All eight fatalities from the May 31, 2013 El Reno, OK tornado occurred in vehicles, including the fatalities of some well-known storm chasers.
Here are the tornadoes with at least 5 vehicle fatalities in the last 6 years:
|Fatalities in Vehicles||Date||City of Tornado (County as Noted)|
|8||5/31/13||El Reno, OK|
|6||5/10/08||Newton/Barry Counties, MO|
|5||5/24/11||Canadian/Logan Counties, OK|
Source: NWS Storm Prediction Center Fatality Summaries for 2008-2013 available here