Assuming my calculations are right, roughly speaking,

1g = 21.937  miles/second squared
1g = 35.304 kilometres/second squared

or

0 to 21.937 mph in 1second = 1g
0 to 35.304 kp/h in 1second = 1g
0 to 60 mph in 2.735 seconds = 1g
0 to 100 mph in 4.56 seconds = 1g
0 to 100 mph in 1 second = 4.56g
0 to 100 kp/h in 2.83 seconds = 1g



gforce = acceleration
         -----------------
         21.93685 miles/second squared
     (or 35.30394 km/second squared)

where

acceleration = final velocity - initial velocity
	       ----------------------------------
	       time


So to work out 0 -60 mph in 2.735

gforce = ((final velocity - initial velocity)  / time
         ------------------------------------------------
	 21.93685


           => (60 - 0) / 2.735
              ----------------
              21.93685 


           => 60 / 2.735
              ----------
              21.93685 


           => 21.97784
              --------
              21.93685


gforce =  1.00



So reasonably easy to work out gforce in a straight line, whether accelerating or decelerating.

But harder going round corners without knowing radius of turn, as

acceleration = velocity squared
               ----------------
               radius