Due to the length of the lever from the fulcrum point (club shaft length) a greater head speed can be expected with the driver. Other variables include the width of your swing with driver versus other clubs. So there isn't one ideal relationship to determine actual, but 77-82 and 97-102 could be typical, or something in the ballpark of 80%. In both cases, a 1.5 ratio of ball speed to head speed is a maximum. A 1.2 is well off the sweet spot.

Hi,  I spoke with Matt over at Trackman and here are some sample data point examples that he was able to give us regarding percentages of 6 iron speeds to driver speeds.  This is using optimal for mid-trajectory preference. 

Ball speed:

6 iron / driver

• 94.6 / 126.4 = 74.8%
• 110.4 / 146.0 = 75.6%
• 129.4 / 169.4 = 76.4%

The percentage is higher for higher club speeds.