Ha, I learned in the US in a Toyota Sienna driving from SF airport, round the city on the freeway and to a suburb. This was a Sienna full of peoples, mainly sleeping! At least I was able to follow Smallhorses (he doesn't hang about you know

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Hey, even in a borrowed Dodgy Caravan I can still mosh hard with Bay Area traffic! Especially when I've got my co-driver giving you the walkie-talkie tour as a you're driving along!
Mark, you shouldn't be too worried about anything really.
As a UK trained driver you've probably got about 50% more wits about you than most drivers on the roads here. (I say that loosely and I know that the vast majority of the US GTPlanet members don't fit into the stereotypical US driver category, so I'll thank you to recognise that I recognise you over the general populus and not get offended by this remark.)
You're going to get rented an Auto. Not because no-one thinks you can't drive a manual here, but because the rental companies here don't rent anything but Autos.
The way you've been taught to drive in the UK is more than enough to teach you to drive here, picture the UK, but with 50% less traffic, although 40% of that concentrated into 3 large areas, SF, LA & SD. The rest of it, and sparsely populated Nevada is easy pickings.
In general, the drivers here are much less inclined to signal when turning or changing lanes, and are more inclined to "lane-hog" on the freeways, which necessitates undertaking, which though technically illegal becomes a viable necessity if you're to get anywhere. (I hate to stereotype, but this'll generally come in the form of Lincoln Towncar, Ford Crown Vic, some sort of SUV or a beige Toyota. Why is it always beige?!!!

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In CA the posted speedlimit is 65 - 70 mph, but shy of 85mph in all but the remotest spots you're unlikely to be noticed as everyone around you will be doing the same.
Check both mirrors and shoulder-check your blind spots on both sides before changing lanes, and be wary of traffic 2-lanes to your inside being about to make a lane change without signalling before you move.

Beware of lane-splitting motorbikes, if you see a bike approaching fast in your mirrors, you should probably let it go. You aren't obliged to, but you should also reference a thread here within the Motorbike forum that addresses this issue.
Most importantly, and a great thing I've found when I used to frequently alternate between driving in LHD & RHD when I was returning to the UK, was to keep the steering wheel next to the centre line of the road (one-way streets excepted.) and this way you'd be guaranteed of success. Car parks are a nightmare as it's easy to forget this, so pay special attention when you're in them.
Now that I'm naturalised to LHD & US it's no big deal, but regardless of how long I've been out of UK, I always have no trouble slotting back into UK traffic and in general the UK drivers seem to be friendlier (apart from the odd road-rage arsehole) and more adept to signal, create gaps for merging traffic, and acknowledge a good deed.
In US, don't flash lights to let someone in, it's perceived as an aggressive move like beeping horn.
No-one waves when you do them a favour such as letting them into traffic, or waiting on the priority side of the road for them to pass. Don't expect it.
Here, and especially CA, you should drive with your mirrors, but drive aggressively, if you see a gap, go for it, if you try to leave a safe distance on the freeway someone will get in there, so be wary.
Nevada (with less than 10% of CA population) will be a little different as you'll be down on traffic volume, although Vegas (which is in Clark County, and therefore home to almost 50% of Nevada residents) is understandably a tad busier than most places in NV.