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Rear gear ratio

76_Rambler
Explorer
Explorer
I bought a 24 ft class c 1976 Holiday Rambler (E-350 chassis) last year, and recently had the motor rebuilt (ford 460). I ditched the motorcraft 4350 carb, and went with a holley 650 dp. I had them install a bigger cam on rebuild. It absolutely has plenty of power taking off from a stop, or climbing hills with no downshift. The transmission is a c6. The good thing is that it's a c6. The bad thing is that it's a c6, and has no overdrive, which means I'm running 2700 rpm at 55 mph and 3150 at 65. It was built when the federal speed limit was 55, and geared for that. I'd like to put a taller rear end in it to reduce my rpm's at 65. I'm just looking to see if anybody else has been through this scenario, and how far is too far on swapping the ratio. I know I'm going to lose power on the low end, and at the lower end of a shift. I'm sure I can make up for any bogging down after shifting gears with a stiffer shift modulator spring to shift at a higher rpm. I haven't lifted it yet to determine what's in it now.
26 REPLIES 26

PatJ
Explorer II
Explorer II
I'm on my third RV which is a 2019 Ford C with 4.56 and a 6 speed with double overdrive, so NA.

My previous RV was a SB Chevy with 4.56 axle and a Chevy TH400 (the GM version of a C6.) It was exactly as you describe. It did really well at 55 MPH (even with light towing,) but dropped off hard above or below that speed. When I had that rig I fantasized about re-gearing or adding an OD trans, but in reality I probably didn't have the power to support it (stock SB Chevy roughly 175 HP.)

My first RV was also a small block Chevy but 3.73 axle. IMO this was the ideal ratio for a non-lockup 3 speed trans with 3rd direct. This was my shortest and lightest RV of the three, but also was the oldest (1975) had the least HP of the three. But still it did very well on the highway 60-65 MPH and was a joy to drive even by modern standards. If you re-gear keep stock size tires and a direct-3rd trans I would shoot for 3.73ish. That's with me assuming Ford and Chevy from the same vintage are roughly equivalent. Just my opinion
Patrick

Bordercollie
Explorer
Explorer
Our 72 Dodge Van camper conversion also came with a low ratio rear end. Wife would beat kid's cars from stop signs.

time2roll
Explorer II
Explorer II

ppine
Explorer II
Explorer II
Try some larger tires.
If you can afford a new rear axle, consider a different trans.

76_Rambler
Explorer
Explorer
It won't let me upload a pic. It's taller in the rear end by probably 5 inches.

pnichols
Explorer II
Explorer II
76 Rambler wrote:
I can only imagine what that'd look like. It already has the muscle car stance!


"It already has the muscle car stance!"

How so? i.e. is it a 4X4 Class C?

(After putting larger diameter tires on our 2WD E450 Class C, it definitely has lost it's OEM look of an RV "riding on donuts", but not quite like a muscle car.)
2005 E450 Itasca 24V Class C

76_Rambler
Explorer
Explorer
I can only imagine what that'd look like. It already has the muscle car stance!

time2roll
Explorer II
Explorer II
You would need a 6" taller tire to go 4.56 to 3.80 equivalent ratio. Yes a 30" to 36" tall tire.

www.4lo.com

76_Rambler
Explorer
Explorer
What sucks about that, is that I just put new shoes on it!

pnichols
Explorer II
Explorer II
Of course the OP can change to larger diameter tires to drop RPM versus speed - without messing with gearing changes.

I did this on our small Class C MH in order to counter it's 4:56 ratio rear differential gearing.

As a bonus, you also get more ground clearance and improved tire wear from larger diameter tires.
2005 E450 Itasca 24V Class C

Grit_dog
Nomad III
Nomad III
Depends how much snort it feels like it has now and where you're driving it.
From KC, anything within a 700mi radius of you minimum, I'd put 3.73s behind a healthy big block and call 'er good.
If planning on hitting the Rockies, the elevation and grades will turn your fire breathin 460 into something much less desirable and the reason it came with 4.56s to begin with.
JMO
2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold

76_Rambler
Explorer
Explorer
I found a gear ratio / rpm calculator on line, and that's about what it said it would do, which is still a little high, but I'll settle for. Im just hoping that going from 456 to 373 isn't too much loss on the low end. I did look into gear vendors and it looks like around a 3k upgrade. Between the motor rebuild, exhaust, and new carb, I'm in the mechanical for alittle over 3k. Dollars are starting to stack up, and I haven't started on the inside yet. It's not in desperate need, but definitely needs updated. Exterior is in surprisingly good shape. Paid $1500 for it in unstable running condition.

time2roll
Explorer II
Explorer II
And what RPM do you want to be running at 65? 3.73 would put you at 2580 rpm.

76_Rambler
Explorer
Explorer
Pretty sure its a 456