In an Effort to keep the engine as stock as possible how much slower do 20"rims and a hard bed cover make the L? If there is a big diff then maybe i need to look into some engine mods.

 

The hard cover should be about 100 lbs. so rule of thumb of weight reduction being 100 lbs = .1 second, then 100 lbs. more would be one tenth more. Theoretically at least. 20s will hurt a little too, both from increased weight and less traction.

 

Won't the 20s make the gear ratio higher? In turn, slower low end.

 

Only if the overall diameter is increased. However, many opt for lower profile tires (40 series I believe) when they switch over to 20s, thus the overall change in wheel diameter is negligible as far as gearing.