For an S-76, it is about 5 knots at 125 knots, while holding constant power - and therefore constant fuel flow. Calculating the fuel savings is easy, just multiply the speed change by the fuel tank capacity, and realize how much fuel you save with each full tank. Multiply by the number of refuels per year, and realize why skids and fixed wheels are not very popular any more.
For an S-76, 5 knots /125 knots = 4% x 1850 lbs fuel = 72 lbs per tank, or 3000 gallons per year (1,000 hr x 500 lb/hr = 500,000 lbs per year x 4% = 20,000 lbs fuel saved = 2,985 US gallons)