65 teeth on the wheel, rather than 60, will lower the overall gearing to 60/65 of what it was - so same revs at 30 mph as it used to do at 32.5 mph.
Not a huge difference, and would help on hills, but lower top speed and economy.
Gearing would be returned to the original if the drive sprocket on the engine could be increased in similar amount. So, for example, a 20-tooth sprocket increased to 21 (still slightly lower gearing) or 22 (giving very slightly higher gearing).