Gears

Changing the gears are probably the best bang for the buck mod you can do, but you want it done right, and you want to understand what you are doing so you make the right decision. First thing is by" higher gear" do you mean numerically higher as in 4.56 or do you mean a higher gear ratio as in 3.31?. In order to make it as difficult as possible for some one without automotive knowledge to understand the higher the number the lower the gearing. So a 4.10 is a lower gear than a 3.31. next you want to figure out what you want out of the car. A lower gear ratio (higher #) will improve acceleration, increase the RPMs and any given speed, reduce mpg, and potentially lower top speed. A higher gear (lower #) will basically do the opposite. I assume you want to improve acceleration. The first thing to do is verify that you car still has the factory 3.31 gears. you don't want to waste the time and money on gears only to find out that a prior owner already upgraded them The easiest way to do that is safely jack up the rear of the car, and put a mark on the rear drive shaft yoke. Have some one rotate one of the rear tires 1 complete revolution, using the valve stem as a reference point. While you count how many times the mark comes around. The stock gears are 3.31 so the mark should come around 3 and a 3rd times. So assuming the gears are stock, time to pick a gear. 4.10s are in my opinion the lowest streetable gear ratio. I would not go any lower (higher number) unless you are building are car strictly for drag racing. Personally I would go with 3:73s. I think they give you the best balance. You will notice a dramatic improvement in acceleration over the stock gears, but still retain the streetablity of the car. But there are some things to keep in mind. many mods often require supporting modifications and gears are one of them. With a lower gear ratio you speedo will run faster, so you either have to buy a tuner or have a local tuner correct it for you. Traction off the line which is an issue stock, will become worse so you need to think about upgrading the rear upper and lower control arms, (which is something you should do even if you don't change the gears) Also I don't know how many miles are on the car, but since you are opening the rear, it would probably be a good Idea to replace the bearings and possibly the clutches. Sorry for the long post, hope I helped. American Muscle put together a video and a FAQ about gear ratios you should look at that as well
https://www.americanmuscle.com/mustang-rear-differential-gears.html