When we did the North Klondike Highway in our non-diesel truck with a 19-gallon tank, we stopped at Pelly Crossing for gas--156 mi after our departure from Dawson City and 168 miles before our arrival in Whitehorse. But we generally tried to follow the conservative old Alaska Highway adage of fill up when your fuel gauge reaches 1/2 empty. That's just in case the next roadhouse-gas station happens to have closed since the Milepost was published, which happened to us twice.
I tried to make it from Watson Lake to Toad River (205 miles) before filling up, but got uncomfortable at Muncho Lake. So I bit the bullet and paid nearly US$8/gallon for enough gas to go those last 30+ miles without worry. Gas at Toad River was US$2/gallon cheaper than Muncho Lake, which I knew before I got there and is the reason I tried to go that distance.
On the Cassiar Highway we did the 150 miles from Bell2 Lodge to Dease Lake on one tank. We paid a lot at Bell2, but knew we couldn't make the full 248-mile leg from Stewart to Dease Lake. We didn't even want to try the 196-mile leg from Stewart to Iskut, just in case the Iskut native store-gas station happened to be closed (it wasn't). The 146 miles from Dease Lake to Watson Lake was easy because we stopped for gas at the Junction 37 station to fill up and camp for the night.