Optimal transmission type for fuel-efficient cars

In this report, we're going to respond on two questions, which will allow to select proper transmission type for a most efficient car, in terms of miles-per-gallon:

1. Is an automatic or manual transmission better for MPG?
2. Quantify the MPG difference between automatic and manual transmissions

The latest regression model from the section below - has all important variables as confounders - and shown us almost 2 miles more per gallon in case of automatic transmission. So, cars with auto transmission more efficient in MPG aspect, than manual transmission cars - considering equal weight & horsepower of the car (all other properties highly correlated with these two, or transmission type itself).

mpg <- mtcars$mpg
trans_type <- factor(mtcars$am)
weight <- mtcars$wt
horsepower <- mtcars$hp

regression_model2 <- lm(mpg ~ trans_type + weight + horsepower - 1)

Call:
lm(formula = mpg ~ trans_type + weight + horsepower - 1)

Residuals:
   Min     1Q Median     3Q    Max 
-3.422 -1.792 -0.379  1.225  5.532 

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
trans_type0 34.00288    2.64266   12.87  2.8e-13 ***
trans_type1 36.08659    1.73634   20.78  < 2e-16 ***
weight      -2.87858    0.90497   -3.18  0.00357 ** 
horsepower  -0.03748    0.00961   -3.90  0.00055 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 2.54 on 28 degrees of freedom
Multiple R-squared:  0.987, Adjusted R-squared:  0.985 
F-statistic:  538 on 4 and 28 DF,  p-value: <2e-16