Measure the same reference points
Read the accessory diagram and identify whether it measures bar center-to-center, usable clamp area, or another datum. Measure the actual bar setup, including bar shape and obstructions, before tightening anything. Check the required bar spread range, front/rear direction, overhang, and torque instruction.
Do not convert “fits this spread” into a load promise. The interface can fit while the vehicle or accessory rating remains the limiting factor.
Limits
No bars or accessory were mounted. Exact spread, torque, clamp location, and load remain product-specific.
Sources and related reading
See Also
If you want to move from general advice into actual product choices, start with Roof Rack for Flush Rails: Measure the Mounting Interface First, Loose Roof Rack Noise: A Torque, Bar, and Fairing Diagnostic, and Roof Rack Strap and Tie-Down Basics: What to Know Before You Hit the Road.
For a wider picture after the basics, Soft vs Hard Tonneau Cover Water Resistance: Which Offers Better Protection from the Rain? and Extang Trifecta 2.0 Tonneau Cover Review: Fit, Features, and Trade-Offs are the next places to read.