A lot depends on What Modifications have been done.. Stage one?
better Air, less restrictive exhaust, fuel device????/bigger bore/Cams?
ALL the modifications will make a difference here..
If you can ask the old owner or a Knowledgeable friend close by you to take a look at your bike OR if you have a friendly HD service, I would go there to ask about what they see...
It is almost impossible to answer the question you are asking WITHOUT a lot of "what has been done to the bike"
*****If it is running good? NOT burning Heavy BLACK out the exhaust and the Plugs look to be a tinge of Grey to brown and not Black.
Try this out and maybe you will/can give us More information...
The mileage you say is NOT way out of line IF the bike answers *****
signed....BUBBIE
Note you state SOMEWHERE??? you need an accurate check to see mileage..