ive got 3.875's in mine. original tired l16, with a 4 speed. on the freeway it scooted right along, honestly i have no clue how fast though lol. the speedo is off and not sure how far, but the other trucks in my group didnt catch up to me and they said they had cruise set at 70 to 75 so i was happy with that. i drove a load of yard debris( 1st load 420 pounds 2nd load i forgot to look but it was moar) from river road, took theisen, to mcfarlens bark had enough power to make it over all of those hills.
i did run into an issue on concord though. they put in a speed bump and its not painted yet, so everyone i know that drives the road has shat their pants when all of a sudden they hit a big bump. none however have flown off of it in a 70 521 with 500 pounds of yard debris in the back. i was going 40 and its a big ol speed bump. a group of like 10 people saw a datsun fly. it blew out my tire from impact and destroyed my passenger side shock. im taking this up with the county. schwab was cool and swapped my tire for free, i showed up 5 min till they closed too.