My Chall has 255/60/15's on the rear,auto and sure grip .First off my spedo is 20mph off till about 50 then climbs to 30 miles off consistantly.At 75 on the GPS my speedo is on 120.Any ideas??Alos guy i bought the car from claims it has 3.91's but 4000rpm's at 70 mph seem right??.Thats flat azz screamin!Thanks for any help.