Slick 60's

Any idea what the "ideal" 9" rear end ratio would be for a Mustang T-5 behind a basically stock 292 Y-Block? Mostly street driving, with occasional highway driving. Just curious.

The best way to answer that is to say the ratio that gets you around 2000 rpm at highway speed.

The factory T5s were around .7 or .8 overdrive. Other things like tire diameter will also affect the rear end ratio required.

Probably the easiest way to determine it is to get the T5 installed, see where you are at at highway speed and make any necessary adjustment from there.

My t5 final is .68... 3.50 gears... P255/60R15..about 25" tires gets me 70 at a little over 2k. steep hills generally sees me down shift to fourth but moderate hills do not.
Lots of pep in all gears including a good pulling 1st.. although I have not pulled a trailer with it yet.

All Mustang T5's had a 0.68 5th gear starting in '85. I would shoot for no less than 2200 rpm at 70 mph in 5th gear regardless of your tire height.

A 255/60R15 is around 26.5" when sitting with a load. About 27" unloaded.

Let's assume you have 28" tires. To get 2200 rpm at 70 mph you use this formula.

gear ratio = (rpm x tire height)/(336*70)/0.68

the result is 3.85. So with a 9" I'd run a 3.89 ring and pinion.

just remeasured and exactly 25 1/2 inch on the tire unladen,3.50 gears unless installer lied to me,2100 rpm= 70mph on the button, according to my tom tom gps

Thanks, guys. That gives me what I needed. Trying to see if I have a pumpkin in my stash that I could swap out in my 80 F100 9" (CV swap).

#.50 was the standard gear ratio behind the T85N overdrive that could be had in these trucks. If you need to pull any weight , then I would not go any higher than that.

Rich