(Updated to add googlesheets
methodology and new data points)
I’ve seen some posts online suggesting that the miles per gallon estimates that your car provides are not accurate. I collected a few observations to see if this was true for my car.
Methods
Each time I filled up the gas tank, I collected the number of gallons that were dispensed until the first automatic shutoff. I wrote down the estimated mpg, the trip odometer reading since the last fill-up, and the overall odometer reading. I calculated the mpg and mpg difference from the estimated mpg. I used a paired t test to see if there was a difference between the estimated and actual mpg.
library(dplyr)
##
## Attaching package: 'dplyr'
## The following objects are masked from 'package:stats':
##
## filter, lag
## The following objects are masked from 'package:base':
##
## intersect, setdiff, setequal, union
library(googlesheets)
suppressPackageStartupMessages(library(dplyr))
my_sheets <- gs_ls()
## Warning in strptime(x, fmt, tz = "GMT"): unknown timezone 'default/Pacific/
## Honolulu'
miles <- gs_title("Prius Gas Mileage")
## Sheet successfully identified: "Prius Gas Mileage"
x <- gs_read(miles)
## Accessing worksheet titled 'Sheet1'.
## Parsed with column specification:
## cols(
## date = col_date(format = ""),
## miles_total = col_integer(),
## miles_trip = col_double(),
## mpg_est = col_double(),
## price = col_double(),
## gallons = col_double(),
## mpg_actual = col_double(),
## ratio = col_double(),
## notes = col_character()
## )
Results
x <- x %>% mutate(mpg_calc = miles_trip/gallons) %>% mutate(mpg_diff = mpg_calc - mpg_est)
x
## # A tibble: 9 x 11
## date miles_total miles_trip mpg_est price gallons mpg_actual
## <date> <int> <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 2017-07-20 90208 358.1 48.7 19.54 6.860 52.20117
## 2 2017-08-19 90596 387.5 53.7 26.02 8.677 44.65829
## 3 2017-09-03 90984 383.5 55.9 22.10 7.730 49.61190
## 4 2017-09-28 91748 359.3 51.6 21.82 7.796 46.08774
## 5 2017-10-13 92216 467.0 54.6 26.08 9.029 51.72223
## 6 2017-10-26 92661 444.0 54.7 24.74 8.683 51.13440
## 7 2017-11-05 93072 410.9 54.5 24.42 7.879 52.15129
## 8 2017-12-01 93996 461.2 55.7 25.95 8.800 52.40909
## 9 2017-12-10 94442 446.3 53.0 28.15 8.940 49.92170
## # ... with 4 more variables: ratio <dbl>, notes <lgl>, mpg_calc <dbl>,
## # mpg_diff <dbl>
x %>% select(date, mpg_est, mpg_calc, mpg_diff)
## # A tibble: 9 x 4
## date mpg_est mpg_calc mpg_diff
## <date> <dbl> <dbl> <dbl>
## 1 2017-07-20 48.7 52.20117 3.501166
## 2 2017-08-19 53.7 44.65829 -9.041708
## 3 2017-09-03 55.9 49.61190 -6.288098
## 4 2017-09-28 51.6 46.08774 -5.512263
## 5 2017-10-13 54.6 51.72223 -2.877772
## 6 2017-10-26 54.7 51.13440 -3.565599
## 7 2017-11-05 54.5 52.15129 -2.348712
## 8 2017-12-01 55.7 52.40909 -3.290909
## 9 2017-12-10 53.0 49.92170 -3.078300
From looking at the data, the estimated mpg from the car’s dashboard was higher than the calculated mpg based on the odometer and gallons readout on almost all occasions. It’s not clear why that first one was different, but I included it since I didn’t have a good reason to throw it out.
The results of the paired t-test are as follows.
t.test(x$mpg_diff)
##
## One Sample t-test
##
## data: x$mpg_diff
## t = -3.1712, df = 8, p-value = 0.01317
## alternative hypothesis: true mean is not equal to 0
## 95 percent confidence interval:
## -6.2374623 -0.9852476
## sample estimates:
## mean of x
## -3.611355
x %>%
summarize(tot_gallons = sum(gallons), tot_miles_trip = sum(miles_trip), est_miles_trip = sum(gallons * mpg_est)) %>%
mutate(overall_mpg = tot_miles_trip/tot_gallons, overall_est_mpg = est_miles_trip/tot_gallons) %>%
mutate(percentage = overall_mpg/overall_est_mpg - 1)
## # A tibble: 1 x 6
## tot_gallons tot_miles_trip est_miles_trip overall_mpg overall_est_mpg
## <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 74.394 3717.8 3995.747 49.97446 53.7106
## # ... with 1 more variables: percentage <dbl>
Discussion
The additional data since the last time I looked revealed a significant difference between the estimated and calculated mpgs. On average there was a -3.6 mpg difference between the estimated and actual MPG figures. It’s a small difference. When the total miles and total gallons are added up, the difference is less than about 7%. It’s significant but maybe not that far off that I might be upset.