changing gearbox/diff ratios for highway driving

Hello everyone.
I'm a newbie to this forum and anything transmission wise with 4WD. I have been playing with hatchback cars for years.
What I would like to know is there any simple modifications to gearbox or changing final drive ratios that would help with highway driving (110KmHr)
Currently 3800rpm @110k I would like to bring it back to 3000rpm @110K I guess changing diff ratios to 3.900? or finding 0.80 or 0.90 for 5th gear?
The reasoning for wanting to do so is the engine is being turbo'd over summer holidays. and power band will be changing. plus I only use car for travelling to and from work on highway,
Car details: Suzuki Jimny, 2006, M13A VVT, manual, standard tires.
I hope someone can help?
i'll post up details on turbo progress when it starts.

thank you
Rowan

Tbh go for some slightly larger tyres...

As above, use a tyre calculator to work out the present rolling diameter, then select a tyre that is 3800/3000 or 1.27x the rolling diameter, but they won't clear the arches.

If you check out the info on the home page there are different differential ratios between auto and manual, pre VVT and post VVT cars, also a different 5th gear ratio I think, but you wont get a non VVT gearbox on a VVT engine.

Thanks guys but I don't want to put larger tires on car. looks like all source a 2nd hand gear box for parts and help with design for different gear ratios made for 3,4 and 5th gear.
Cheers Rowan