Here are some numbers for you to roll around in your head awhile concerning GMC V-drive powertrains:All Models -
Bevel gear ratio in the 4-spd manual gearbox is 0.808:1 for all 8V71-powered units, starting with the 1961 PD4106.
Bevel gear ratio in the V-730 is 0.87:1
Bevel gear ratio in the 6-71-powered models was 1:1, such as your old TDM. 35-foot Coaches -
All the Parlors in this size used a 4.125:1 rear axle. They were basically geared to run 60 mph @ 1650 rpm using tires that turn 495 revs/mile with the manual gearbox.
The overall final drive ratio with the OEM powertrain is figured as follows: 4th gear x rear axle ratio x bevel gear ratio = final drive ratio. Plugging in the numbers gives you:
([1:1] x [4.125:1] x [0.808:1]) = 3.333:1
If you install a V-730, you get:
([1:1] x [4.125:1] x [0.87:1]) = 3.588:140-foot Coaches -
This size Parlor used a 4.375:1 rear axle ratio. This works out to about 100 rpm higher at any given road speed than the 35-foot models. Makes sense - the 40-footer's heavier.
Plugging the numbers into the above formula gives you a final drive of 3.535:1 for the stick shift, and 3.806:1 for the V-730.Real World Mileage -
After 25+ years in the bus industy, IBME that most stick-shift, 8V71-powered 35-foot GMs get 8-9 mpg overall. Some owners, running in the flat mid-west and keeping the speed down to around 60, have reported 10, but that's rare. V-730-equipped models return 1 - 1.5 mpg less.
Most 40-foot stick GMs get 7-8 mpg, and again, the V-730s about 1 - 1.5 mpg less.
It really does depend on the routing, winds, weight of right shoe, highway vs city, overall weight of coach, etc.
Because of the limited amount of powertrain options for a V-drive GMC, about the only way to improve mileage (besides parking it and never driving) is to install 24.5" wheels with the tallest 11R24.5 tires you can find. Bridgestone, for example, makes a drive tire that turns 470 revs/mile. This tire will just about bring a V-730-equipped coach back to similar mileage as a stick-shift unit.
The heavier 40-foot MCIs will return 1 - 1.5 mpg less on average than a comparable GMC. Simple physics.
Where a bus really shines compared to a stick 'n staple is in range. A PD4106 with the OEM stick shift powertrain, for example, will go over 1,000 miles on a tank of fuel, and still leave you enough to find a truck stop. So will the 4905, for that matter, but the reserve margin's a little tighter.
Finally, you don't buy a bus for fuel mileage. It's just like the old joke about the Mercedes salesman telling the prospective customer after being asked about a model's fuel consumption: "If you cannot afford the fuel mileage, you cannot afford the car." You buy a bus for the safety and convenience, not mileage.
FWIW & HTH. . .