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Discussion Starter #1
I am new to this forum. I have an all original 66 GTO, 389 with four barrel, four speed and 3.55 rear. I bought the car to drive and enjoy, at 10.5 miles to the gallon, gas prices are keeping me from driving very much. What is the best rear end for mpg and performance? I have thought about a 3.08 or 3.23, what can I change to with the least expense?
 

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the price of the job will be the same. still have to buy ring n pinion gear set, bearings n seals, gear oil, cover gasket, Prussian blue, n labor to install. don't expect that much better miles to the gallon.
 

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With your 4-series carrier (3.36--4.33 gear ratios) you will need to replace the carrier assembly as well as the gears. $$$$$. Easier to find a complete rear end to swap in. If you're going for the best mpg, with a stick, a 3.08 is as low as you want to go. Simply swapping 3.36 gears into your present carrier will make a noticeable difference....I did that on my '65, and added 27.5" tires for a corrected ratio of 3.23. The car gets 15-ish MPG and cruises 70-75 mph just fine. 3.55 gears were ok when the national speed limit was 55 mph, but with today's higher speeds, they just don't cut it, unless you like a 60 mph cruise speed.
 

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Discussion Starter #4
Thanks for the information. I already have 215/75R/15 tires that measure 27.7 inches on the car. My first car was a 65 GTO that ran real strong on top end and would still beat a 396 Chevelle or a 390 Ford in a street race and get 15 mpg. Geeteeohguy I think your recommendation is the way to go.
 

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If you don't have to have original, I would suggest a 1967 or later intake with a Q-jet to replace the factory intake & Carter AFB and set it off to the side. The smaller primaries of the Q-jet will improve your gas mileage (as has been my experience), and still have the larger secondaries for that big kick in the pants full throttle run.
 

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Jim is correct. No better fuel miser than a well-tuned Q-jet. Better mpg than a 2bbl carb, even. Getting 20-21 mpg in my heavy '67 GTO on the open road at speed, but I'm cheating with a 2.56 rear gear.
 

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Have you considered a 5 speed swap? The overdrive gear will give you approximately 30% better fuel mileage, depending, without sacrificing performance. Might be more $$ than you want to spend though.
 

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To answer the original question, which was 'what is the best gear for mpg and performance for a '66 GTO 4 speed", the answer is simple: 3.23:1. It does everything well. Good off the line. Good on the highway. Converting his current gear to a 3.36 and running 27.5" tires will accomplish this. Simple, cheap, and durable. No mods to the floor pan of the car. No $4000 expense. Still retains originality and a real Hurst shifter. Easy-Peasy. Overdrive trannies are great, but very expensive and you have to cut the floors. Not for everyone.
 
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