All depends on the ratio in the auxiliary transmission, as well as engine RPM, axle ratio, and tire diameter.
Some auxiliary transmissions are still 1:1 or direct drive in high, no increase in top speed. Others can have overdrive ratios up to about 0.73, other common ratios are about 0.85 and 0.80 plus or minus a little given different makes and models.

To calculate road speed input the values here.
http://www.onlineconversion.com/bigger_tires.htm
Put your auxiliary ratio in for the "transfer case" ratio.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-