Best southern driving route Boston to San Diego
 

I’m moving from Boston to San Diego and need the fastest southern route (leaving the day after Christmas) and also dog friendly stays! I’ve been researching the typical routes, just looking for thoughts on doing it in 5 days (two drivers) 6 days tops and in the winter, if it’s possible or do I realistically need an additional day?

I'd head down I-95 to I-20 and then take that west. Overwise, you might run into snow.

Going south to Atlanta, then east would certainly make your chances of encountering snow less, but at the cost of a longer trip. From my own experience, when snow is forecast, the interstates are kept snow-free. It isn't always forecast, though, and then it can be gnarly. Make sure you and your vehicle are equipped for winter driving. It's better to have stuff you don't use than to need it and not have it! We always used La Quinta hotels when we travelled with our dog. I use them now without the dog! They usually have a decent breakfast.

I would look at the weather projections for the week I plan to travel. If you want the fastest route, put in Buffalo, Cincinnati, St, Louis, Albuquerque etc and see what will probably be happening in terms of weather were you will be if you start out 90 then head down ( the fast way) and if it looks like snow/sleet/freezing rain take a slower route. I would still pick cities along any route and check weather projections.

in any case, try to get a little beyond a city before you stop for the night to avoid rush hour going through the city. Daylight hours are short in the winter so be kind to yourself and quit when visibility is diminished.

Your major concerns will be snow at high elevation out West, then freezing rain anywhere. There can be falling or blowing snow in the central/lower Midwest but there aren't that many hours/year that that happens so your odds are low that it will be a problem. Sometimes staying north in the below freezing zone when the sky is clear is better because less chance of falling precipitation that freezes on the pavement.

Get yourself to Columbus by whatever route you prefer, could be I90 or I80 to Cleveland, or I76/I70 via Pittsburgh..

Then from Columbus, route through Oklahoma City, connecting to I40.

Then through Phoenix, connect to I-8 the rest of the way.

You can fill in the dots in between...

Oklahoma City, OK - Albuquerque, NM - Flagstaff, AZ is all high and there is a good chance of encountering snow. This would be the most direct, quickest, route. You could use I-8 or I-10 to I-15, depending on where in San Diego you're going.