there are two kinds of parallel signals: ONE-PART (such as "and", "or", "but"), and TWO-PART (such as "not only ... but also", "both ... and").
when you have PARALLELISM WITH A ONEPART SIGNAL, the only words that are "locked in" are the ones directly FOLLOWING the signal. as long as you can find the corresponding structure in the other part, then the parallelism is fine.
examples: i worked in nevada and florida. i worked in nevada and in florida.
BOTH OF THESE ARE FINE.
reasons: in the first, the part that's "locked in" by the signal and is just florida. therefore, the parallel construction would be just nevada. since that construction is there, the sentence is parallel: i worked in nevada and florida.
in the second, the part that's "locked in" by the signal and is in florida. therefore, the parallel construction would be just in nevada. since that construction is there, the sentence is parallel: i worked in nevada and in florida.
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for completely analogous reasons, this sentence would be fine either with or without your second "that":
an increase that would amount to roughly five miles per gallon and would represent...
an increase that would amount to roughly five miles per gallon and that would represent...