Im not sure I fully understand the concept of rear end gears and speed, accel. , etc. I currently have 3:45's and I want to get some new gears in it. Now if I were to go with 4:10's I would lose/gain what? I have a 2.3 5-speed, what is the best ratio?

Oh you will notice a lot of improvement with 4.10s. You may lose the ability to go 105, but chances are you can't go that fast anyways. Oh and make sure to get limited slip if you get them out of the junkyard. I am about to do this, but I am going due to bigger tires.

What size tires are you running?

You would gain acceleration and lose top speed and highway gas mileage. With a 4 banger higher gears are better because you don't have much power to begin with.

higher number = lower gearing
- faster acceleration
- more torque
- higher GVWR
- worse gas mileage

lower number = higher gearing
- higher top speed
- less torque
- lower GVWR
- better gas mileage

Hillyard's right, it will depend on the tire. For tires close to stock and a fair amount of highway driving, I like 3.73 in my '93 2.3L. Cruise RPM will go up by the ratio of the two gears, for instance a 2500 RPM cruise with 3.45 will become 3.73/3.45*2500=2700 RPM with 3.73.

Im currently riding on 235/75/15, will 3:73 make a difference in my acceleration? I usually dont push it past 65 on the odometer which is about 75-ish with the larger tires I think, so will I notice a difference with the top end around this speed?