Sounds like you've got a good build there. Well done

 

This has to be a troll post. What retired couple is road tripping the entire north america with a S2000??

Anyway, skip the turbo. Get fresh gapped plugs, fresh air filter, get the taller gears, get a hardtop, get some efficiency tires from a Prius, get a standalone and tune the ECU for efficiency (i.e. stay out of vtec).

Or, go buy a 4 cylinder Camry for your road trip and sell your S2000 to somebody that wants to drive it as designed.

my 2 pennies

 

Hopping the car up to 350bhp isn't going to use less fuel, easy as that. Give it a service and make sure the tyre pressures are up and call it good.

 

It appears his idea is to dramatically raise rear end gear (lower the gear ratio) to improve hwy mileage by lowering revs, well beyond what stock engine torque would allow. Then raise engine torque with turbo, so engine can pull those gears.

This could work. Maybe. But then it adds another dimension to working out the kinks that many here have already mentioned. Plus you'd be going down a largely unpaved road. No one to copy off when trying to plan build, no one to answer questions when sorting issues. Plus if you have trouble on the road...

Most of all, in the end it may not actually improve mpg.

Instead i'd just raise gear ratio a reasonable amount, get a hardtop, and fit lower rolling resistance tires (not prius tires, just something less extreme than the typical summer tires).

You'll probably get 35 mpg hwy or better at reasonable hwy speeds.

 

That generally only works if you drop the engine size so you get into high load area's with the turbo to take advantage of the energy pulled from the exhaust stream. Hence all the 1.0L turbo engines around in euroboxes lately.

 

This is an angle none of us thought of looking at this thread from