The safe speed depends on the actual driving conditions: - Tire size and protector. With a full-size spare, you can maintain the commonly accepted speed on the road, avoiding racing. Drive at 50 mph max for 10 miles; then, slightly accelerate. On a donut tire, don’t go faster than 50 mph limit (preferably move slower). - Tire inflation. The spare tire might have been used just a few times or never for quite a long period, which could result in air loss. Having less pressure than in other tires might cause skidding at high speed or urgent braking. - Environment conditions. With a spare mounted, the road imperfections might cause more harm to your car suspension and chassis. The car becomes less controllable than usual, especially on a slippery road.