What is the weight capacity of this rack both on highway and off road?

We do not believe in locking a moving load limit of the rack as there are too many variables. Any fixed figure would be kind of a vapor number. Here's how we roll: The Front Runner Slimline II Rack Tray is designed to carry heavy loads and is one of the strongest, if not THE strongest out there. Our engineers have dialed in a safe load rating at 660lbs (300kg) but it has been known to carry more. Have you seen us load 1540 lbs of water on a Front Runner Rack mounted to a Land Rover Discovery 4? Check out our YouTube channel. Note: this rack is so darn strong it can exceed the factory load rating of some vehicle's roofs. Please contact your vehicle manufacturer to check your vehicle’s specific load ratings. And always load your Front Runner Rack with evenly dispersed weight and lots of common sense!