View Full Version : Gas Millage On 62 Hardtop, 390
08-05-2013, 08:29 PM
Are they any owners who have attempted to monitor the gas mileage they are getting on their cars. I curious to see if my car is burning rich and i need of an a adjustment. These cars are not as efficient as the modern fuel injection but I was hoping to get a rough highway/city comparison. Thank you to all that respond.
08-05-2013, 08:42 PM
There are so many factors involved that affect gas mileage that two identical cars of this vintage will never get the same mileage. You have to take into consideration the condition of the motor, drive train, wheels, tires etc. As far as determining if your car is running rich the best way is to use a vacuum gauge to adjust the mixture screws on your carburetor. If you don't have one they are relatively inexpensive.
08-05-2013, 09:45 PM
I don't count on much more than 13mpg - and our gallon is Imperial, bigger than a US one!!
08-05-2013, 11:05 PM
The best way to determine if your engine is running rich is to take the plugs out, line 'em up on your bench in the order they came out, and compare them to published photos. You want to see a nice light tan color on the electrode insulators. If you see a darker tan or black you're wasting fuel.
08-06-2013, 12:52 AM
There are so many factors involved that affect gas mileage that two identical cars of this vintage will never get the same mileage. You have to take into consideration the condition of the motor, drive train, wheels, tires etc...Oh, how about changes in barometric pressure, humidity, altitude, temperature, cylinders in front usually run leaner than the middle ones. Carburetors don't offer a fine mist of atomize fuel under high pressure. More commonly, they pass UNBURNED fuel that dribbled in but was ushered thru so fast it had no chance for evaporation. The list goes on.
John's right. The same car will have 'good' days and bad, depending on what kind of day it was tuned on. These engines fly completely blind. There is NO feedback from any exhaust sensors. Idle mixture adjustment screws are only useful at idle. When you step on that pedal, jets and metering rods take over.
Everything affects how your engine performs. When you tune, it's usually with the air cleaner off and the hood up. That's not how the car runs down the road. Normally, the air cleaner chokes off the hot air your carb sucks in from a hot radiator.
If you live in altitude, you can go with smaller jet sizes but beware, if any cylinder runs too lean detonation and pre-ignition can burn holes in your pistons. (Don't ask me how I know.)
A rich mixture will help cool your engine and slow down the burn. Slightly retarded timing will help too. These are IN-efficient 'tricks' that classic car owners use to cope with today's fuel formulas. They cost a little more to run but will add longevity to a classic engine that was designed to much different specifications. - Dave
08-12-2013, 10:03 AM
Very Interesting. Thank you. I guess pulling the plugs is the best way to determine if the engine is burning lean or rich? I have read that most fuels today are junk with the ethanol causing chaos inside the engine. If I switch to the highest octane gas I can find (94) do you think gas additive is still necessary?
08-12-2013, 10:48 AM
The only advantage of a higher octane gas is to avoid pre-detonation (pinging) and run-on. If you don't have hardened valve seats you should use a lead substitute additive to keep them lubricated.
08-12-2013, 11:47 AM
I have owned the car for a month. It is the original 390 6.4l says the P/O but I don't know if the valves are hardened.
08-12-2013, 09:39 PM
We don't have lead in the gas anymore here in NZ - haven't since about '94.
I have not worried about lead-additive or hardened valves in any of my cars.
Only once have I seen valve-seat recession and that was in the '63 Anglia my wife had which had a 1,300cc Escort motor in it.
Had to adjust the valve twice due to valve-lash disappearing, otherwise it ran fine, it had solid lifter whereas the F100, T-bird etc have hydraulic lifters so maybe there's valve-recession occuring in them without it being evident.
When it shows its head THEN I will get hardened seats and new valves fitted.
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