1000 kg is always going to weigh 1000kg.
As for what I think your question is...
When pulling 1000kg, you are really pulling very little weight (force for the purist) (you could push it by hand). If you were to tow up a vertical slope you'd be pulling 1000kg.
Calculating an approximate "pulling" component of 1000kg on a given angle would be what that "SIN" button on your calculator does.
20 degree slope (SIN 20) .34 x 1000 = 340 kg plus normal rolling resistance
Want to see how wimpy your car really is? Watch this....
Or perhaps how strong duct tape is!
The tow rating on a vehicle takes all of this into consideration, with a rope over a cliff holding 1000kg your car could not actually "tow/pull 1000kg"... It can however pull something that weighs 1000kg on roads of reasonable grade.