OK maybe this is a stupid question but here goes...According to the Ford advanced towing guide my 08 F250 crewcab 4X4 with 5.4 and 3.73 gearing has a GCWR of 16000lbs but if it had a 4.10 it would be 18000 add the V10 and it goes to 21000lbs. WHY...is this a horspower vs gearing issue or are the axles different? I thought only gearing was changed and rear axels were the same otherwise. Please educate me, I posted this question in the towing section as well since I recently bought a fifth wheel toy hauler and am close to my GCWR with it attached with my current set up.

Scott

 

Axles are the same. Gearing makes a significant difference in tow capacity. For example, I have an Explorer. It's rated at something like 1500 pounds because it has 3.55 or something gears. Put 3.73 in the same vehicle and it doubles the tow rating, or close to it.

So, it's a combination of things probably. The V10 has more power, torque and hp, and then gearing will be 4.10 or 4.30 in the V10 (no 3.73 offered for years in the V10).

 

You also have to add tire size to the equation. Tire size, trans ratio and gear ratio add up to the final drive ratio, which isn't as low as you might think. The final drive ratio and the HP/TQ curves plus the cruise speed make the difference. For my equation, I use a cruise speed of 60mph and an ideal Engine RPM of 2000. This defines my final drive for picking gears, and it makes for a decent tow ratio and take off.

Then you have to examine the transmission and engine. Too much weight with too high of a final drive will make both work harder and create more heat, Can the rad remove it from the engine and will the transmission survive? Those numbers are going to be on the conservative side for transmission life.

 

gear ratio in the rear is a direct reflection of how much leverage your engine has over your vehicle is the simplest way to put it.
my truck is 08 super duty with the 5.4 w/4.10 manual trans. tows my 32 foot camper with narry a wimper and slows down nice on mountains as well.
but you know automatic is rated higher in towing.