Back calculate it.What's the max cruiser rpm you are comfortable with? What's your max cruise speed? What's your tire size or how many revolutions of the tire per mile?

With those three numbers you should be able to calculate it. Trick is that 60mph is one mile per minute. So revolutions for a tire to roll one mile become tire rpm at 60mph.

For example, 3000 rpm at 80 with 27" tall tire.

27 x pi /12 = 7.07 feet per revolution

5280 / 7.07 = 747 revolutions per mile

Turn ideal rpm at 80 into rpm at 60

(60 / 80) x 3000 = 2250 rpm at 60

calculate gear ratio 2250 / 747 = 3.01 That's optimum gear ratio assuming the car is either 4-speed or converter slips only a little bit. If converter slips 3% then multiply ideal gear ratio by 0.97, giving you 2.92 as the ideal ratio.
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Now pick your closest factory gear ratio.

Done.

R.