rear ratio to match peak torque??

[RapidRide2]

I have been reading on here that is best for economy to have peak torque at your desired cruising rpm. My desired rpm is between 65-70 mph. An average mostly stock sb chevy achieves peak torque at or around 22-2300 rpm. With a 700r4 and 235/75/15 tire the chart from bowtie overdrives calls for a 3.90-4.11 rear ratio. This seems somewhat steep??? However it DOES put me right around 22-2300 rpm at 65-70 mph. My question is why have factories for many, many years,,, circa 82' all the way up to now still put 2.56's 3.08's 3.42's etc. behind overdrive cars and trucks??? If a taller (numerically) gear ratio would have given the best mpg on small block cars and trucks???? seems to me you would see alot more 3.73-4.11 factory rearend gears. Thanks to all who reply.