upgrading to 20" rims will this put extra pressure on my tranny?

[Lookin4Trouble]

short answer is yes
long answer is yes, but probably not by very much.

The more sprung weight (weight of rotor, wheel, and tire) you're asking the transmission to move, the more work the transmission has to do, the more heat it will build, and heat kills all things mechanical. That said, you're not significantly lowering the lifetime of your trans because the trans was built to handle that kind of load. You're likely to notice a difference at low speeds in the amount of gas you have to give it to get up to speed.

Also, agree with LeiZ, you will shorten the lifetime of your brakes. Every pound of sprung weight roughly equates to 3 pounds of unsprung weight. So if you're adding, say, 10 pounds to each corner, your brakes have to do the extra work of stopping as though you had a 120lb skinny dude (like me) in your passenger seat.

Just curious, how much heavier than your old wheels/tires are the 20's? I was pretty surprised at the weight difference when I went from 16s to 18s on my last car, went from 38 pounds at each corner to 54 pounds at each corner...

//Edit - I went to 18's, not 20's, and it increased the weight 16lbs each corner. I wasn't awake this morning when I typed up my reply =oP - tidE\\