The research found that the tiered structure of rates means that people lending a large sum of money may pay less interest than those borrowing smaller amounts.
Mr Black told consumers that by researching and comparing deals, they may have the chance to save "a considerable amount" of money.
"Obviously choosing a longer period to repay a loan does exaggerate the differences and so the length of the loan is another factor to be considered," he added.
"Large tier rate differences do not necessarily equate to uncompetitive rates, so borrowers need to be on their toes when it comes to taking out a loan."
Established in 1994, Defaqto is a leading research company which offers information on thousands of products including pensions, savings and loans.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.