My 2012 Ram 1500 4WD needs leveling, the difference from the tires to the body is 3" lower in the front. The 2.5" leveling kit has a spacer that's only 1.5" thick, how does this raise the front 2.5"?

Consider it a ramp. Placing the spacer inboard increases the outboard lift. Its all in the geometry.

But I want the space in the wheel wells (from the top of the tire to the center of the wheel well) to be equal and the springs are right at the wheel wells, so the geometry at the front bumper won't be the same as it is at the wheel wells.

I do not know what and the heck you are talking about. A leveling kit does just that. Placing the spacer inboard will raise the chassis increasing the distance at the tire to the wheel well. Just like a screen door. The closer you plave the door stop to the hinge the more open the door will be. Its not rocket science here.

I understand that, my question is, how does a 1.5" spacer raise the vehicle 2.5" ?

The pivot point of the A arm and the spring’s location in relation to the wheel end is why a 1.5” spacer will raise the front end more than 1.5”.

The further inboard the spring is, the smaller the spacer needed to get the same lift at the wheel.

Okay, That makes sense, thanx.

I just leveled my 1500 with Bilstein 5100 front struts. This is apparently the preferred method. Works great and the ride is noticeably better. This is a 2022 with 300 miles. Little more money than spacers though.

im at a dealership, I see these daily, My answer to you is DO NOT USE A READY LIFT KIT WITHOUT CHANGINE THE UPPER A ARMS!!!!! I use ready lift only as a term, not the brand, any spacer kit puts crazy strain on the upper ball joint, they pop out, or shift over in the control arm bore. they come in all the time with upper ball joint clunks, bangs, or issues. IF you are going to go the level kit rout go with a strut from Fox, or Bilstein, or another reputable company and get a heavy duty upper control arm kit that increase the amount of movement the upper ball joint will get. a stock ball joint cant move enough for the tighter geometry a leveling kit makes.