I need some help figuring out a fair dry rental price for a 1968 Cessna 310P with VFR avionics. The owner flies it about 30 hours a year and would like the plane to fly more. I recently got my commercial AMEL rating and would like to build up some multi time. The owner isn’t trying to make any money on the deal, but wants to make sure he is covering his costs. In addition to the 310, I am also looking at a rental Grumman GA-7, Piper Seminole, or Piper Twin Comanche, all of which rent for $240/hr wet. Any experienced 310 owners out there that can help us figure out a fair dry rental price?
As is so often the case, the answer is "it depends". If the owner just want's to end up where he would be financially whether you fly or not, all that needs to be accounted for beyond fuel is his hourly operating cost with no contribution from you for the fixed stuff like hangar rent, insurance (you'd still need to cover any increase in premium due to your usage), debt maintenance, database updates, and annual inspection (minus repairs). You should include things like engine and prop overhaul reserves, accessory maintenance reserves, tires and brakes, oil changes (maybe you could contribute the labor for that), and something for unscheduled maintenance.
On a NA 310 I'd suggest something like $35-40 for engine/props, $5 for accessories (add $2 if it has boots), $5 for tires/brakes, $10 for oil changes, and another $20 for unscheduled maint. for a total hourly dry rate of $75-80. Maybe another $20-25/hr thrown on top of that for general wear and tear TTAF depreciation etc. would be good bringing the rate to a nice even $100/hr.
But if the owner wants to actually save some bucks by having you share in the fixed costs it will be difficult to come up with a meaningful hourly number since that would greatly depend on the number of hours per year and the total of those fixed costs (which vary considerably).
BTW, personally I think that basing such a number on the cost of fuel used is way overrated. For one thing when fuel prices jump 25% because of unrest in the middle east, that shouldn't really have any effect on the rest of the operating costs. Second, right now you can pay anywhere from $3.80 to almost $7.00 for a gallon of gas and I can't see how you'd even pick a fuel price to multiply by some factor. If you used your actual fuel receipts the owner would suffer if you were good at finding cheap gas and would see a windfall if you paid through the nose for fuel. Finally, for a wet rate, a change in fuel price doesn't create anywhere near as big an error in the total cost since fuel is such a big part of that but to come up with a dry rate the errors are a much greater percentage of the non-fuel cost.
Bottom line is that when fuel prices are stable and relatively consistent there is a fairly strong correlation between fuel cost and operating cost, especially if you include the fuel in the total rather than paying for it separately, but if any of that's not the case (e.g. wide fuel price range and/or computing a dry rate), that relationship contains too much error to be practical.