To help with specifics, we need the OP to tell us not just "1985 Class C" but also the Chassis Make, Model and YEAR. Our "1984 Class C" turned out to be on a 1983 Ford E350 chassis which had 16.5" tires on "Coined" Lug-Piloted wheels, hard to find 16' replacements for. From what I understand, Ford went to Hub-Piloted 16.5" wheels in 1984 models. THOSE would interchange with later model E350/450 wheels, all the way to current date. Then not much later, 16" wheels, but I don't know that year.
What I DO know, is that if it's a 1980's Ford, transmission is the famously durable C6. Three forward speeds and no locking torque converter. It also means that without overdrive, standard since 1992, it'll turn up the same engine RPM at 60-MPH that overdrive will at 85-90. I didn't drive it much over 60. I don't think the OD really helps that much with MPG. The E350 was carbureted till about 1988 or 89, then EFI, then MPI till the 460 ended in 1996. My guess is that OD helps MPG by no more than 10%, same for MPI. Our current Class C has OD and MPI and gets 1 MPG less than the carbed one without OD. But it's 7' longer, 1' higher, 6" wider and weighs 3000 pounds more...
