Be interesting to know the gear ratios for the GA16 though, could probably give a more informed answer then.
Ask and you shall receive!
A few GA engined gearboxes:
GA16DE (n14/b13)
1st: 3.063
2nd: 1.826
3rd: 1.286
4th: 0.975
5th: 0.810
reverse: 3.417
GA16DS (carb n14 for comparison to GA16DE)
1st: 3.063
2nd: 1.826
3rd: 1.207
4th: 0.902
5th: 0.756
reverse: 3.417
GA14DS (carb, n14)
1st: 3.333
2nd: 1.955
3rd: 1.286
4th: 0.902
5th: 0.733
reverse: 3.417
GA14DE (N15)
1st: 3.333
2nd: 1.955
3rd: 1.286
4th: 0.902
5th: 0.756
reverse: 3.417
GA16DE (N15)
1st: 3.063
2nd: 1.826
3rd: 1.286
4th: 0.975
5th: 0.810
reverse: 3.417
Just for completions sake on the last two boxes:
GA14DE (N15)
Top Speed in 1 gear = 33.622 MPH
And changes into 2 gear at 4047 RPM dropping 2853 RPM
Top Speed in 2 gear = 57.320 MPH
And changes into 3 gear at 4539 RPM dropping 2361 RPM
Top Speed in 3 gear = 87.139 MPH
And changes into 4 gear at 4840 RPM dropping 2060 RPM
Top Speed in 4 gear = 124.236 MPH
And changes into 5 gear at 5783 RPM dropping 1117 RPM
Top Speed in 5 gear = 148.229 MPH
GA16DE (N15)
Top Speed in 1 gear = 36.585 MPH
And changes into 2 gear at 4113 RPM dropping 2787 RPM
Top Speed in 2 gear = 61.370 MPH
And changes into 3 gear at 4859 RPM dropping 2041 RPM
Top Speed in 3 gear = 87.139 MPH
And changes into 4 gear at 5231 RPM dropping 1669 RPM
Top Speed in 4 gear = 114.934 MPH
And changes into 5 gear at 5732 RPM dropping 1168 RPM
Top Speed in 5 gear = 138.347 MPH