ROMULUS, Mich. — If you thought expensive plane tickets and rising gas prices were the last of your travel woes, think again. Car rental costs also are rising, adding another financial jolt to the journey.

“Rates are up substantially year over year,” car rental consultant Neil Abrams said. His company, Abrams Consulting Group Inc., developed a travel data rate index that compiles information from eight different rental companies and tracks rates from week to week.

Looking at the price of a rental on a mid-size car, the Purchase, N.Y.-based consultant found a $20-a-day increase from last year. For example, he said, the cost of a one-day rental, booked one day in advance, was $82 this week, compared with $62 during the same week last year.

The reason for the price hike, Abrams said, is that rental companies are decreasing the number of cars in their fleets, going from more than 2 million cars to 1.7 million nationally in the past three years.

Ned Maniscalo, a spokesman for Enterprise Rent-A-Car, said he sees prices increasing, but not significantly. “I would be surprised if they’ve gone up 10 percent,” he said.