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).

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:

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?
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?).

Solution 1:

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.

Solution 2:

If this were programming, parentheses would help to disambiguate:

"I am driving (more than 5 mph) under the posted speed limit."
"I am driving more than (5 mph under the posted speed limit)."

Thus the same sentence can be parsed such that it means both what you're intending, and the opposite. Same goes for the second phrasing, with the same grouping logic.

That being said, English is not programming. Your first statement ("more than 5 mph under") sounds more intuitively appropriate for the meaning that you're trying to convey.

You could also try to avoid "more than" or "less than" entirely, in favor of a disambiguating word choice:

"I am driving slower than 5 mph under the posted speed limit."

Solution 3:

You were driving more than 5 mph under the speed limit. To get there, you subtracted your speed from the limit, giving you 7 mph under. This is more than 5, so there you go.

Solution 4:

While one of the two could be considered correct (I would say the first one), there is as you observe a potential for misunderstanding. You can remove that by slightly permuting the first phrase:

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

This also has the advantage of first stating the main point (your speed was under the posted limit) and then quantifying that statement by an estimate of how much you remained below the limit, rather than starting with a quantification for a statement that has not yet been made.

Solution 5:

I don't agree with most of the answers. Firstly, there is a syntactic ambiguity, but everyone has shown the parentheses incorrectly.

Secondly, there are three interpretations, not two.

The syntactic ambiguity only makes a difference between the third interpretation hitherto not discussed, and the other two that have been discussed.

We can have two independent clauses which modify "driving":

( I am driving ) ( {more|less} than 5 miles per hour ) ( under the posted speed limit)

This makes two comments about driving: it says that I am driving at some speed that is more or less than 5mph, and that I'm driving under the posted speed limit.

In writing we would use a comma, which should translate to a difference in speech, such a pause or intonation, but these could be lost. Also, we are likely to use a coordinating conjunction: "I am driving more than 5 miles per hour, yet under the posted speed limit". One reason we reject this interpretation is that 5 mph is small. If the sentence were: "I am driving more than 30 mph, under the posted speed limit" this interpretation comes more strongly into play.

The other parse is like this:

( I am driving ) ( {more|less} than ( 5 miles per hour under the posted speed limit ) ) )

and the remaining ambiguity is purely semantic.

The phrase "5 miles per hour under the posted speed limit" has two semantics: it can denote an absolute speed related to the reference (the speed L - 5), or the difference relative to the reference (the difference 5). The focus is either the 5, or the absolute speed formed by subtracting 5.

Perhaps there is a subtle syntactic difference between the two interpretations, but the point is that the whole phrase functions as a unit so anything of that sort is internal to the phrase.

Then the comparative quantifier "more" or "less" applies to one of these two interpretations of the entire unit.

Under the absolute interpretation, "more" means greater than that absolute speed.

Under the relative interpretation, "more" means a greater relative displacement below the point of reference, and so a lower absolute speed.

This is why saying "faster than" or "slower than" instead of "more" or "less" resolves this semantic ambiguity. "Faster than" and "slower than" confirm that the interpretation must be absolute speed, because we never describe a smaller difference in speeds as being faster or slower. 40 mph is 10 below 50, and 39 mph is 11 below 50. We wouldn't call this 11 "faster" than the 10. (However the syntactic ambiguity remains: "I'm driving slower than 5 mph, below the speed limit".)