How far back do we have to go to get to cars that just showed percentages, cause I have literally never experienced one in my life.

Even my model year 2000 clunker has instant MPG, average MPG, miles to empty and a gas gauge with actual gallons.

My 2011 only has the gas gauge, without any gallon markers on it and numerous 2016/2017 cars I've been test driving haven't had it. It's certain classes of cars that have had it for quite a while, nowhere close to all.

My 2007 Toyota Matrix just has an analog fuel gauge that goes from Full to Empty with marks for each quarter. No gallons, no MPG calculations.

My 2003 VW Golf has a gas gauge that shows percentages - the individual markings represent 12.5%, not individual remaining gallons (it's a 14.5 gallon tank, not an 8 gallon one) My mother's 2009 Jetta has similar. Whether or not the instrument cluster implements a multi-function display that shows instant/average MPG or miles to empty, every car I have ever driven, from the 80s clunkers of my youth to brand shiny new rentals has had such a fuel gauge.

Such a style is cheaper to implement. Resistive float sensor in the tank, run a wire to a mini 90 degree needle gauge in the instrument cluster, done. Instant/Average MPG and miles to empty are additional fanciness that up until recently, came standard only on higher-end cars, or was part of an optional package that may or may not be avail on lower end brands/models.

I thought that the gallons calculation used the same cheap mechanism, just adding a trivial percentage left * capacity calculation.

My 1999 Camry has a dial that shows full, 3/4, 1/2, 1/4 and a red line.

2009 Civic LX - gas gauge with 20 squares, no mpg at all, no actual gallon number.

The only car I've ever had that showed MPGs was a 1990 Audi.

Everything else (1998 Toyota Camry, 2002 Chevy Impala, 1997 Ford Ranger, 2005 VW GTI) have only had an analog fuel gauge.

Not far. My last car was a Toyota Yaris 2001, and the display (center top) only has percentage, and it's not very fine-grained: http://i.imgur.com/ESXap6F.jpg

2005 Ford Focus, just a gauge. It has percentage fuel on the OBDII interface but not MPG estimates. Some cars might have it on the OBDII if they don't have a gauge.

You have to go back pretty far. In the wake of the 1973 Oil Crisis, most new cars had MPG displays.

The 2012 Mazda 6 doesn't show MPG, but it does calculate it and expose it over ODB II