I know 20's having a slightly different outer wheel diameter will throw off some dynamics with the engine gear ratio etc, is there any simple way to compensate for that? How bad really is it for the car?

Thinking of getting some Vossen 078 20's, any advise on that manufact.

 

What year is your car and what size tire are you going to run.

 

it's not that bad, if you run the proper tire sizes you wont have a problem!

 

i have a 2005, stock 17's.

 

I think stock size is 215/55/17 which has a height of 26.3"
The 245/35/20 has a height of 26.8" so it shouldn't be to bad with an extra half inch in height.