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Re: [GMCnet] Final Drive Options and Recommendations [message #220515 is a reply to message #220505] Fri, 30 August 2013 11:17 Go to previous messageGo to previous message
James Hupy is currently offline  James Hupy   United States
Messages: 6806
Registered: May 2010
Karma:
Senior Member
Find a slight uphill slope covered with wet grass. Back your GMC on to the
grass with the front end upslope. Wait a few hours. Then attempt to leave.
Then you can decide if you need limited slip or not. I have a 78 GMC Royale
with a 403. It was upfitted by Coachman in May of 78 so it is a fairly late
coach. All hardwood cabinets, 6.5 KW onan, lots of golf cart batteries, Dry
bath. I carry lots of spares and tools. Net weight over 10,800 pounds. We
travel over mountain passes any which way we go except North to Seattle. I
drove the coach in 2008 and 2009 with 3:07 final drive. Then changed to
3:70. That one modification did more to improve the driveability of the
coach than anything else I have done to it. 3000 RPM equates to about 65
mph. The 403 is Happy, Happy at anything over 3000 rpm. It will easily make
80 MPH indicated on flat level ground. How much faster you choose to drive
than that is up to you. I would never go back to the 3:07
Jim Hupy
Salem, OR
78 GMC Royale 403


On Fri, Aug 30, 2013 at 8:57 AM, Mickey Szilagyi
<mickey@apex-internet.com>wrote:

>
>
> Thank you all for the various opinions on our final drive situation. My
> son is convinced to go with the 3.70 since that seems to get the most
> votes. I'm having a harder time deciding between the 3.55 and 3.70. We
> have a 77 Kingsley with the 403 and it appears from what some of you have
> said it is on the heavy end as far the GMCs go. We may also be towing
> either a small utility trailer or small vehicle - something to transport a
> family of 5 when needed. I can't see that we would travel faster than 70
> even going cross country. I pull a 21 foot boat with my 1999 Durango and I
> like to go between 60 and 65 so I don't see changing much with the GMC.
> Maybe 70 mph on the open road to make a little time. My son drives about
> the same way. Other than getting to our destination vacation areas on the
> interstates we'll be doing a fair amount of driving on two lane roads at
> lesser than interstate speeds. Probably not much cold/winter weather
> driving.
>
> So, in trying to come to a decision, what is the peak performance RPMs
> and/or torque range of the 403. I'm guessing it's between 3000 and 3200
> but please verify or correct me if that's not right. Maybe this is not as
> important as I'm assuming as there are too many different driving
> situations.
>
> Also, we are undecided on the limited slip add-on. It's expensive but now
> would be the time to do it and it should be a one time expense. How
> important or how much of a factor would it play in the overall performance
> and use of the motorhome. Is it only helpful once in a blue moon or more
> often than what I imagine. Is it something that could fail or be a problem
> as we see it only has a 2 year 24,000 mile warrantee, less than the final
> drive warranties. Is that telling us something?
>
> Finally, does the final drive assist in downhill driving and braking on
> long grades? Would one have an advantage over the other - 3.55 vs 3.70.
>
> Are there any other factors that we should consider? We realize there
> isn't a magical sure fire combination but we rely on the forum consensus,
> experience and expertise for a lot of what we have already done to our
> coach.
>
> Thanks again to all, each of your comments are read and greatly
> appreciated.
> --
> Mickey
> 1977 Kingsley, 403, Lansing, MI
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