Which Rider finished with a speed of 81.91mph?
CREATE TABLE "1971_isle_of_man_sidecar_750cc_tt_final_" (
  "rank" real,
  "rider" text,
  "team" text,
  "speed" text,
  "time" text
);
SELECT "rider" FROM "1971_isle_of_man_sidecar_750cc_tt_final_" WHERE "speed"='81.91mph';