What is the average speed of rider Peter Williams?
CREATE TABLE "1970_isle_of_man_production_750_cc_tt_fi" (
  "rank" real,
  "rider" text,
  "team" text,
  "speed" text,
  "time" text
);
SELECT "speed" FROM "1970_isle_of_man_production_750_cc_tt_fi" WHERE "rider"='peter williams';