Don't trust my tachometer.

I have an '89 Evinrude 40hp on my Tracker. While running, I can top out around 27mph with myself in the boat and only lightly loaded. I'm not terribly disappointed in the performance, but the tach only shows 4500rpm at WOT. I'm not sure that I trust the reading on it, as the gauges on my boat were all in relatively poor condition when I redid the interior.

Additionally, I replaced the lower unit and prop with one from another boat as the original was damaged pretty badly. I think the current prop is a 12 x 11 3/4.

Any easy tricks to gauge the speed of the motor while it's running? I know that props are very dependant on conditions, but what are other people with similar setups running for props?

First....Does the Tach go over 4800 in Neutral or on the trailer with the Hose hooked up.
Then, go over the connections for Corrosion etc.
For a really serious Witch Hunt, find a friend with a Frequency Generator. He can run that tack all over it's span.
You could get a cheap Tach/Dwell meter and see what it says at WOT.

Something is wrong cause I just plugged your numbers in here https://www.go-fast.com/Prop_Slip_Calculator.htm and it is impossible to run 27mph at 4500 rpms. At 27 mph you should be pushing around 5500 rpm's with that set-up. What is your tach set at on the back? Do you know the gear ratio of the new lower unit?

Thanks for the replies. Sorry, bad info on the propeller pitch. The correct info is 11 3/4 x 17. I really screwed that one up.

I haven't really watched the tach in neutral, so I don't know what the limit is. Is it ok to rev it that high without a load???

As to the lower unit, I don't really have a good answer as to the gear ratio. My original lower looks like someone bounced it off a rock and tore out the bearing carrier and prop. It was re-welded and held oil, but when I got the boat, the shaft bearing sounded like a bag of marbles. I got the new lower unit off of a bad salt water motor (thrown rod). Any way to easily see the gear ratio without disassembling the lower unit from the motor? I don't think the tach setting should be a problem, as the motor is still the original to the boat (Evinrude Tracker), but I will check that too.

Is it possible to use my clamp on multimeter (Craftsman) that can detect frequency on one of the plug wires or the signal wire running to the tach to see what the engine speed might be?

Well that changes everything! There is no way a 17" pitch is going to turn near max rpm's on that boat. Running that prop in the slip calculator leads me to believe your tach is reading correctly. That prop would be better suited on a 60 - 70 hp motor. A 13" pitch should put you around 5300 rpm's. BTW the operating range on that motor is 4500 - 5500 rpm's

On those 2/c 2/s motors .Its Ok to run on hose at low speed 1000 rpm ..4800 rpm can cause damage ( you have no load /and no backpressure on exhaust....Tachs are cheap,portable or permenant..Motor repair AIN'T cva34

Thanks. That was my understanding as well. In my experience, most motors don't like being run at high speed without a load.