What is the tyre size of Triumph Speed 400?

Its a 398cc liquid cooled DOHC single cylinder that churns 40PS@8000 rpm. It redlines at about ~8500 rpm so all the power is delivered at the top end. 80% of Torque is available from 4000 rpm with a maximum torque of 37.5Nm@6500 rpm, so it feels very torqee, because of its light and compact body. Vibrations are there after 6000 rpm but it feels more pleasant in 4th to 6th gear in that upper rev band. The engine can easily sit on 6-7K rpm for long hours and cruise at about 120kmpl. Not much heat related concerns even in traffic, the radiator does its job well, but yes it does heat up a little bit, which is why prefer riding on highways.

The Speed 400 uses an all-new 398.15cc liquid-cooled single-cylinder engine. It produces 40PS at 8000rpm and 37.5Nm at 6500rpm, working in conjunction with a 6-speed transmission with a slip-and-assist clutch. The claimed mileage stands at 28.3kmpl.

The seat height of Triumph Speed 400 is 790 mm.

Yes

Triumph Speed 400 has a Saddle height of 790 mm.