Sonata vs Elantra
 

What are the main differences between the Sonata and the Elantra? (in terms of MPG, speed, features)

You need to be more specific. There are plenty of differences because the Sonata is a much larger car. What are you looking for? Are you considering purchasing one of the 2? What are your needs? What model years for the cars?

Base model 2011 Sonata GLS loaded with extras
 

Mine was $21,050. with shipping and had more extras than you can imagine included. Power seat and alloy wheels add $700. but I didn't get them. You are gonig to have a real hard time finding so much value in a vehicle as the 11 Sonata. It drives like a sports car, 4 banger engine unbelievably fast & purrs like a kitten, rides fantastic and seats so comfy I feel like I'm in a much more expensive car. Have to be careful and use the cruise control because at 60 mph I feel like I'm doing 40. If you can swing it, DEFINITELY go with the Sonata. Review a window sticker and you will be impressed.

The main difference is that they are two different class cars. You're trying to compare apples to oranges.

Sonata: http://www.hyundaiusa.com/ebrochure/..._eBrochure.pdf

Elantra: http://www.hyundaiusa.com/ebrochure/..._eBrochure.pdf

Go see 'em at a dealer.
 

Best thing is go get up close & personal with them at a dealer and you'll see it is NO CONTEST.