My mariner runs pretty decent but i need opinions. At wot i get about 6500 rpms top and the boat tops out at 45 mphs , does that sound right and will a 150 add on a good amount of speed?

Woah, 45mph at 6500 rpm sounds a bit slow, this is trimmed up all the way on a plane? What pitch prop? I have a Mariner 150 that is overpropped and I'm getting 65mph on a plane trimmed up all the way at 5100rpm.

But that being said, what you're getting might be right for a 90.... but 45 mph still seems a bit low.

I would bet your tach is wrong because you wont be together long at that rpm.

6500 rpm is very alarming... I know mine should max at 5600rpm. There is a chart around here somewhere with max rpm per engine.... I might be able to turn it up

here you go:

http://www.go2marine.com/productcenters/propellers/engine_rpm.shtml

there are some 90's on that chart for Mercury, see if you can nail down which one you've got.

5,500 is all that motor needs to turn. Check your tach settings. When you look up stuff it's the same as a Mercury 90ELPTO.

wow , i thought it was running pretty good but it sounds like its time to take it to the shop and get it checked out! That is trimmed all the way up and on plane. What a way to start my monday! Its a 89 mariner i will try to find the pitch when i get off

Thank you guys for the input btw. Ive learned some things today. I have been running 6500 often when i need to get across the lake. My speed is taken off of the lowrance so who knows how accurate that is.