I've noticed that Jolteon's agility can make up for their lower HP. Rather than go toe-to-toe with a Snorlax, Vaporeon, Slobrow, etc. and make it a blow for blow bloodbath, Jolteons can compete because they are best at dodging. The Jolteon can "float like a butterfly and sting like a bee."
That said, to bring diversity to a gym defense which is often a high HP based on water defense, I like leaving Jolteons. But, there is such high turn-over at gyms, I can't tell (yet) if diversifying defense with Jolteons adds value. Maybe I should ignore diversity and just leave the highest HP guy I have.
The only way it makes sense to leave Jolteons for defense is if Niantic exploits their ability to dodge. Does anyone have experience or knowledge of this? I bet few have ever even had to face a Jolteon in a gym.
Best Answer
Defending pokemon don't dodge, unless I'm terribly mistaken.
Also, unfortunately for people who like quick pokemon, speed is a very poor stat in the game and renders pokemon like Jolteon almost useless. It doesn't affect the time it takes to dodge, so speed in Pokemon Go is close to being a wasted stat. If you've ever had to fight a Vaporeon with a Jolteon you'll see how terribly unbalanced it is. A Jolteon should destroy a Vaporeon, but Vaporeon has such high HP and atk/special atk that Jolteon is totally outclassed despite having a type advantage.
The way CP is calculated is based off of the base stats in the pokemon games...due to the formula used, Speed (from the pokemon games) barely increases the pokemon go damage/defense values compared to how much the Attack/Special Attack and Defense/Special Defense values do.
Notice how the two have the same relative formula with regards to the two normal/special stats and speed.
taken from: https://www.reddit.com/r/TheSilphRoad/comments/4t7r4d/exact_pokemon_cp_formula/
If you viewed this with varying numbers, you'll see that speed has a pretty insignificant effect on BaseAttack, and the best combination of numbers would be a high Attack and high Special Attack (as opposed to only one of those values being really high):
for example, if you had to distribute 300 stat points around, you can see how valuable each one is when calculating BaseAttack (this also holds exactly true for BaseDefense, since it's the same formula with Defense/SpeDefense and Speed):
All equal values:
BaseAttack = 220
--
One high stat (not speed):
BaseAttack = 214
--
One high stat (0 speed):
BaseAttack = 224
--
Equally high attack/special attack:
BaseAttack = 300 (!!!)
--
High speed:
BaseAttack = 128
pokemon with both high attack/special attack (or defense/special defense) are going to be significantly stronger than a pokemon with high speed.
In game values that are used to calculate how strong a pokemon is:
Optimal movesets for Vaporeon/Jolteon are:
Water gun/hydro pump and Thundershock/Thunder.
Type effectiveness / not very effectiveness is 1.25 multiplier.
Water gun on Vaporeon vs Jolteon will do 12 DPS, Thundershock on Jolteon vs Vaporeon will do 13 DPS. Vaporeon has twice the HP of Jolteon and Jolteon's speed barely adds to its defense. Thunder will do some extra damage (after type effectiveness it does about 1.5x the dmg to Vape as Hydro pump does to Jolteon) but it doesn't make up for the equal DPS in quick moves & HP disparity. It's no wonder Vaporeons beat Jolteons handily.
In conclusion, because of the formulas used in the game, fat, slow, pokemon with high HP, equally high Atk/SpecialAtk & Def/SpecialDef are the strongest in the game for battling. Speed is extremely weak regardless of whether or not you're attacking/defending. This is why pokemon like Vaporeon dominate and pokemon like Jolteon are terrible.
*Same type attack bonus. 1.25 multiplier because move type is same type as pokemon type.