A Prius always starts moving on battery power and engages the gasoline engine when acceleration requires it, The speed at which this happens varies by model and load. At 90,000 miles the car should have no problems but is approaching the limit of the hybrid battery warranty. The battery should last close to 200,000 miles but some have failed around 120,000. Driving a hybrid is different from driving an ordinary car. Watch the Consumption Display and it will help you get the best results. Good luck.