To do 46 mph on 26" wheels with a 36t rear sprocket means the engine should be turning 8778 rpm. You're doing 9082 that's a 304 rpm difference. Possible explanations are:
The wheels are slightly smaller than 26".
You're riding into a strong head wind.
You're riding in an upright position instead of a tucked position.
The thread pattern on the tires hinders the rolling resistance.
You're using cone hub bearings instead of sealed bearings.
You're having around a 3% loss in efficiency; which is really good.
You may be able to do some slight tweaks to get the efficiency loss even lower.