Oil prices will persist near or above $50 a barrel for years and force a shift to more fuel-efficient cars and alternative fuels, the government said Monday, discarding earlier predictions that costs would drop to around $30 a barrel.

The analysis reflected a significant change from the department's projections a year ago when it predicted oil prices in constant dollars — not counting normal inflation — would retreat in the long term and settle at about $31 a barrel by 2025.