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Old 08-17-2003, 01:58 PM   #1 (permalink)
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What would the gas mileage be approximately on a 69 mach I with a 351 4v windsor engine, close ratio 4 speed toploader, and 3.25:1 rear gears(city driving)? I drive about 40 miles in the city every day so gas mileage is important. I would drive really carefully about 90% of the time to save gas and maybe once in a while put my foot on the gas to enjoy the power. What could i do to improve gas mileage besides installing stiffer springs for the secondaries?

Thanks for all the help.
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Old 08-17-2003, 02:07 PM   #2 (permalink)
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Install a 5 speed tranny , install 1 2.80 rear end , remove the 4bbl intake and install a 2bbl + the carb .
Might help the mileage a little , but nothing will help more than keeping your foot out of it or just getting a 2nd car with a 4cyl engine .
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Old 08-17-2003, 02:17 PM   #3 (permalink)
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If the carb is vacuum secondaries with stock calibration, and is driven for economy most of the time, the secondaries probably arent going to get used to speak of.


Economy can be enhanced by ensuring the car is in a good state of tune.....idle mixture and speed set properly, ignition in good shape (electronic ignition is a BIG plus).


And I believe lugging the motor is almost as bad for gas mileage as crusing at 50 mph in 3rd.


In stop and go traffic, a V8 car isnt going to get very good mileage period.
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Old 08-17-2003, 03:57 PM   #4 (permalink)
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What gas mileage can i expect with the setup I mentioned above?
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1969 Mach I,
rebuilt 351w, bored .030 over,
RV Torque cam, Holley 4160 600 cfm carb,
new 2.5 inch dual exhaust system with x-pipe and
terminator mufflers
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Old 08-17-2003, 04:10 PM   #5 (permalink)
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Its all how you drive the car thats it. no one can tell you how to drive it and if you granny it you will get the most outa the gas you use.
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Old 08-17-2003, 04:28 PM   #6 (permalink)
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Around the city, you can probably expect about mid-teens mpg. Installing a overdrive tranny isn't going to help in the city & a 4bbl carb will get better mpg's than a 2bbl as long as you don't get into it (since you're only using half of the 600 or 750cfm, it's still less then a 2bbl cfm which can be between 375 & 530cfm) Ultimately, if mileage is your biggest concern, I recommend considering a swap to fuel injection, but if this is out of the question, then make the engine as efficient as possible by (1) Keep it in good tune, (2) definately swap to electronic ignition (at least a pertronix), & (3) make sure air can get in & out of the engine as freely as possible (K&N filter & free-flow exhaust). I would install a vacuum gauge in the cockpit to monitor the tuning of the carb. Lastly and very importantly, maintain proper air pressure in your tires, & you can even go a couple psi higher to decrease rolling resistance.

Fuel injection will net you the best mpg (prob 22 mpg city, 28 hwy), and it will eventually pay for itself. If my stang was a daily driver, especially with your kind of mileage, I would certainly have fuel injection.

Good luck
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Old 08-17-2003, 06:47 PM   #7 (permalink)
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Im used to getting about 14-15 MPG so that is sort of what i am hoping to get, if i could get higher then that would be great. Do you guys think I could get 14-15 MPG city driving or is that too much?
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RV Torque cam, Holley 4160 600 cfm carb,
new 2.5 inch dual exhaust system with x-pipe and
terminator mufflers
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Old 08-18-2003, 05:47 AM   #8 (permalink)
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While rebuilding my Holley, I borrowed a book that said some 2V carbs may get worse gas mileage compared to a 4V due to the 4V having small primaries. As long as you stay out of the secondaries, of course.
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Old 08-18-2003, 08:26 AM   #9 (permalink)
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Time to diet. Lighten the car as much as possible.
Keep tire pressure at the upper limit. Set toe in about 1/16". Make sure that brakes don't bind ... and so on.

I have 600Vac that was tailored to 318-360 '77 Mopar Van. 61 main jets and black! secondary spring do the trick with 2-stage power valve + D.U.I. distributos and veryvery long legs at rear. I don't know city mpg but highway is about 24mpg. When I pass everybody on 2 lane road with gusto I get about 15-16 mpg, the car is much lighter 65. So I guess 15mpg would be reasonable for 69 in the city.
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