It shouldn't take much more than 20-25hp for most modern cars to hold a steady 65mph on flat ground. I'll use my Civic HX as the baseline. We'll presume it requires 25hp. My Civic turns 2500rpm at 65mph, and since torque = (hp/rpm)*5252 then we can calculate it's making about 52.5 ft-lbs of torque at the crankshaft at that RPM.

If you really, truly wanted the torque at the ground, then you are correct that you'd need to multiply it through the gearing and make a rough estimate for how much is lost due to mechanical inefficiency. Most chassis dynamometers suggest 13% loss for a low power, FWD car, so we'll use that. My HX has a 23.4" diameter wheel, so the radius when accounting for tire squish is perhaps 11.5", or about 0.96 feet. 5th gear is listed as 0.70 and the final drive is 3.72.

The math should thus be 52.5 ft-lbs * 0.70 * 3.72 * 0.96 = 131 ft-lbs. If you figure in 13% loss, then only about 114 ft-lbs actually made it to the ground.