If your mostly racing and NOT doing much hwy driving go as low as you think the combo will allow. But if you do intend on driving it on the street 3.73 is as low as I'd ever consider with those short tires. Remember tire height has alot to do with final drive gearing. Your super short 25" tires with a 3.91 gear is about like a car with 28" tires and a 4.30 gearset.
