Warning, technical post ahead...
According to the laws of conservation of energy, the potential plus kinetic energy at the top of the lift will equal the kinetic energy at the bottom of the first drop (this is a rough and simplified example, so we'll ignore friction and drag). Kinetic energy is equal to half the mass times the velocity squared, or 1/2mv^2, and potential energy is equal to mass times height times the acceleration due to gravity at a point, or mgh (we'll use 32.2 ft/s^2 as an approximation). Therefore, by applying algebraic concepts, the speed at the bottom of the first drop (in feet per second) is the square root of the velocity of the lift (in feet per second) squared plus twice the height times the acceleration due to gravity, or sqrt(v^2+2gh). This will give the answer in feet per second, and to convert it to miles per hour, multiply the answer by 3600/5280 (approximately 0.682). Applying this math, and assuming the speed of the lift is negligibe, you get:
The maximum possible speed of a 91 foot tall coaster (such as Grizzly) is approximately 52 MPH
The maximum possible speed of a 70 foot tall coaster (such as Giant Dipper) is approximately 45 MPH
The maximum possible speed of a 108 foot tall coaster (such as Gold Striker) is approximately 56 MPH
So while some speeds are likely exagerrated, most of them are probably pretty close to the actual speed. This method won't get you the true top speed, but it can prove if the "official" speed for a ride is high (however, keep in mind that lift speed can play a role, especially in coasters with cable lifts, and the full drop length must be taken into account if the ride drops below ground level). If the laws of physics don't allow that speed to be achieved in an ideal case, it won't occur in a real world case.
Top 5 Steel: Bizarro (SFNE), Nitro, Apollo's Chariot, Maverick, Banshee
Top 5 Wood: Boulder Dash, El Toro, Thunderhead, Gold Striker, Lightning Racer
Top 3 Parks: Holiday World, Busch Gardens Williamsburg, Cedar Point
TPR Trips: IntimidaTour 2010, North East Trip 2011, LeviaTHON 2012, Mini Tours 2014 (New Hotness & East Coast)