We have a 2015 R25SC with the Volvo D-3. It is suppose to run at 3,000 RPM which ours does when lightly loaded. We have no bottom paint and the boat is stored on the trailer with a clean bottom. It gets wiped down when put back on the trailer and if the water is warm enough to swim gets cleaned, at least in part, during the week. End of the season the bottom gets waxed. With a week or two of clothes, food, gear, two people and a 70 lb labradoodle, full water and full fuel it struggles to get above 2890 or 2900 RPM and does about 17 miles per hour. At 2400 RPM it does 11.8 to 12 mph on the GPS. At 2550 to 2600 she will do 14 mph. As the week goes on and the water tank empties, the fuel tank empties, and the refrigerator empties, things improve. By the middle of the week when the fuel tank, water tank and refrigerator are half empty we get 13 mph at 2400 rpm, 15 mph at 2600, and 17.8 mph at 2930 RPM. By the end of the week, with less water, fuel, and food, top speed improves to 18 plus mph and max rpm gets to 2980 to 3000. (speed at lower RPMS also improves slightly.) Give me a slight tail wind and she will hit 19 MPH. I pay for a pump out whenever my holding tank gets close to half full because these boats, like most boats in this size range, are very sensitive to extra weight. I try to unpack many items and re-pack for the next trip because otherwise we end up carrying more stuff down the highway and across the water than necessary. (My figures are based on relatively clam seas, no head or tail winds, and no current.) (My last rip home from the lake I got 6 MPG towing, rather than the typical 7.5 MPG because I had a steady head wind and was driving just a little faster than I typically do - lots of wind resistance for these boats.) All figures are for fresh water. Like most engines she will run faster and better when temperatures are cooler and the air is more dense. I watch my speed and nautical miles per gallon figures closely to find the sweet spot for my trim tabs at whatever RPM I am running at. There is always a sweet spot for the tabs on the R25SC and too much or too little can make a big difference. I find that my nautical miles per gallon are pretty steady from 2400 RPM to 2700 and they are between 2.0 and 2.3 depending on the load. At wide open they are only about .1 NMPG less. Per the Volvo tech's advice, I try to exercise my turbo charger and run for ten minutes every hour (if conditions and location allow) at some where between 2800 and 3000. We will typically cruise in flat seas for hours at 2400 to 2500 RPM, with occasional variations up to 2700 to keep the turbo exercised, in addition to the ten minute runs at closer to full throttle. I spent 35 years driving Sea Rays with IOs and had no complaints, but when it rains, and for live aboard comfort and the type of cruising we do this inboard diesel is a great little boat. At 11 MPH the R25SC is not very efficient and digs a nice hole in the water that is perfect for wake surfing, but she is 10 to 12 mph too slow to water ski. Every boat is a trade off. Two weeks ago I did 2 hours of 3 to 4 footers, close together, with steep sides, do to the high winds and shallow bottom on Lake Simco, and I was too busy trying to see through the wall of water on the windshield to even care what my NMPG were, but the Volvo and the Ranger Tug never missed a beat and when we got to the lee side she ran wide open like she just came out of the garage after a nap.
