WASHINGTON  Consumers who buy minicars to economize on fuel are making a big tradeoff when it comes to safety in collisions, according to an insurance group that slammed three minimodels into midsize ones in tests.

In a report prepared for release on Tuesday, the Insurance Institute for Highway Safety said that crash dummies in all three models tested  the Honda Fit, the Toyota Yaris and the Smart Fortwo  fared poorly in the collisions. By contrast, the midsize models into which they crashed fared well or acceptably. Both the minicars and midsize cars were traveling 40 miles per hour, so the crash occurs at 80 m.p.h.

The institute concludes that while driving smaller and lighter cars saves fuel, “downsizing and down-weighting is also associated with an increase in deaths on the highway,” said Adrian Lund, the institute’s president.

“It’s a big effect  it’s not small,” he said in a telephone interview.

Yet the institute did not quantify how many more highway deaths might be expected statistically from any increase in the use of minicars.