Had the coach out on a 260 mile round trip this week, so tried to follow up on monitoring the gauges. Turbo boost never exceeded 9# and that was on a long climb when the transmission down shifted to 3rd gear. I checked the air filter when we got there. It doesn't look that dirty but I'm going to replace it anyway. Since I have not replaced it previously, I have no idea about it's age.

I checked the manual I have for the transmission and it says it is a lockup converter. I verified this by watching the tach on a long downhill decline. The engine RPM stayed locked up with the speed of the coach even when I had my foot off the accelerator and on the brake.

Now for the matter of the engine RPM the coach turns at highway speeds. AND PLEASE CHIME IN AT ANY TIME IF I'M WRONG ABOUT ANY OF THESE CALCULATIONS. According to the Gillig manual the coach is equipped with a 4.10 geared differential. 4th gear in the Allison transmission is 1 to 1. The transmission is equipped with a lockup converter so assuming no slippage the engine is turning 4.1 rpm for each rpm of the rear wheels. According to Toyo tires website, 255x70R22.5 tires turn 568 revolutions per mile. If I figure 60 MPH which is equal to one mile per minute, then the engine would have to turn 4.1 times 568 revolutions per mile to maintain an actual speed of 60 MPH. That calculates to be 2328 RPM. That appears to be about what it's running.

Also checked the revolutions per mile of 10R22.5 tires, which I plan on using to replace the tires on the coach . Because of the larger diameter the revolutions per mile are 520, which should reduce the engine RPM at 60 MPH to 2132 RPM, a reduction of about 8.5%.

Nick

Nick: My coach and manual specifies 295-75-22.5 tires which explains my lower RPM at 60 mp. My experience however is that I can run about 1890 at an indicated 60-63 mph. Could be speedo error but how did you get such small tires?
Tom

quote:

Could be speedo error but how did you get such small tires?

They were on the coach when we bought it. Don't have any idea why someone wanted such a small tire. Maybe they were short and wanted to lower the coach.
Nick

quote:

Originally posted by Nick Cagle:

quote:

Could be speedo error but how did you get such small tires?

They were on the coach when we bought it. Don't have any idea why someone wanted such a small tire. Maybe they were short and wanted to lower the coach.
Nick

Better cornering?