grammaticality - Was I driving more than 5 mph under the speed limit, or less than 5 mph under the speed limit?

Suppose I am driving 38 miles per hour in a 45 zone. This, of course, is seven miles per hour under the speed limit. Of course, I am driving this slowly because the road is wet, and safe driving habits suggest that when the road is wet, one should consider the safe speed limit to be five miles per hour less than the posted limit.

For this situation, which is correct?

• "I am driving more than 5 mph under the posted speed limit"


• "I am driving less than 5 mph under the posted speed limit"

To support the first case: my speed discrepancy from the posted limit is more than 5 mph. TO support the second case: my total speed was less than 45 less 5 (equals 40).

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Edit: As many pointed out, there are certainly ways to assemble a phrase to remove any ambiguity. But that's not really the point of why I asked. My reason for asking is two-fold:

1. The scenario came to mind when I was driving after a recent snowfall. I was minding my speed, but thought about accident situations. I wondered what would happen if I got in an accident and told the officer, "officer, I was driving more than 5mph under the speed limit." Would that indicate careful or safe driving? Could that phraseology be used against me? Would the insurance company clamp down on that usage of a phrase to deny a claim?


2. More generally, how do comparisons work? There are (at least) two comparisons in either construction: "under the posted speed limit," which unambiguously means "slower than the legal limit," and "more/less than 5 mph." However, in this case, "5 mph" isn't a speed at all; it is a discrepancy. One cannot drive at a speed discrepancy; in fact, the only reason the quantity "5 mph" has units at all is because of the mathematical property that units be additively consistent (i.e. you cannot add 5 mph to 17 bananas). Restructuring the phrase to use a dimensionless quantity also removes ambiguity: "I was driving slower than 89% of the speed limit."

So my question could really be more abstractly framed as: when relatively comparing something to another comparison, does the "relational arrow" point in the direction of the inner-most comparison?

Other examples:

• It is less than 5 degrees below freezing (29 degrees, or 23 degrees?).


• I am more than 5 pounds below my target weight of 180 lbs (177 lbs, or 172 lbs?).

Answer

I think the other answers have missed a key factor in how this would be interpreted. And that's that it's a lot more common to be talking about your speed in relation to the national speed limit than to be talking about your speed in relation to 5mph less than the speed limit.

In other words it is unusual to say that your speed was "less than (national limit - 5)", whereas it's fairly common to quantify how much less than the national speed limit you were.

So I think that

I was driving more than 5 mph under the posted speed limit

is very clear that you mean that's how much you were under the limit by. In fact it took me a little while to find your other meaning at all.

Edit: David Richerby makes another excellent point: if you were indeed saying that your speed was "less than (national limit - 5)" you would say "I was driving at less than (national limit - 5)". Not "I was driving less than (national limit - 5)". Further evidence for the ambiguity being negligable in this instance. Do read his answer.