What speed should I get (rough idea)

Have been researching a lot on the web and read a lot of advice on prop selection but would like a wee bit of advice....

I have a 20ft x 8ft Cape Island boat, all fibreglass (its quite heavy).
On it I have a 45hp Johnson commercial (no tacho) and a Johnson 8hp kicker.
I get around 5.5 kn out of the kicker and around 7.5kn out of the 45hp (gps).
The 45 kicks up much more fuss than the 8 with little extra speed! and at a much greater rate of Gas!!

Should I be expecting more from the 45? Could it be a problem with prop choice? she seems to run smooth enough.
(its std dia but not sure of pitch ... can I measure the pitch?)

I know I havent given all the info required in the read first, but am just looking for a little guidance, not specific recommendations.

Re: What speed should I get (rough idea)

You will need a tach. It could be fitted with a stock tach or you could get an induction tach like the Tinytach an optical tach could be used temporarily.
What year is the Johnson?What year is the boat?
Near as I can tell the Cape Island is not intended to plane.
So 5.5 knots is probably very close to or above its most ideal speed.
Pushing it at speeds close to planing would take a lot of power.
Maybe you could get 10- 15 with the right prop.
Could you post some pictures.You'll need to use a host site.
The prop numbers should be near the base of the blades or under the prop nut.
Without a tach you can't really tell anything.

Re: What speed should I get (rough idea)

Thanks for the reply. I will see if I can find the prop id info. As for a photo of the boat,can I just give a link to my website where a picture exists?
It just seems strange to me that the little 9" prop on the 8hp pushes it along at 5.5kn with little fuss, while the 45 moves so much water but does not achieve much more in speed. The 8 is a longshaft off a sailboat I had some time ago. Maybe the pop was matched to that type of vessel.