Change rear axle ratio on 94 Marquis

[Dale Wiest]

I have a 94 Marquis Le Sierra with the 3176 Cat and Allison HT-747 4-speed trans. According to the brochure the rear axle ratio is 3.42
According to the tach I'm at 1800 rpm at 63 mph which seems to high for my liking. I will assume this coach was built to run 55 mph as that was the general speed limit in the usa in 1994. In my research I've found different axle ratio gears of 3.32 or 3.07
Which gear set would be best considering this is only a 4 speed trans and my max cruise speed on flat ground would be 70 mph as that is our local speed limit.

[Steve Huber Co-Admin]

Dale,
Dropping the rpm will probably cause the engine to lug when accelerating and pulling. Also, 70 seems a bit high for your tires as I think RV tires are rated at 70 max.
Steve

[Edward Buker]

Steve, Dale,

The M144s from Toyo were rated up to 80Mph, not that 80 comes into play realistically. I have hit 70+ at times on downhill before hitting the Jake.

Dale, I lived with a 4 speed Allison 3208 and a Cat motor and share your issue regarding pushing the RPMs in 4th. I think the answer hinges on how much power and acceleration you have now. The third to 4th as I remember it was a tall shift dropping the rpm so if you have plenty of power when you now enter 4th gear you may be fine. If not then perhaps someone could fit a 5 speed Allison in there which would be more ideal.

Later Ed

[Dale Wiest]

The 3176b engine is rated at 365 hp and 1350lbs torque and moves the coach quite well. It is a tad smaller than a C12, but a lot bigger than a 3126 engine and way more torque. My thinking is at a cruise speed of 60-65 mph it should rev around 1600-1650 rpm, and be easier on engine and fuel mileage, rather than my current rpm of 1800-1850 at 60-65 mph.
What are peoples rpm at 60-65 mph with the C12 engine in high gear?
My loaded weight with full fuel and water is under 30,000 lbs with no slides
Ideally switching to a series 4000 6speed auto which was available with this engine in 1995 would be nice, but try and find all parts needed to make the switch, will not be easy.

[Steve Huber Co-Admin]

Dale,
My C12 runs 1450-to just under 1500 when running 60-65mph. Coach weighs about 35 - 36k.
Steve

[Dale Wiest]

I have a mechanic friend with his own shop and hoist in shop, so it would be an easy gear swap, with minimal cost.