How much energy does unretracted landing gear consume?
While it’s not a true air to air fighter, following are some comparison numbers for the EA-6B Prowler that might help provide a useful ratio for a tactical &quot;fighter type&quot; aircraft.  (Information comes from the NATOPS pocket checklist, under the constant altitude “bingo” fuel divert tables.)
Theses figures are for a flaps-up aircraft, fully loaded with 5 external stores, and both engines operating.  (There are multiple configurations and distances covered in the tables).
Fuel required to go 100 Nautical Miles:




Altitude
Gear Up
Gear Down




Sea Level
3970 lbs
5220 lbs


10,000 ft
3580 lbs
4590 lbs


20,000 ft
3270 lbs
4160 lbs





NOTE:  The gear down figures are worst case and presume a hydraulic failure.  In the event of a hydraulic failure the gear will be blown down pneumatically and the forward main landing gear doors, (that normally re-close to cover the empty wheel well) will remain open.  This imposes some additional drag penalty over and above normal flight with the landing gear down.

You should be able to extrapolate this into something useful, but if you would like more data points please let me know.