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Whether they are "significant" at 35mph depends on what we are talking about exactly.
Every time you double your speed you use 4 times the amount of energy per mile to overcome aerodynamic drag. (At the same time rolling resistance doesn't change much and engine efficiency usually goes up, at least to a point.)
If going 35mph your car uses 2hp to overcome air drag and you travel 35 miles then you'd have used 2hp-hours of energy, or about a 2/3 cup of gasoline if the engine were 100% efficient.
If you travel twice as fast at 70mph you'd need 16hp to overcome air drag but at the same time you'd get there twice as fast meaning you'd use 8hp-hours of energy, or about 2 2/3 cups of gasoline to travel the same distance.
If you drove half as fast at 17.5mph then you'd use approximately 0.5hp-hours, or about 4 teaspoons of gasoline to overcome air drag. You'd also take twice as long (2hrs) to drive those 35 miles.
Of course in those three examples you still need to add rolling resistance, energy loss due to acceleration and braking, as well as inefficiency losses, which is why driving twice as fast isn't actually going to quadruple the amount of fuel used (it only quadruples the amount of energy needed to overcome air drag over the same distance).
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