Rebuild bike converted to a single speed. Using a tensioner on the chain. Have a three ring chainring and using the high gear (46T) and a 16T cog. Bike spins great on the work stand but riding it I can't get the pedals to work. Can't understand why I can pedal on the stand but not on the pavement. Thanks for help.
There are three things to check (well, more, but let's start here):
Chainline. Is the front cog so offset from the rear cog that the teeth are catching on the chain? Is the offset so great that when you introduce variance i.e. take the bike off the rack and put it on the road, it's overly-resistant to turning.
Chain tension - per Carel's comment above, make sure your chain is not overly tensioned. This will also cause seizure. I'm guessing this is it.
And because I'm so smart, I have first-hand knowledge of this one, bmx versus road chain, cog, etc. i.e. make sure if you're using a road chain that you're not using a BMX cog and visa versa.