2001 Hyundai Elantra Mileage is way off.

We just bought this car. The third cylinder housing on the distributor was allowing spark to jump to engine block. The dealer filled the area between the third cyliinder distributor housing and the engine block with silicone and the missing issue stopped. But now the engine does not get even 20 miles to the gallon on the highway. The air filter is new. We are suspicious that the third cylinder has a jumping spark issue at high highway rpm's when it would be harder to detect it as you drove. There isn't much to these vehicles so it has to be something fairly simplistic. We know others who get high 20's city driving and high 30's to low 40's mpg highway with the Elentra. What are your thoughts?