I’m still debating whether I like the subtle look of the rear lower lip diffuser or the entire diffuser that covers the rear valence (I think it’s called). I’m curious if anyone here is using the small lip diffuser. Im looking at some on eBay and Amazon but they appear to need to be mounted on a flat surface. I’ve noticed on my car it’s not flat underneath the back but has 5 ridges.

 

This is the style I’m looking at.

 

What year is your car? My 2014 IS 350 is flat back there.

 

Mine is 2015 IS250. It’s F sport not sure if that makes any difference.

 

The rear bumper and diffuser for 2014-2016 are all the same regardless if it's an F Sport or not.
Maybe you can post a pic of what your car looks like.

 

I was thinking they would all be the same.

 

It’s kinda faded/dirty under there but there’s 5 groves on that panel. After looking at it now I don’t see any good way to mount the lip diffuser.

 

I think the ebay part you provided will simply mount to the current diffuser and undertray (that has the 5 grooves) to bridge the gap between the two parts.

 

I hadn’t thought about that. I’ll measure the gap to see if it will fit.

 

Well huh. After shoveling todays snow, I took a peek under there, and yup, I do have that same under tray. Forgot all about it, as I must have seen it while installing my trailer hitch.