My car was a 305 auto with 215 x 65 x 15 tires from the original factory owner manual/ handbooks.
the diff ratio was 3.08:1


I put a 408ci in and a TH350 and 255x60x15 tires


i calculate the taller tires now bring that 3.08 diff up to 2.96:1 ratio due to the 27 inch tires.


I then want to bring the diff ratio between a 3.08:1 and 3.36:1 to get bottom end speed with the tall tires i want to run.


I then calculated a 3.42:1 gear set with 27 inch tires is the same as the original 215x 65x 15 tire setup with a set of 3.29:1 gear set.
TRUE OR FALSE.


i use the equasions on the below site


https://tiresize.com/gear-ratio-calculator/


are these number correct.


i want to run the 255x60x15 tires like a true early 70,s muscle car would run

 

Keep in mind that your new transmission has different gear ratios than the old. My advice is to just forget about what gearing you had before. Choose gears that work with your current engine/trans combination and nevermind trying to make wheel speed same as before.

An axle gear somewhere in the 3.50 to 3.73 range is quite reasonable for a decent engine with a TH350.

 

Connecting this thought with your other thread, you just swapped to 3.42 gears so I would run those for a while until you get the engine running right. You simply cannot judge what to do next until you get it running right as is.