I am starting a kitchen remodel and I want to change to a Gas range and also install a gas tankless water heater for an adjoining bath. The home has a 1" gas supply line from the meter to the 120,000 BTU gas furnace and 40 gal water heater. Can I 'T' off this line to supply the gas range and 117,000 BTU tankless water heater, and still have enough gas volume to supple the furnace and 40 gal water heater?

Probably but look at the exact requirements of the tankless. They need a boat load of flow to work. You need to size the line for all units as if they are running at the same time. I also add 20% to the total just in case.

Here is a do it yourself chart
Just use the chart it says generator but it does not make a difference just fill in your specs off your gas equipment and allow for the exact requirements of the tankless.

Natural Gas Generator Pipe Size Chart (http://www.uscarb.com/natural-gas-chart.htm)