will 20x8.5 on 245.40 work or 245.35...which is better for a drop?

 

i think, both too wide for the front

 

Big question will be what Offset on the wheel? You might want to look at a 42 to 45 offset to clear the fender.
I have a 20x8.5 with a 225/35/20


Neat tool to use to see size difference for Tires..

http://www.miata.net/garage/tirecalc.html

 

Yes, it's true, I'm posting this again.

Skip the miata.net tool; this is the tire and wheel calculator we all use, and will help you understand how the changes from stock may or may not work, and what the resultant changes will be:

http://www.rims-n-tires.com/rt_specs.jsp?postId=906

Keep in mind that the stock tires are 205/55x16" with +45 offset.

Stock weight is approximately 22# for tire and 22# for steelie = about 44# total at each corner.

Have fun playing with the calculator/comparer!