Rpm vs speed

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I have a 2011. 2575 rli. With a 5.0 with bravo 3. Not sure tach is working correctly it show 4000 at 30mph. Does anyone have a similar set up ? Maybe you could confirm this rpm.
 
So many factors impact speed. It can vary a lot at the same RPM from one boat to another, even when the pontoons are fairly similar. IMHO I just don’t see it being reliable to dial in ones RPM based on a precise MPH reading.
 
Have built and raced cars my entire life. Just looking for a base line i understand not to adjust to anther boat. Trying find out if tach is off. This is first tritoon or boat with this small of an engine. Doesn’t sound like it is turning 4000.
 
Get the Mercury Vesselview.
See my post on tachometer accuracy.

 
No I don’t know if it’s broke trying get an idea. Have bought a new one guess i will just install it and see.
 
Not to many people running inboards. But it does seem slow to me. Did you gps the speed?
 
But they don’t rev as high as an outboard so it may not be far off if it is actually in the low to mid 4000 range.
 
Speed on gps is within 2 mph. Most boats I have own would run nearly one for one. At 3000 it was 30 mph. I guess I will try a new tach.
 
I think you may be on the wrong track. The ratio you are attempting to use may not be applicable to your power package/boat. (Your motor should be achieving something near 5000 RPM @ WOT. Are you nearing 50 MPH at WOT?) Before replacing the tach, I'd try a couple things. If a tuning tool is available, use it in the tach mode to check the boat tach. ( I have a small Snap-On that works great for that) Also, many tachs are adjustable to 4, 6 and 8 cylinder motors (or an ECM mode). The setting is usually a rotary switch on the back of the tach. Over time the contacts can get dirty/corroded causing the tach to malfunction. Frequently just operating and resetting that switch can fix the problem. While at that job, check all the tach connections for corrosion and that they are snug. BTW: it has happened that tachs are installed with the wrong setting.....
 
Yeah your correct. When other boats at 5000 were running 48 -50 then 4000 on this boat would be around 4000. Have change setting on back of tach. it did correct the bouncing to 5000 problem. Thanks
 
Good to read the problem is fixed without unnecessary parts swapping.

Just curious, what version of the 5.0L do you have? Without specifics, the performance sounds pretty good for a sterndrive power pack that weighs almost twice what a comparable HP outboard would. Is the motor factory stock?
 
Have built and raced cars my entire life. Just looking for a base line i understand not to adjust to anther boat. Trying find out if tach is off. This is first tritoon or boat with this small of an engine. Doesn’t sound like it is turning 4000.

What kind of racing did you do? I am an SCCA member and hold a IMSA credential, we built and raced F5, FF, and DSR. I really miss the life. Boating is OK, but nothing like racing.
 
Ran SCCA regional formula vee. And auto crossed an RX7. I miss it too. Sold them then went into restoration then started buying corvettes. Then wife chose to buy at the lake. Didn’t need extra cars sitting around. Have messed with friends off shore boats. Have driven twin 600 hp to 75 mph.
 
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