I need a small single speed gearbox to run a tractor PTO driven pressure washer with a ratio of about 1:3. That is 3 turns output for every 1 input.
A diff from a IRS RWD car with the diff locked could be just the job but as they are not designed to have the crownwheel turning the pinion all the time would it last? The load would be fairly light, a 2hp motor would power the pump ok.

have a look under a volvo 340 with CVT .

they have a small diff which transfers the power to another diff from the CVT wheels , maybe worth a look .

I didnt even know they did 340 with CVT!

The 340 only came with Variomatic when it first came out
Volvo 340 = DAF

I reckon you'll have no problems at all as long as you make sure to drive the correct side of the crown wheel so as to keep driving on the front of the gear teeth. I know what I mean

I know what you mean, so the same sides of the teeth are contacting as would have been in normal operation.
So has anyone got a diff kicking about with bolted flanges that they dont need?

The front diff from a sierra 4x4, dead cheap and if it take the torque in it can take the torque out