According to this
Tire Chart your tires are 26.7" tall. You said 3spd so I assume its a 3-on-tree? With a manual transmission we know that in 'highway gear' you are at 1:1 ratio through your gearbox. I know there are better calculators out there but I used the one found at
Dirt Road Magazine I scrolled down to the one that was titled "Find RPM" I plugged in 60MPH, axle ratio 3.54 transmission ration 1 transfer case ratio 1 (since you don't have one) and tire diameter of 26.7 (found above) it says you are turning 2673RPM at 60, 3118RPM@70mph
I'll give you a couple sets of numbers based off that second website.
With your 3.54
2673@60MPH
3118@70MPH
With 3.25
2453@60MPH
2862@70MPH
With 3.00
2265@60mph
2643@70
It's not always about rear gearing either, let's say for example you decide it's time for new tires and wheels with your current 3.54:1 rear, I've never seen a galaxie that couldnt hold a bigger tire.. so
With your 26.7
2673@60MPH
3118@70MPH
27.7 (215/75R15)
2577@60
3006@70
28.3 (225/75R15)
2521@60
2942@70
Or even a combination of tires/gears, or you can chose a gear then 'fine tune' your RPM with tire selection. This is just me, I'd go with the 3.25 then swap to a taller tire later on.
FWIW using 3.25 and a 27.7" (215/75R15) would put you at 2759@70mph, which is darn close to your current 3.54 with 26.7" tires.
Things to think about.
-ron
