According to my gearing chart, a 7:33 with a 120/90x18 rear tire will give you 140 MPH at 7750 RPM. Its possible a breathed on motor will pull that. With an 8:33 and the same rear tire, your bike would go 160 in 5th if it could pull it. It takes something like the Bill Ross's Mandello Meteor Land Speed Bike to pull those kinds of numbers. Does that mean the gears wont work, of course not. Expect to have to slip the clutch a bit more off the line. FWIW, with the 8:33, your 4th gear will be just about the same (138.4) as 5th with the 7:33 (140.3). Think of your bike as a 4 speed, ignore 5th, and you will hardly notice the difference.