The gearing kinda depends on the camshaft. I believe you said a “built” engine, so I assume an aftermarket camshaft.
Since you said you want to rev to 7500, I’ll assume the camshaft will make power up there.
Let’s look at the current setup.
50T : 11T = 4.55 : 1
With the 30 series and a 6” driven pulley (comes standard), overdrive (0.9 : 1), the final gear ratio would be 4.1 : 1 (if the TC ever fully shifts). Let’s assume the TC does shift.
With 19” tires, 4500 RPM would be 62 mph.
3600 RPM is around 50mph if the TC shifts out.
If you’re doing 50mph at 4500 RPM, the TC is not fully shifting.
7500 RPM would be over 100 mph if the TC shifts out with that gearing.
Now let’s switch the 11T to an 8T.
50T : 8T = 6.25 : 1
With 30 series overdrive, gear ratio would be 5.625 : 1.
Let’s still assume the TC shifts out.
4500 RPM is 45 mph.
Now let’s keep the 8T, but also switch the 6” driven pulley to a 7” driven pulley.
50 :8 = 6.25 :1.
With 7” driven pulley (highest gear is 1.12 :1 instead of 0.9 :1), gear ratio after fully shifting would be 7 :1.
4500 RPM would be 36 mph.
6250 RPM would be 50 mph.
If the 7” pulley fits, you’ll need a longer belt.
So with 19” tires and a 50T rear sprocket, your best bet would be an 8T front sprocket and the 7” driven pulley (if it fits). I’m assuming you have #40,#41, or #420 chain.
If you really want to make use of 7500 RPM, consider a 60T rear sprocket, an 8T front sprocket, and the 7” driven pulley.
7500 RPM = 50mph.
It will wheelie like crazy with a tiny twist of the throttle, but you’ll reach 7500 RPM at 50mph.
---------- Post added at 01:13 AM ---------- Previous post was at 12:54 AM ----------
Another option is keeping the 6” pulley, using an 8T front sprocket, and replacing the 50T with a 60T.
4500 RPM = 37.7 mph
5975 RPM = 50 mph.
19” tires makes it difficult to make use of high RPMs at a reasonably low speed. For off-road I like the NR 280-0211 cam which makes its power at 5500 RPM and below.
