Can my truck tow this?

valhalla360 wrote:

If you are pushing the truck to it's absolute towing limit...yes, a deeper rear end will increase the tow rating. But if you are within the ratings, it won't improve the towing.

If you compare two trucks identical except: one with a 10k tow limit (& lower numerical rear end) and one with a 12.5k tow limit (& higher numerical rear end) and you are towing 7k...it really doesn't matter with modern 8-10 gear transmissions. Basically, the transmissions will pick different gears to effect the same overall gear ratio (or at least close enough it won't be noticeable when looking at engine RPM and efficiency).

This is different from the good old days of the 3 or 4 speed transmission. Yes, in those days the low numerical rear ends were about getting better MPG. Most miles ran empty and the set up the transmission around that.

Problem as soon as the engine can't hold top gear...with a 4speed, you are going from say 1.00 to 1.36 or a 36% change in gear ratio (some transmissions can be upwards of 60%). That was often WAY MORE of a drop than the engine needed, so the engine would race like mad (36% increase in RPM) to keep the RPM up high enough to maintain highway speeds. By upping the rear end, the truck could hold top gear and stay at a reasonable RPM...when towing... but will run higher RPM than needed when not towing hurting MPG.

But say the modern Ford 10 speed transmission, dropping from 10 to 9 goes from 0.63 to 0.68 or about a 7.5% change in gear ratio. So if the engine can't hold top gear and drops down, the engine sees a 7.5% RPM increase to hold the same speed and that will often be enough...if not, it can drop to 8 or 7 and so on but it can dial it in with small percentage changes. It can do this with either rear end, you will just see different gears being used for the same conditions but for the same speed the engine RPM and load will be within a few percentage points...the rear end simply doesn't matter if you are within the tow limits.

Is the engine the only part of the vehicle that takes a strain? Yes, the more verity of ratios in trans is better for engine. Just pulling numbers off the donkey, I don't know what would be real on a RV. If it takes 1000ft/lbs of torque at the wheel to move the load, with 4:1 rearend, the driveshaft is only carrying 250ft/lbs plus friction in rearend. But if the ratio is 2:1 the driveshaft must carry 500ft/lbs plus more friction in the rearend. And that increase in force is also applied to frame, motor mounts and more.