Which sense of the term "mileage" is intended in the phrase "your mileage may vary"?

This is mainly an answer to a different question — ”When did the phrase switch from a literal to a figurative meaning?” — but by documenting it I do answer the question, so pause a second before downvoting me.

A Google Books ngram search for “mileage may vary” brings up a bimodal graph:

The first portion, starting about 1970, peaking about 1980, and bottoming out in 1990 almost completely consists of literal usage in terms of miles per gallon, e.g.:

“Your mileage may vary due to how and where you drive, truck's condition, and optional equipment.” Popular Science, June 1978

In the second, later, peak one finds the term used mainly in the figurative sense mentioned by the poster. The first linked page for the period 1998-2011 includes books on

bed bugs:

“And in dealing with animals, the phrase your mileage may vary must be considered”

childcare:

“As with anything else kid-related, your mileage may vary!”

web programming:

“But your mileage may vary. If you view a Web page…”

and real estate marketing:

“Your mileage may vary, but not by much. If you rely on yourself to send out those letters…”

Shows how useful Google Books ngram can be. But then again, your mileage…

Your mileage may vary fits your first definition, "Mileage as a measure of fuel efficiency."

In the US, the EPA has a procedure for how to estimate a car's mileage in the city and on the highway. These tests were run under laboratory conditions on a machine called a dynamometer.

Source

In the US, new cars are sold with the laboratory results on a sticker.

But the individual who purchases said car will drive in cities and on highways that are different than the laboratory conditions. S/he may also accellerate more quickly than the EPA lab procedure or run the air conditioner. This will change the individual's mileage compared to the lab results. Thus "Your mileage may vary" is a common warning that was voiced over in 1970s new car advertisements and is in the fine print today, and it refers to the fuel efficiency.

(It could not refer to the miles traveled over a lifetime, as it is employed for new cars, which are by definition low in mileage.)

Here is an image from a 2020 Nissan Maxima sales brochure.

The fine print (about two-thirds of the way through) says:

2020 EPA Fuel Economy Estimates 20/30/24 (City/Highway/Combined). Actual mileage may vary with driving conditions.

As it turns out, at least according to Oxford, mileage has another (informal) definition.

mileage, noun

Actual or potential benefit from something.

I would say that this is the most accurate definition for this usage.