im getting ready to order rear gears for my 1967 mustang w/ 289 and a c4 tranny and a 8" rear. i was thinking 4.11 but i would like to know how high my rpm's would be at say 65mph. i wont be doing to much freeway driving but but it might. would i be better off with 3.80 or 3.55?
i went with 3.55s with my c4. i have a mild 289 and i was turning about 3200-3300 or so at around 65. it was pushing it a lil, i would stick with 3.55s, dont go any higher unless you plan to get an overdrive or 5spd.
i wish i would have gone with 380s because i just swaped to a 5spd, but gears arent to much money so i may change them out again later on.
3.55's are the most you want to go on the street, You will be around 3200 rpms depending on tire size. Even if you have a 5 speed they will drop to 2300 or so.
I couldn't decide so I went with a 3.80. I run 275/60/15s (28") on the skreet which gives me an effective gear of 3.56s. Then at the track I switch tire sets to a 275/50/15 ET Street radial to use the extra gear.