I think a petrol 2.25 in good fettle should be able to hold 25mph on anything but a very steep hill. A diesel is another matter though, I'd think 15/20 on a long hill. My 2.5 N/A90 can hold 30 on the Glenshane pass, and that's a fairly steep mile long hill.
As above, a good service can make a big difference, if it's petrol, I'd check/change the points too.
My old 2.25p with uprated head and electronic ignition was good for 65 on the flat, and 40 on most hills. They can be a realistic daily driver, if you can stand the fuel bill.