What for? Do you know that Toyota 5-speed ans 6-speed transmissions have their ratios almost equal sa last gear?
For example: Toyota V160 transmission (yun lang ang nasa isip kong RWD 6speed ng Toyota eh)...
Gear ratios from 1st to 6th: 3.830 2.360 1.680 1.310 1.000 0.790
Di ko sure kung ano ang tranny ng Revo diesel, but if yung Toyota G52 sya:
Gear ratios from 1st to 5th: 3.930 2.330 1.191 1.00 0.830
Difference: 0.040
%diffrence (0.830 to 0.790) = 4.82%
<-- do you think 4.82% increase on your top speed is worth the trouble?
Conclusion: More gears does not always mean so much higher top speed - higher accelaration sure yan...
Good luck din if you can get yourself a 6-speed RWD transmission to match your bell housing/engine block.... If you find one, please tell me where...
If you would like higher top speed talaga, change the differential ratio.... mas practical and less hassle...

Your stock is 4.3:1 if i'm not mistaken. You can try looking for 4.1:1 or 3.909:1...
