3.55 vs 3.73 gears for 90 mustang 5.0 5 spd.?

Thanks for the reply. The car actually has 2.73 gears(stock) in it now. I am aware of the change in speedometer reading. As far as gas mileage, it isn't a daily driver, it's just a weekend,fair weather toy. To be honest the only gear that seems to have any torque is first, after that it's unimpressive. My only real question as to which gears to go with is more based on running at highway speeds. Will 3.73 be too much rpm @60-70 mph? Thank you again.

Buy a used differential or rear axle with the gears you want and try them out. The cost is not high. I expect you will like it. Most cars are geared primarily for economy.

With that high of a gear the 3.55 will make a huge difference. Gear ratio and speed calculator. https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Doing my own analysis shows that at 56 mph your engine (2.73 gears) should be turning 2,000 rpm with a 1/1 gear ratio (probably 4th gear). With 3.55 gears you would need about 2,600 rpm at 56mph. I would find this acceptable for a fun car. Highway cruising for long distances at high speeds might be somewhat tiresome.

I looked it up. 4th is 1.00, 5th is 0.68. Use these numbers to decide on what final drive ratio you want. https://www.cjponyparts.com/resources/ford-mustang-t5-transmission

Greatly appreciate all the good info. I plugged values into a rpm calculator and messed around with different gear ratios. With the tires I'm running(245/45/17) and either the 3.55 or 3.73, the overdrive(5th) rpm is running at 1895(3.55) or 1991(3.73) @60mph. For reference stock(2.73) is 1457 rpm. I don't have a lot of experience with this kind of thing, but doe 450- 530 additional rpm tax the engine that much? I am leaning to the 3.55. Thank you.