bought a 19 foot nitro bass boat with a 1995 Mercury 150 EFI Black Max. Tach wasn't working. Came with a 25 pitch tempest prop. Took it to shop, they put on a 6" fixed jackplate, set the height, installed a new tach and a hotfoot. When i take it out it runs 55mph on gps but tach is only showing 4500rpms. Shouldn't it spin up to 5500 rpms? What could be causing this? Don't want to spend $$ on a new prop just to guess. Thanks

Have them check the tach settings to be sure tach reading is correct before changing props.

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Running the figures you supplied thru prop/speed calculater, that is correct @5 % slip on prop. I would have dealer check the hot foot to see if stop was properly adjusted and engine is getting wide open throttle.