Used to sell one that was accurate within 1%. They're made by an old funny car racer friend of mine. Bench-tested every one. He also sells 'em to Summit, Jeg's, maybe Speedway, which they re-brand as theirs. You can PM me if you like for his phone #.
The Proform tester I bought was JUNK. The one I bought from topside some years ago is consistent and pretty accurate, based on what my calibration spring and other "known quantity" springs read.
