When trim tabs are installed, it can be seen that their effect of raising the stern and lowering the bow allows the boat to reach planning speed at a much lower rpm around 1900rpm. The result is less engine strain and less gas will be used. Typically this allows planning speed to be reached without using full throttle, this again reduces the amount of gas used.
You may consider these just claims and statistics by RocketFlap, but every trim tab manufacturer will justify and claim the same facts.
What RocketFlap trim tabs do.. is not revolutionary but how it does it, is......

Additional trim tab information:
www.boatus.com/boattech/trimtabs.htm
www.bennetttrimtabs.com/whatdo.htm
FS with optional red tabs
FS 'Fixed System'
For boat owners that wish to quickly set-up their boats running attitude. These manually adjustable trim tabs correct any conditions such as a slight listing or stern heavy drag due to slow planning speeds, heavy fuel loads, gear or passengers. Even boats fitted with heavy 'Four Stroke' O/B engine.
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will it work on my boat?

calculate your boats transom angle

what the system does and doesn't do!
When a boat accelerates it climbs over a wall of water that is pushed up in front of it. This is referred to as 'getting onto the step' or 'climbing onto the plane'. It's this action which lifts up the front (bow) and causes the back (stern) to sink lower as you accelerate. This is the time when the engine is put under the most stress and fuel consumption is at it's highest. The more a boat is loaded with gear or passengers the more extreme this becomes and the longer the time it takes the boat to get on the plane and the more gas used...Installing trim tabs on a boat will help lift the stern and lower the bow of any boat. This correction of a boats attitude will give improved acceleration and fuel economy as well as passenger comfort.
The gray zone in the two graphs represents the test results of a typical 19' to 20' runabout boat with a V8 engine.
The red zones in the graph show the effect on the acceleration/mph and fuel consumption (miles per gallon) when trim tabs are installed.
Without trim tabs you can see that up to 2000 rpm the boat is only moving relatively slowly at 10 mph. It is not until the rpm has passed 2500 that the boat gets to it's planning speed. In order to get to 20mph full throttle is often used, as soon as the boat reaches this speed. Generally, you reduce the throttle setting unless you wish to carry on accelerating to maximum speed.