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From: Daven Anderson
Date: February 08, 2002
1. The highway gas mileage would mainly be determined by axle ratio,manual or automatic would make virtually no difference. Since the standard ratio for an automatic '58 Fury is 3:31 and for the manual it's 3:73,the automatic cars would get better highway gas mileage with both setups having 'normal' ratios. The '58 Furys are dual four-barrel cars (both 318 and 350) and dual four-barrel cars can really pound down the gas. MOTOR TREND magazine in 1958 reported that a 1958 Fury with the 350 Golden Commando engine and A/T got 8.4 mpg city and 10.6 mpg highway. I used to have a '69 Sport Fury 383 4-bbl. (stock,3:23 ratio and auto,no O/D) that got 13-14 city and 17-18 highway. The single most effective way to improve your Fury's mileage would be to put a modern single 4-barrel carb and (383 LB) intake manifold. Your performance will not suffer,you should GAIN Horsepower and Torque doing this (as well as the huge MPG gains!). The OEM carbs put together flow little more air than modern 4-barrels,and even in 1959 Plymouth changed the Golden Commando to a single 4-barrel (so even with the small-flow four barrels of the time,dual 4's weren't worth the trouble to keep using!) Bonus: the original intake setup looks really cool as a display and you can put it back on at any time! Now about the manual trans. swap: FORGET IT! It won't help your mileage or performance,unless you got a 5 or 6 speed WHICH would cost you more to do than the Gear Vendors fitment to your car's Torqueflite. Sure,the 1958 Corvette has a 4-speed,but the '58 Borg-Warner T-10 is notoriously fragile and only a '58 Vette owner doing a 'Bloomington Gold' show-car restoration would want one. Yes,your rpm at 65 MPH should be around 3000 rpm. To sum up,I think changing the car to a modern 600 cfm single four barrel ALONE should give you the gas mileage improvement you seek. I HIGHLY recommend you do that first!