Ay Sir,
few notes and facts.
I moved from Finland to US in 2009, moved my Chally to US. Drove first 1,5 years with Finnish plates. In practise, once you establish a local address one should register the car locally + local insurance. But in your case you're only visiting so you're OK with your plates and your insurance, because you just can't have one unless you've "established locally". Just make sure your local insurance company covers also abroad (all in all, you'll drive around Europe..or Canadians drive in US.. nothing specific in that field).
other than that... for speeding "there's time and place" as the CHP officer said when we discussed this over at beer and debated about his 600hp Vette performance and "time/place"
In practise highway patrol/local police are not that interested on your plates as far as your car is not a complete mess, they usually appreciate oldies in good shape and do not bother about your plates. That's what I've experienced in California.. and also discussed about this issue with quite many CHP and local officers (well, happened that my neighbor was SJPD el capitan, and couple of other in that field).
About the gas, as already mentioned is just plain crap. I was ok with V-Power back in Finland, and refueling first time V-power locally in CA resulted = reset timing advance, and stiffer advance curve = crappy output. Resolution, 50% V-power / 50% Sunoco 100 at local 76, reset advance springs and total advance back to original setup, all good. rock'n'roll. Cold hard fact, tested.
