Hello from germany.
 

Hello from germany, the country of unlimited highway-speeds and expensive fuel.
I've been looking around in this forum before to save some fuel.
This has resulted in me archiving a record of 5,93L/100 km (40 mpg) in a Toyota MR-S (5-speed manual 1.8L 140PS DOHC VVL mid engine RWD) under not exactly ideal conditions.
Your EPA rates it at 22 city/27 highway/24 mixed, wich is rather easy to beat.
I hope to improve on my current record by learning a few more techniques as well as slightly modifying my car.
Sadly I can't do as much as some of you guys do or the Police and TÜV get angry...

Parts interchangeability with the Corolla may help overcoming this issue :D

I miss that beautiful country! When I came home I wondered why every Prius owner drove like Audis in Germany.

Welcome to the site!

Yes, I've looked a little into it:
A modification I've got an eye on is to use parts form a different C-series transaxle to get either a taller 5th gear or do a 6-speed conversion with a realy tall 6th gear from a diesel.
I expect some not insignificant gains as the 5th gear is rather short in the Spyder, causing it to rev almost exactly 2900 rpm at 100 km/h.
And it's not like the TÜV checks the casting number of transmissions, so that should be fine.
But it would be a lot of work as I'll need to find a donor gearbox and convert it with parts of mine and just getting the gearbox out is a rather complicated process...

So perfect car! As I see on Wiki, it is below 1000kg.

Welcome to the club!

Sure it's not the easiest job, but it sounds the most likely to provide effective results.

When I said effective, I was also considering how under the radar it would stay.

I think the results should be noticeable.
When driving on the Autobahn at ~90 km/h in the right lane, the engine is reving ~2500 rpm at a realy low load.
The BSFC is likely rather bad at 2500 rpm and low load.
Shifting it from 2500 rpm and low load to 1800-2000 rpm and higher load should do something.

However that mod is likely going to wait for a while.
Before that I'll experiment a little with the aerodynamics and likely swap tires.

Are you sure you won't have any trouble with the police or TÜV regarding some aeromods?