Gearing for economy post?

[5B Steve]

   After reading the previous post about tire size does it really make that much difference on preformace. If indeed it does I need

   to purchase a new set for my 5B.  I have (what it came with when I bought it) 11-R 22.5.  Is that factory size, and what

  is the best buy for the money?


   Steve 5B......

[belfert]

I certainly wouldn't buy a new set of tires in search of fuel economy unless you already need new tires.  You would have to drive the tires 100,000 miles and save at least 5% to even think about breaking even.  You probably already do pretty well with an MC5 so you would probably need a lot more than 5% to break even.

[TomC]

Tire sizes affect the overall economy of the bus.  Some of the tires sizes with the revs per mile- 295/75R-22.5 or 275/80R-22.5=517rpm; 11R-22.5 or 285/75R24.5=500rpm; 12R-22.5=485rpm; 11R-24.5=476rpm.  If you have a direct drive transmission with 3.73 rear end ratio, as an example, then your engine speed at 60mph will vary with the 517rpm tires of 1928rpm to 1775rpm with the 476rpm tires.

Another influence on fuel economy is what model of tire you use.  Tires a much more then big round rubber things that hold air.  As an example-Michelin makes over 50 models of tires just for trucks/buses.  Use an off road tire, and you'll have a rough riding, noisy tire that won't get as good fuel mileage as their fuel efficient over the road tires-because of tire to road friction.  If you use 11R-22.5 tires, Michelins X-One super singles at 455/55R-22.5 is a direct replacement.  It is proven that at least with a tandem axle on a truck, replacing all 8 tires with 4 super singles will up your mileage by 3-5%.  Now that would be significant.  I know if I had a tandem axle bus like a Crown or Gillig, the X-Ones would be my choice. I would run them on my truck, but there isn't a replacement super single for the 11R-24.5. 

Course the biggest influence on fuel economy is the loose nut behind the steering wheel.  Flooring the engine all the time and cruising at 75 will not get the same mileage as feathering the accelerator to let the engine gradually bring the bus up to speed and cruising at 55mph.  The difference between cruising at 55 compared to 75 can be as much as 2.5mpg difference.  Good Luck, TomC

[bevans6]

It's also unclear to me what the effect of raising or lowering the over-all gearing on a bus will be.  I know, for example, that when I went from a 3.7 diff ratio to a 4.11 in my van (towing my race trailer) I got an improvement in over-all fuel mileage.  I thought I would when towing heavy, but I was surprised that I also got an improvement around town.  the only place I got worse mileage was highway unloaded.

In the bus context, I think that just going to a taller over all ratio may not improve mileage if the current setup is optimized for your engine, load, speed, route and terrain, driving style.  Maybe a lower ratio would do you better if you drive in town or on two lane roads a lot, like Sean does with Odyssey.  Maybe the taller ratio will only work if you  are all about 80 mph on the Interstate, with a powerful engine.  I pondered the idea of which drive ratio would work best for me, but with a 3.7 and the hills I find in my road I felt like slowing down and running 60 on the Interstate and actually making it up the hills at 40 mph was the best compromise.

Brian