SPend $100 and get a motor vac cleaning
a basic cleaning is not the same
new plugs are good but so is a new rotor and cap
i have bosch 4's i don;t think they are amazing but good enough
i won;t know until i dyno different plugs but i really couldn;t care less
Keep the tires as much as there max is if dong only highway then a few lbs over won;t hurt either , the reduction in rolling resistance is significant
synthetic oil is great for reduction in gas
a cold air intake is your best way to spend a few hundred and save way more in gas and get performance out the box
i have a 70 litre tank which gets me 500-600 km