Assume a GVWR of 19,500 lbs. At sea level in a Ford 6.2 with 3.73 diff. Does this mean the truck can perform highway speed in 6th, 5th or ? If a 3% hill climb is encountered is the truck expected by Ford to drop to 3rd, 2nd, or ? Or is the rating solely for safety in regard to suspension, brakes, drivetrain? Just wondering if the GVWR could be defined as driving 60 mph in 2nd gear even at sea level. The manual states to decrease the GVWR by 2% for every 1,000 ft of increase in elevation. So at 5,000 ft the GVWR drops to 17,550 ft. So is 60 mph on accomplished only by using 2nd gear for during a 3 mile climb? (Just as an theoretical example).

 

GVWR has nothing to do with how well the motor pulls the load. There are too many variables such as wind and frontal area of the towed vehicle.

 

J2807 seems to include engine performance in actual driving conditions. But I have some difficulty in relation to gearing. BTW GCWR is what I should have used in the post.

 

I think that since the specs refer to time, speed and grade plus the maximum weight any gear is acceptable! For the truck maker the highest GCWR is their goal so if 1st gear meets the test criteria then that is the gear you must use for that truck. Spec also states that all critical mechanicals must be met during the test.

 

I don't know if it's different your side of the Atlantic, but here in the UK the GCWR is determined by what the vehicle can pull from a standing start on a 10% (IIRC) incline, I think it may be the same there as you get a greater GCWR with numerically higher differential ratios on the same truck.

 

UK standard sounds simpler and it is. SAE test is a bit complicated. Not only does the tow vehicle ascend the incline 5 times it also must meet same criteria going backwards! Each time traveling a few yards within just 5 or 6 minutes.

 

At 6%, my 15,300# 40' fifth wheel trailer on my 10,000# F450 (9250#truck+hitch+5 people) takes all of my 440 up to hold 65 MPH. At the top of siding hill, it's close to 7%, so even in my F450 I can't hold the speed.

Here's the issue. Many hills are 3%-4%. At 65 MPH, they'll need around 300 HP (ish) to hold speed. My 6.7 F450 spins at 2000 RPMs in 6th at 65, where it makes 925 ft-lbs of torque. 925*2000/5252 is just over 350 HP, so I cruise up quietly without even downshifting.

Now the 6.2 needs to spin about 4800 RPMs to make 350 HP. We are only talking smaller 3%-4% hills. 6.7 = easy and quiet in 6th. 6.2 = loud and fast in 4th or often 3rd, downshifting again and again. I've towed gas and diesel - if you don't mind pulling the mountains slower and climbing many hills loud at high RPM, the 6.2 is fine. Running loud and spinning fast bugs me, so it's a diesel for me.

 

I run about the same going up Sliding Hill. In Colorado going up to the Eisenhower tunnel on I70 55 is about it. That is with 16K behind me.