My Prime severely underestimates my distance to empty range

it isn't calculated using the whole tank. there is a safety measure so dum dums won't run out of gas.
it is usually a couple gallons, and should be specified in the owners manual

 

Gas light comes on with around 2 gallons left in the tank and the meter says about 30ish miles left of range. Its intentional so you don't run out of gas. 2 gallons could get you 120 more miles if the meter was accurate at 60mpg (it's not. Toyota overestimates mpg) but it could just as easily only be 80 miles if you're driving in really cold conditions on the highway getting close to 40MPG for that last 2 gallons.

 

As commented, DTE is based on the ~9.3 gal of gas. (11.3 gal full tank but the empty light coming on when 2 gal still in the tank). It is an estimation based on the previous driving record usually prior tank-full driving not the most recent driving on the current tank. Also, the mpg displayed on the dash is inflated. Toyota routinely reports anywhere from 5-10% more optimistic mpg than actual. You can use the DTE at fill-up to estimate your HV portion of mpg. DTE after fill-up above 500 miles is a good number, representing about ~53mpg.

Read more on this thread:
What is your DTE after fill-up?

 

60 mpg sounds a little optimistic. On a 2000 mile road trip averaging about 70 mph with no EV, I averaged about 45 mpg. Some of the long stretches were at 85 to 90 mph, though.

 

So I drove mine today until it said I could only go 1 more mile on gas.

How much further could I have actually gone on that?

My battery was full as well.

 

You have about 1.5-2 gals of gas and full charge. It depends on how you drive, but using an EPA estimate of 53mpg and 25miles EV range you can drive 100-125 miles further.