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Will changing my gearing make a diff? Joe 2-23-04  
I have a 1985 22foot class c motorhome with a chevy 350 small block, 400 turbo tranny, 28" wheels and a 4:11 rear end. My engine is turning 3000 rpm when I'm running about 55 mph. I get about 8 miles per gallon and my rig weighs out at 9220 gross. I'm worried that my engine won't last at those kind of rpms and I'd like to get better fuel economy. Can a small block run 3000 to 3200 rpm for hours on end without starting to come apart? If I dropped my ration down to a 3:73 would I see a significant change in my fuel economy. Just curious, I hate to spend alot of money for nothing.
Re: Will changing my gearing make a diff? David 2-24-04  
I have your same problem with my 85 Minnie Winnie...

The thing has already gone thru 2 motors, however, I am pretty sure they went for reasons other than high sustained RPM.

From what I know about the Chevy 350, there are trucks out there- snow plows for example that are turning their Chevy 350's at 5-6K RPM for super duty applications...

Mine typically churns at about 2750 at 55 RPM, but I think I have a sweet spot at about 3300 RPM where I am travelling approximately 64.1 mph....

Instead of changing out the gearing (which may be cheaper) you may want to find an overdrive/underdrive unit. It will cost you around 3K, but, you will see savings in gas (if you drive a lot) and you will have the same power (and more) on the low end of your gears, instead of sacrificing power by changing your gears...

Just a thought! I chose to not do anything yet...maybe some day when I can afford it...
Re: Will changing my gearing make a diff? Mark O. 3-9-04  
Hook up a manifold vacuum gauge in order find out if you have enough HP to be able to use a higher speed rear end.

If you are currently running 10" or less of vacuum at your cruise speed it is very unlikely you will see any increase in your fuel mileage. You may in fact experience an increase in fuel use if your manifold vacuum drops below 5-7" at cruise speed.

It takes a certain amount of HP to move the mass and frontal area you have in your RV down the road. A small block is going to have to work pretty hard all the time to maintain highway speeds.

The installation of an overdrive/underdrive may actually be the best choice. If you drive by the vacuum gauge you will be able to know when it is best to up shift or down shift.

Good luck.

Mark O.
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