Originally Posted by pengyou
Thanks. So a diffy that is 3.x is likely to provide better mpg on the freeway but slower acceleration?
In general, that is what you would experience. But if you go thru any hill country, it will also make a difference.
The school bus was only made for local, short distance runs with a million stop and go's. So high numerical ratios make sense. If you wish to go long distance, as in rv usage, then yes, you are on the right track to find the best ratio for your intended usage.
There are various ways to find your current ratio. Some may methods may even be on youtube now days. But when all else fails, you can get a friend to help and jack up one side of the rear axle on a level surface, chock well the other wheels and put a heavy duty jack stand under the axle for safety. Always remember safety is your top priority.
Then with a piece of chalk, mark a spot on the tire that you can index pretty accurately with something on the buss exterior. Mark the drive shaft the say way. While the friend rotates the wheel two full revolutions very slowly, you count the drive shaft revolutions. Then divide the drive shaft count by 4 and you'll have your ratio.
An example would be 2 tire revs and just a tick under 16.5 on the drive shaft would be a 4.10:1 ratio.
You'll have to find the axle brand and model to be able to find a replacement gear head or gear set. Likely your bus would have a heavy truck axle under it. Likely a mainstream brand like eaton or rockwell which is now known as merito. Could be as low of rating as 20,000lbs, but should be a 23,000lbs rated on a the full size buses. Just depends how old it is and what the requirements were at that time.