If the range at best L/D from 2500' is 5 miles that suggests the L/D is 12 or thereabouts. Any genuine glider with that as a best L/D probably has a polar like a camel's hump. 10 knots slower and it will stall out of the air, 10 knots faster and it will fly about half the distance.
A more modern 40:1 ship has a glide range of about 15 (nautical) miles from 2,500. Flying faster probably won't make a huge difference as the polar is probably fairly flat, flying slower will reduce the range but not by as much as for whatever the glider you were thinking of. It will probably be a longer duration though as 10 knots below best L/D speed will probably be close to min. sink.
Notice all the 'probably's in the above? Unless you state what the glider is, so the polar is known, any answer can only be a slightly educated guess.
Last edited by cats_five; 28th Jul 2008 at 18:52.
Reason: dratted smileys...