MPG Calculator: Shiny App Using Vehicle Weight as Predictor of Fuel Efficiency

MK
July 2, 2016

Background

  • Data from mtcars show that vehicle weight is the number 1 predictor of fuel efficiency.

  • Compared to all other potential predictors, the correlation between 'wt' and 'mpg' is the strongest at -0.86: 

        wt        cyl       disp         hp       carb       qsec 
-0.8676594 -0.8521620 -0.8475514 -0.7761684 -0.5509251  0.4186840 
      gear         am         vs       drat        mpg 
 0.4802848  0.5998324  0.6640389  0.6811719  1.0000000
  • A more detailed analysis is available on RPubs.
  • The Shiny App presented here uses a linear regression model to predict MPG based on vehicle weight.

Building a Linear Regression Model

Assuming a linear relationship between the dependent variable 'mpg' and the independent variable 'wt,' a simple regression function would allow plugging in weight in tons to calculate a predicted mpg for a given vehicle:

mpgPredict <- function(weight) {
        mtcarsWeight<-mtcars$wt
        mtcarsMPG<-mtcars$mpg
        lm1<-lm(mtcarsMPG ~ mtcarsWeight)
        coeffs = coefficients(lm1); coeffs
        estimateMPG = coeffs[1] + coeffs[2]*weight
        names(estimateMPG) <- "Miles Per Gallon"
        round(estimateMPG, digits = 1)
}

Building the Shiny App: ui.R

  • Basic page with the app title
  • Space for entering a vehicle's weight in tons between 1 and 6 tons at 0.1 increments
  • Space where the calculated mpg estimate is shown rounded to the nearest 0.1.
mpgPredict <- function(weight) {
        mtcarsWeight<-mtcars$wt
        mtcarsMPG<-mtcars$mpg
        lm1<-lm(mtcarsMPG ~ mtcarsWeight)
        coeffs = coefficients(lm1); coeffs
        estimateMPG = coeffs[1] + coeffs[2]*weight
        names(estimateMPG) <- "Miles Per Gallon"
        round(estimateMPG, digits = 1)
}

Building the Shiny App: server.r

The file server.r contains the function used to predict mpg based on weight and rendering instructions for the information on the web page: 

mpgPredict <- function(weight) {
        mtcarsWeight<-mtcars$wt
        mtcarsMPG<-mtcars$mpg
        lm1<-lm(mtcarsMPG ~ mtcarsWeight)
        coeffs = coefficients(lm1); coeffs
        estimateMPG = coeffs[1] + coeffs[2]*weight
        names(estimateMPG) <- "Miles Per Gallon"
        round(estimateMPG, digits = 1)
}
shinyServer(
        function(input, output) {
                output$inputValue <- renderPrint({input$weight})
                output$prediction <- renderPrint({mpgPredict(input$weight)})
        }
)