manual advises a sealing compound to be used when assembling the exhaust pipes and attaching the muffler. Have not been able to find this stuff anywhere. Is it still made, or are there any substitute materials that can be used?

Thanks for your advice

John Farren

IIRC you can get a tube of "muffler cement" at your FLAPS. Connect the pipes and apply a bead around the joints after clamping assembly, then wipe off any unsightly goobers. This stuff sets up real FAST but no more leaks.
We used this stuff when I worked at the MAremont/Gabriel R&D Tech center.

DonO

[Ed S.]

Re: Gas Mileage - 66 coupe

John,

You did not mention if you had a 4 speed or automatic. Also, differential gear ratios will have a significant impact in fuel mileage. Do you know what ratio yours is? The higher the number (the lower the ratio) and the lower the fuel mileage, 4:11s or lower would probably consume close to a gallon in 10 miles. Finally, what type of driving and how you drive will impact mileage - but I suspect that you are well aware of those variables.

[Timothy B.]

Re: Gas Mileage - 66 coupe

John,

The stock 585 CFM Holley carburetor calibrations should not be so rich to yield those MPG numbers. If you feel like it pull a few plugs and see how they look as that will give you a indication where to check.

[Mark E.]

Re: Gas Mileage - 66 coupe

As John suggests, your mileage should be bettervassuming you don't have 4.11 gears, mostly drive WOT until the thermostat opens and then shut it down.

My '70 454 with 4 speed and 3.36 gears manages 14-16 MPG on the open road; 11-14 MPG going to the local cruise night and back.

Things I would check (in this rough order):
- Vacuum Advance Unit. Make sure it moves freely, doesn't leak, and is the proper unit for your car.

- Ignition Timing. Initial, centrifugal, and vacuum are all important. Make sure the centrifugal mechanism works freely, and the timing curve is correct for your engine. Check out one of Duke's many posts about setting up the curve.

- Thermostat. Make sure it isn't stuck open.

- Heat Riser. Make sure it isn't stuck closed. Also look for other exhaust obstructions.

- Choke. Make sure it fully opens.

- Distributor gear. Is it clocked properly on its shaft?

- Poke around. And look for less obvious stuff like PCV, vacuum leaks, marginal ignition wires, dirty filters, brake drag, low tires, frozen fan clutch, slipping clutch, ...

- Check the air/fuel ratio with an exhaust gas analyzer. That takes the guess work out of how rich you're running.

- Check the phasing of the cam and condition of the timing chain.

- You're not one of those guys who drives with his left foot on the brake, are you?

And lastly, think through how you measure the amount of fuel used and calculate MPG. Some methods are more accurate than others.

[John F.]

Re: Gas Mileage - 66 coupe

John,

You did not mention if you had a 4 speed or automatic. Also, differential gear ratios will have a significant impact in fuel mileage. Do you know what ratio yours is? The higher the number (the lower the ratio) and the lower the fuel mileage, 4:11s or lower would probably consume close to a gallon in 10 miles. Finally, what type of driving and how you drive will impact mileage - but I suspect that you are well aware of those variables.

Ed - I have a wide ratio 4 speed with 3.36 rear end. My driving is usually to and from local car shows, and some cruising in rural areas - I live in a small town in Western North Carolina, and my driving is probably 75% highway, 25% around town. I still suspect something mechanical, just not sure which direction to go in.

John

[John F.]

Re: Gas Mileage - 66 coupe

Good advice Mark - I'll try some of these ideas and see what happens. Thanks for taking the time.

John

P.S. Definitely not a brake rider!

[Tom B.]

Re: Gas Mileage - 66 coupe

My 66 L 79 gives me 20 mpg on hwy doing 65 mph with 3 36 gear and 4spd

[Jim T.]

Re: Gas Mileage - 66 coupe

John you mentioned your distributor was checked. Was the vacuum advance source of vacuum checked to insure that it receives full time vacuum from a full time vacuum port on your carburetor? At idle your 1966 I should have 18-20 inches of vacuum from my experience of my Corvette. The supplied vacuum will not do any good if the vacuum canister has a rupture and will not hold vacuum to operate. Your centrifugal advance may need some attention. Removing the rotor cap and activate the advance function and see if it moves freely. A previous owner could have installed after market strong springs for the centrifugal system which would inhibit the function of good advance from the centrifugal system. A replacement vacuum advance unit that provides more advancement with your 66's supplied vacuum may help. Your reported gas mileage of 10 for a 327/300 can improve.
Reference Tom's post above of 20 mpg, my best ever for my 1968 327/350 L79 was 19 mpg and is has the lower 3:70 gears

[Mark E.]

Re: Gas Mileage - 66 coupe

John,

Jim's advice about the VAC source can make a real difference. Back in the day, I saw so many cars with a VAC that was disconnected (they thought this was a hot rod mod), leaking, or too lazy. Their gas mileage was lousy. If you search through Duke's posts, he has great information about selecting a VAC and hooking it up properly. Be sure to check and configure your initial and centrifugal timing curve as well. These all must work together.

I saw this thread under "similar threads", which has more mileage data points for you:

https://www.forums.ncrs.org/showthre...43-Gas-mileage

[John F.]

Re: Gas Mileage - 66 coupe

John,

Jim's advice about the VAC source can make a real difference. Back in the day, I saw so many cars with a VAC that was disconnected (they thought this was a hot rod mod), leaking, or too lazy. Their gas mileage was lousy. If you search through Duke's posts, he has great information about selecting a VAC and hooking it up properly. Be sure to check and configure your initial and centrifugal timing curve as well. These all must work together.

I saw this thread under "similar threads", which has more mileage data points for you:

https://www.forums.ncrs.org/showthre...43-Gas-mileage

Guys - thanks for the remarks about the VAC. I'm planning on spending an afternoon checking this and several other issues. I also found a large crack in my heat riser and have replaced it. I don't know how much difference that will make, but we'll see.

John