Interpolate between data - NOT WORKING?

Hi, I have lists with data for 3 different vehicles' speed vs grade %. I'm trying to interpolate for a given grade, the speed the vehicle will travel! I get values for the first 2 vehicles (although not very accurate), but for the 3rd one I keep getting errors. I have no idea why. Attached is an extract from my worksheet. Any help or alternative method will be much appreciated. Thanks!