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3.73 to 4.10 gear change

16267 Views 7 Replies 5 Participants Last post by  capt205
My limited slip is going bad. It chadders rounding corners when towing my 8000# travel trailer. Changed rear end grease with additive, and no change in the way it acts. So, I think it is time to go into it. While reworking the limited slip, I was considering changing the gears to 4.10. I mostly tow with my Excursion. It weighs around 8000# and has a near verticle front nose area. Since 90% of my driving is towing trailers, I would like to have more power while in the mountains. What are the cons of making this change? I tow around 60 mph and know rpm will be up around 200 rpm. I know it will hurt my running without trailer mileage. But I think it would help my towing mileage. Let me know your thoughts.
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I tow around 60 mph and know rpm will be up around 200 rpm.
With stock-size tires, you should be seeing around 1,700 RPM at 60 MPH today. If you go from 3.73 to 4.10 rear axle ratio, that's a ten percent change, so your RPM should increase 10 percent at the same speed. 1,700 plus 10% = 1,870, or around 1,900 RPM on your analog tach.

Because of the horsepower curve, 1,900 RPM is a lot better than 1,700 as far as power is concerned, at least on a stock engine. And even with your hot-rodded engine, you should prefer towing at 1,900 instead of 1,700 RPM because the tranny won't downshift for every little bump in the road.

So for towing, I think you'll really like the 4.10 ratio if you keep the speed down to around 60 MPH.

Unloaded, your RPM is around 2,150 at the 73/74 MPH ticket limit, and it would be almost 2,400 RPM with 4.10 rear end. That's why the fuel mileage goes down. But if you can make yourself cruise at that same 60 MPH unloaded as when towing, you'd get a lot better MPG.

My trailer weighs around 8,000 pounds when wet and loaded for the road. I think the 3.73 ratio with stock-size tires is about perfect for everything except mountain climbing. I wouldn't want a 4.10 ratio in mine. I normally cruise at 1,800 RPM or 62 MPH when towing. Even with my hot-rodded engine, 1,700 RPM or about 60 MPH is not quite high enough on the power curve to prevent downshifting too often, so unless I'm in really-flat country, I settle for 1,800 RPM. At 1,800 RPM with the DP-Tuner 60-tow tune, mine rarely downshifts for smaller hills.

I would prefer to tow at the traffic speed of 2,150 RPM/73 MPH, and my rig will gladly cruise at that speed all day without any complaint. But fuel mileage takes a beating if I do that. Mine costs about one MPG for every 100 RPM more speed, so cruising at 73 MPH instead of 62 MPH would cost me around 2.5 MPG.

It's not the engine RPM that hurts fuel milage - it's the increased aerodynamic drag at the higher speeds you get with the higher RPM. So you probably won't notice any change in MPG when towing at 60 MPH with the 4.10 gears
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Back to your limited slip:

Most folks that have tried it prefer the Eaton Detroit Truetrack. The Detroit "Locker" is a lot more famous, but the TrueTrack is a lot more pleasant to drive with in a street vehicle. The cost is very little more than replacing your Ford LS with the Ford unit.
Detroit Truetrac
If I spent 90% of my time towing an 8,000 lb trailer at 60 MPH, I'd put a 4.88 in it.
That would be around 2,225 RPM @ 60 MPH in overdrive, and almost 2,900 RPM when downshifted to direct drive while mountain climbing. Yeah, that could work. :)

But I wouldn't want it in my rig. I like my 3.73 rear end the way Ford designed it, as long as I can have my 60-tow tune to give me a few more horses at 1,800 RPM.
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