What is the difference in a 2.5 and a 3.5 Nissan Altima Sedan
Use the comparison tool on Nissan's site if you want to see the difference on the 2013 model. http://www.nissanusa.com/cars/altima/versions-specs
The sort answer is that the 2.5 is a 4 cylinder engine, and the 3.5 is a V-6.
The 3.5 models have certain options not available on the 2.5 models.
There are several different trim levels available, depending on the engine choice. There is overlap in the middle of the line, too. I believe the SE is available with both, and if you get the SE PLUS on the 2.5, you get all of the equipment of the 3.5SE. It can get very confusing with the numbers and suffixes. That is why the simple answer is that the one is a 4, the other a 6.