I know that if you use a longer rod you can get a longer cast, but does anyone know the relation to this? I know there are tons of variables, like reel model, size and correct lubrication, wind, weight of lure, etc. I am just wondering like for each inch of fishing rod length you may get an extra 2 feet of casting distance or something like that. Is there a big difference from say a 6'9, 7', 7'6 rod to a longer cast if all variables are equal? I hope you can understand my query. Thank You for any and all input.

I prefer accuracy over distance and 7’ rods  is my preferred length for most presentations. I do throw a toad on a 7’4 rod, but it has a really soft tip that makes it load real easy and is pretty accurate for the length/lure weight.

Way too many variables for any kind of meaningful equation. Go with your own gut, experience and preference. 

Rod length verses casting skill and lure weight verses lure aerodynamics all make a difference.

My average casting distance with 6'10" jig rod using 10 to 12 lb mono or FC with 1/2 oz jig is 120' /40 Yds under fishing conditions. Occasion make a 150' / 50 yd cast when the situation requires it. I know casting distance from practicing for 50 years.

Would a 7'4" jig rod with everything else equal increase my casting distance, yes slightly maybe 10'. 

My average cast using a 8' Swimbait rod with 4 oz swimbaits is over 180' /60 yds, but about the same length using my 7'10" Swimbait rod, no noticeable difference.

My average cast using my 7' crank bait rod is about 120' /40 yds, sometime a little further but I can't cast the same crank baits any further using a 7'5" crankbait rod, the lure doesn't have the aerodynamics to travel much further.

Your skill limits how far any rod can cast.

Tom

PS, I haven't fished with a partner cast as far as I do when fishing together, 100' cast is a long cast for most bass anglers.

for me being shore bound, (no boat) I know I love the longer rods. my main casting rod is 7'6 and my main spinning rod is 7'5.