I had a ground wire come off the bottom of my distributor on a 1987 454 and caused similar issues. It was like the spark was scattered in stead of advancing properly. Just make sure you check ALL wires. With tach and such not working properly, just make sure all wires attached properly and engine grounds are good.

The first thing you need to do is spend $100 on a GPS. You have no idea how fast you are going or if the speedo is even consistent. Once you have an accurate way to measure the speed you can see if you have a problem. An accurate working tach is also essential.

OK...weird. My son was annoying the crap outta me yesterday to go for a joy ride, so we went. And i let him go full throttle for the first time (thinking it would do no more than 60 because I was having trouble getting it past 62) and I tell him to hit it, and I look over at the speedo and he's flying past 64...got it up to 69. Could've gone faster too if we had more water (water was too choppy). The only thing he did differently was he didn't trim up as much as i did. When I'm going full, I trim up as much as the handle lets me, which is about halfway on the gauge. He only trims up like 1/3 (on the gauge). And after it was at like 66, i told him to bump it up a bit and it got up to 69. Now when i had the 454, this is what i was doing to. Am I driving wrong? lol. Guess I need to play with the trim more.

Overtrimming probably lead to the mph loss.

Bimini dont tell your son he drove better you'll never get in the driver seat again, well at least you figured it out

Chop vs flat water makes a big difference with a straight vee as well.

your speed wax wore off.. re-apply and you should be good

How about fresh fuel ? I just went throught this problem. It turns out the garbage fuel we get has about a 30 day shelf life. That's from the depot to your engine. I slip my boat, and I go in the water pretty early ( first week of April ). I used my boat on April 15th and only 2 more times until the end of June. I lost 10 MPH. I figured major problems, but I still had my RPM's. Most boats in my marina have had to do a short haul, quick cleaning, drop her back in. Due to the extremely warm water this year, we're getting growth in May and June that we don't normally get until late August. So I went for the short haul, and to my disappointment, the bottom was pretty clean. I was hoping for an easy solution for my loss of speed. As we were lowering her back in the water, I started BS-ing with the kid in the marina. He told me about the poor gas situation and got me thinking. My gas was now almost 4 months old. I went right to the gas dock, cruised around a little, and all problems solved. Speed right back to normal. Hopefully your problem is as easy to fix as mine. Good luck

X2

Water condition and more weight that might shift the center of gravity on a stepped boat might be your issue.