The difference in cruise speed can be seen in Van's aircraft. The first RV to have a nosewheel was the RV6a, it is only 2 miles per hour slower than the otherwise identical RV6. And this is a fairly fast aircraft.
This also shows why retractable undercarriage can be not worth the complexity, loss of useful load, cost, cost to maintain, cost to insure and cost to repair after a gear up. The RV6 in hypothetical RG form would be 4 mph faster, the RV6a would be 6 mph faster.
RV-6 / 6A - Van's Aircraft Total Performance RV Kit Planes
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