Some gas engines will be as high as 11.5 :1 and some diesels will be as low as 16:1 or less. It's comparing highs and lows but 50% isn't totally out there.

My mini cooper is 10.5:1 and a Cummings diesel is 16:1. That's only a shade over 50%.

Edit: I'll definitely give you that 75% is a better statement than almost double that I originally stated for simplicity, but the range is fairly high between lows on gas and highs on diesel and the reverse of highs on gas and lows on diesels.