What he said, It's something along the lines of there is a 'Maximum Score' and then every fault you get subtracts a chunk of points, and for every second/fraction of a second below the 'perfect time' which is usually set at 0 seconds (at least that's what it was in Trials HD) you get points subtracted. But basically agreeing with what don said, low faults is key, once at 0/minimum faults you can obtain, get your speed up
Winding down the achievement hunting...
95%+ Completion (139/161 games complete w/ DLC (Most Recent: Fez)
Halo Fanatic: All Gamerscore (Including DLC) in Halo CEA, 2, 3, 4, ODST, Reach, and Spartan Assault (360, X1, and W8) Just gotta finish Halo Wars and get SA for WP8!