There is no such thing as a "performance and economy axle." There is a tradeoff--lower ratios (higher numbers) give better acceleration while higher ratios (lower numbers) give better economy.
The '64 GTO standard rear axle ratio was 3.55:1 early on and 3.23 later in the production year. The 3.55 ratio resulted in about 950 rpm per 20 mph of vehicle speed. So, at 60 mph in high gear, your engine would be running 2,850 rpm.
That's nearly double the engine speed on most modern vehicles, which results in more fuel usage, engine wear, noise, etc. The tradeoff was that the stock GTO's accelerated faster than most cars built in 1964. Their top speed was limited by engine RPM--120 mph would have the engine at 5,700 rpm.
The optional performance axle in a '64 GTO was 3.90:1, which gave 20 mph per 1,000 rpm.
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BONESTOCK GOATS
'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'99 Bonneville SE Sedan
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