So lets pretend that our cars depending of the level of tune , 3 speed transmission and medium gears can still "probably" achieve 11 to 13 MPG... if you dont, well im sorry for you...
There is a fun little fuel cost calculator on the EPA site, you can enter most fairly modern car and compare two models.
Just for fun I've compared :
The behemoth-esque 1987 Squarebody Suburban 4x4 350 SBC, EPA Combined: 11 MPG
(city 11 and hwy 13)
***Runs on regular gas
I think its a brave equivalent of our vintage iron 60 years old tech no matter how fast or slow it is...
VS
The mighty 2016 Camaro with 6.2 V8, 8 speed automatic, EPA Combined: 20 MPG
( city 17 and hwy 28 MPG)
***Must run premium gas
The cost difference between the two is 10 cents per mile driven.
So if your drive 10 000 miles a year you basically save 1000$ per year going from your old crappy cast iron gas guzzler to a brand new LSX, or in simple you save a grand for every 10 000 miles you drive...
Peter.