RTS Performance Parameters, Engine, Rear End Gearing & Tires

Converting a transit bus to highway use is an interesting topic. Overcoming the engine RPM and short gearing are the first task at hand.

Over weight is a killer to anything that moves. I had to assume that my bus has taken weight reduction opportunities. That leave me with fewer parameters to evaluate.

1. Tires.

The original bus is equipped with 11R22.5 tires 8" rim width. To increase the 55mph to higher speed 65mph or 70, 75mph, some has suggested replacing them with 24.5 dia rim.

* Used steel rim 24.5" can be had for a little as $25-$40 each Maybe there is positive cash flow if the aluminum wheels are traded for cheap steel wheels.

* 11R22.5 turns in 505 per miles, where as 11R24.5 turns in about 485 per miles. Upgrading the wheels, at the same engine RPM and rear ends gears will increase speed by 4%, assuming there is adequate HP to push. 55mph 11R22.5 = 57mph 11R24.5

2. Rear Ends

The standard rear end gear of 5.38 is for urban driving - seeking quick acceleration, but it takes a hit for prolong highway speed driving.

The transit gearing, 5.38 typically turns 2,000 - 2,100 rpm at 55mph - it quickly shorten the engine life.

Many suburban RTS have 4.56. It turns 1,800 rpm at 55mph. If needed, it can be pushed to go 65mph.

One of the popular options is to use 4.10. It turns at 1,900 rpm at 65mph, or 1,600 rpm @ 55mph. Using 4.10 rear end - which runs about $3,000 a set, posses a new set of problems,

* How much power does it need?

* Upgrading the fuel injectors for more HP, to balance out tall gearing, is there a fuel saving after all?

3. Engine (fuel injectors)

The standard 6v92T engine produces 270HP to 330HP. Most transit bus is equipped with fuel injectors producing 270HP. The engine block, various fuel injectors all reached the "270HP" at around 1,500 rpm.

* So, for the transit buses "270HP", there is no need to rev engine over 1500 rpm? If tall gears are used, drivers need to rev and keep the engine over 1500 (say 1,900 rpm) to get that 320HP needed to push the bus to speed?

What is the sweet spot for 6V92TA engine? Fuel burn/distance traveled at 55mph?

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The next step is to figure out how much power do I need to push this bus at 65mph, since I will be likely to a lot of highway driving.. Will I have enough HP to climb a 6% grade?