Change rear end differential gearing to increase gas mileage?

[Coachmen1972]

ent: 8/14/2006

My rear is a 456. It shifts into third at 25mph, and runs at a high rpm at cruise speed. I suspect there is massive fuel savings to be had from lowering the ratio, say to 410 or so, and this seriously lowering the rpms at cruise. The 440 I have is a low torque engine and should be able to handle that....

Any thoughts?

[denisondc]

Sent: 8/14/2006

I would agree with you IF your RV had a manual transmission, with 4 or more gears,(or a lock-up torque converter that could handle the torque). You can calculate your driveshaft speed using the rear axle ratio, tire circumference, etc. With my 7.50-17" tires, they turn 611 times per miles. With my 4.56 axle ratio the driveshaft is turning 2780 rpm at 60 mph. On a level road @ 55 mph there is about 200-300 rpm of -loss- through the torque converter. That -slip- gets much higher on even moderate upgrades.
I imagine that torque converter 'slip' at lower driveshaft speeds would gobble up the savings you expected due to lowered engine r.p.m. I assume there would be more heating to the tranny fluid too.
The 413 has its torque 'peak' at 2700 rpm, though Im sure its not much of a 'peak'. I am pleased at a driveline design that has the engine spinning close to its most efficient rpm, when I am at crusing speed on the highway. The torque peak for the 440 was higher, maybe 2800, and was also a very mild -curve-.
I would have agreed the rpm at cruise seems high, until we drove ours up and over the continental divide. For the last few miles getting up to 10,500 feet we were in 1st gear, the gas pedal floored, going 25 mph - with the tach reading 3600 rpm. Nothing overheated, though tranny fluid,engine oil, and engine coolant were all toasty warm. We also passed plenty of vehicles, from cars to Vanhool intercity buses, that were at the shoulder of the road - smoking.

If you decide to try it, let us know how it goes. First step would be finding a lower ratio gearset to fit.

[jbmhotmail]

Sent: 8/15/2006

What is the size and loaded weight of your RV. Also what size tires are you running. It may pay you to locate someone with larger size tires (for rear axle) that with give you your desire gear ratio and see how you like it before changing your differential gear ratio, this would be for a trial run. Don't forget about changing your speedometer gear.

[Coachmen1972]

Sent: 8/15/2006

Good thoughts.....thanks. I don't know my rpm's,as I have no tach....guess I should hook up the Tach and dwell and go for a ride, or install an electric tach.....then I can talk numbers instead of "a little high"

I like the bigger tire idea, except for the fact that the outside rears are hard to get out from between the hub and the body as it is, a taller tire would be even more of a problem.

There exist electric overdrives....anyone have any experience with these?

[ClydesdaleKevin]

Sent: 8/16/2006

The Ark has 4:88 gearing, and while lowering it might improve gas mileage, I'll take the torque for getting up hills any day!  We tow a fullsize van full of our stuff, and I'd hate to see what 4:10 gearing would be like trying to get over mountains.

Kev

[furnguy]

Sent: 3/28/2007

I have a 78 Chieftain with the 440 and the 3 speed auto transmission. Mine turns 3300 RPM's at 60 mph. It walks up fairly steep hills in third with plenty to spare. I too think this is revving too high and would like to get it down to 25-2800 at 60. My current 30.1 diameter tires give a circumferece of 94.51 inches. If I went to a 34" tire it would give me 106.76". My calculations may be way off, but I figure this would only give me 10% or 330 rpms at 60. There are also problems finding a tire that height as they have to be fairly narrow for the duals on the back.
Overdrive units are expensive and the cost would never be recoverable in my research.
Has anyone tried changing the gears in the rear end and if so what is suggested?
Thanks on behalf of "Barney"

[Slantsixness]

Sent: 3/28/2007

4.56 rearratio is what you should have. If you have the 4.88 that Kevin had (he has a newer 77 Itasca now), then you might consider changing the ring and pinion.

Overdrives in the long run will cost too much. Oversized tires will work, but not by much, typically 200-300 rpm, and there can be clearance problems with the tires, or just getting them for that matter. (and replacing at least 4 rear tires at once will set you back at about 1/2 what the overdrive unit costs... another dilemma...)

You could, with a healthy 440,  switch to something on the order of 3.55 or 3.23 ratio, but you won't realistically be able to tow anything, and the rig will start off SOOOO SLOOOWWW that you would probably regret that low of a ratio too... but the RPMs at 65 would be around 2200.... hmmm... do the math....

Lower gear ratios, while an improvement at highway speeds and gas mileage, will take its toll on the transmission. Also, in order to get the rig moving, you would want to change the torque converter over to something with a 2700-3000 rpm stall (to match enginge power and torque band to the transmission output and rear ratio, which, at slower than highway speeds, will decrease you gas mileage again!

See the vicious loop?

You could most likely go to a 3.91 rear ratio and not notice any appreciable driveability difference, and slightly better gas mileage at highway speeds, but the lower city speeds would work the engine harder, and of course you know that means... worse gas mileage again.

If you are running bias ply tires, the same old "rumor" is true for motorhomes as it was for 50's and 60's cars.... run the tire pressure high, and improve fuel economy, lower it and economy suffers but the ride is better. (but I'll warn you that the tires wear out quickly, and the tires don't handle as well as they do with proper inflation.) (If you are running radial tires, this doesn't work, by the way.)

All right then I'll get of the soapbox.....

(did I mention my '72 Brave gets 15mpg?)

Tom

[Lefty]

Sent: 3/28/2007

Our '76 Chieftan has a GVWR of 14,500lbs. The actual weight with normal gear, groceries, full fuel tanks, etc... runs about 12,000lbs across the truck scales.
While it will never win a drag race, it will keep up with normal traffic with ease, red light to red light. Pulling hills, it will slow down until it gets into 1st. if the hill is really long & steep. But once it gets into 1st. it pulls hard enough that I've never worried about not being able to make the crest. It pulled up Cheaha Mtn. which is a 9 mile 11% grade up to the campground. I was running about 18 mph at the top. & I was behind a 34' diesel pusher the whole way that I could have passed with a fair amount of ease if I had enough room. Also, it runs 75-80mph highway with relative ease. My milage averages around 7-8mpg which seems to be what most of these get.
An overdrive unit, such as one from Gear Vendors, would run about $2,500-$3,500 new. I've never seen a used one. I would never use mine enough to even come close to making up any in savings. While it might be possible to get a different gear ratio, I've never looked to see if anybody even makes one. A higher ratio would help with highway cruising, but at the cost of lower end torque. And torque is the main thing you need with a rig this size. Especially if you will be in hills or towing a dingy or boat. The 440 was a very tough engine, with an almost perfectly flat torque curve that stays above 400 ft/lb through almost the entire range. Making it ideal for heavy pulling. Modifications can improve milage, but any modification that improves one area will always have a negative impact on another. (you can't have your cake & eat it too.) The key, is to improve an area enough that it offsets any negative loss. Such improvements can vary from advanced engine tuning, to porting/polishing the heads, to rebuilding the engine to produce more power throughout the entire operating range.
It is entirely possible to build a 440 that makes over 550 ft/lbs torque from 1,500-5,500rpm and a peak hp of around 500. Note I said "possible", not "Cheap"... The standard Load-flite 727 will easily handle that kind of power without any loss in service life. A higher performance engine will allow the engine to pull the same weight at a lower work load. This will usually result in a fuel savings enough to offset the higher fuel demands of a higher performance engine. For example: I had at the same time 2  1976 Ford F-100 XLT Ranger pick-ups. Both were identical in options & body style (long wheel base, A/C,ps,pb,automatic) One had a 302 2bbl, small block, the other had a 390 4bbl big block. They both got 10mpg running with no load. The 302 truck got 7mpg if loaded or towing, the 390 still got 10mpg regardless of load. It got 10mpg even with 1,800lbs of gravel in the bed & towing a dual axle car trailer loaded with 3 tons more gravel. The difference was, the 390 never had to work hard to pull the load. The 302 was straining it's little guts out! Same with a motorhome, if you can get the engine to not have to work as hard, you get better milage. The higher a ratio, the lower the rpm at cruising speed. But, it also means the engine has to work harder to pull because it loses mechanical leverage.
In short, the factory ratios were a trade-off between maximum pulling power, and decent top cruising speed. They allow a typical rig to cruise effortlessly on even ground at highway speeds between 60-80mph. And yet, retain enough low-end grunt to pull most grades with little trouble.
You can raise the gear ratio & get a higher top end, or, you can lower it to increase pulling power. But each has negative impacts on total performance.
The best approach would be to build the engine to make more power througout the power band. The cost would be about the same as an over drive unit, but it would result in better drivability from the extra power.Such an engine should, theroretically, get better milage under the same load as an engine with less power. Whether or not it would ever pay for itself in fuel savings would be dependant on how much you drive. With the right engine, great tuning, proper inflation, etc.. you could see a 20% savings in fuel (assuming an increase from 8mpg to 9.6mpg) Assuming you filled mine up, a 20% savings on 100 gallons @ 2.50/gal. would be saving $50 bucks every fill-up!
At a cost of $3,500 it would take 70 tanks of fuel to pay for itself. At the rate I have used mine, that would be between  15-20yrs!!! So, as you see, it depends on the amount you intend to use the rig. Somebody like Denison, who travels much more than me, could save enough to pay for the upgrades in maybe 4-5 yrs. making it well worth it.

BTW- There is an old rule-of-thumb on building high performance engines that still holds fairly true. It costs about $10 for every horsepower of an engine. A 400hp engine will cost about $4,000 to build right. A 500hp engine will cost about $5,000, etc... The rule is not hard & fast, but it usually comes pretty close. The 383 stroker small block in my pick-up made 435hp on the dyno when built 7 yrs ago. It cost me around $4,000 to build, with me doing all the assembly, and farming out the machine work & balancing/blueprinting. I did the porting/polishing on the heads & intake, and I also performed the detailing like de-burring the block & champering all the oil holes. Had I just had a shop do all of it, it would have been about $5,000 total. So the rule would have been a fairly good guideline, though not exact. (the engine now has about 75,000 miles on it, & I've never even had to remove the valve covers since installation. Only oil changes & tune-ups )

[Oz]

Sent: 3/29/2007

We did have one member who installed and electronically controlled overdrive.  That would be the ticket for long, level highway cruising. 

There's a great discussion which covers the topic already on the boards.  Just use the Message Search for more info.