Math Fun with Cars

[lalessi1]

I thought it might benefit a few to arbitrarily throw out a few things out here.

How can you determine your car's tire diameter or height?

    Based on tire size it is easy to figure out....... take a 235/70 x15 tire for example
                                            235 is the width of the tire in mm
                                            235/25.4  converts mm to inches, this give you a width of 9.25 inches
                                            9.25 x .70 is the height of the sidewall or 6.475 inches
                                            Double the sidewall and add the wheel diameter (15) and you have the diameter of the tire
                                            6.475 x 2 + 15 = 27.95"


                                            A 235/60 x 15 tire calculates to a little more than 26.1" in diameter

How does tire diameter affect my car? 

     A 27.95" diameter tire will travel 27.95 x 3.1416 (pi) inches in one full turn or revolution. That is 87.81" or 7.312 feet                               
     per revolution.
   
     A 26.1" diameter tire by the same logic travels 6.833 feet per revolution.

     To travel a mile or 5280 ft. the 27.95" tire must turn 5280/7.312 or 722.1 revolutions per mile

     The 26.1" tire must rotate  772.7 revolutions per mile.

     60 mph is the same thing as 1 mile per minute. This means the 27.95" tire is turning 722.1 rpm at 60 mph and the 26.1"
     diameter tire is turning 772.7 rpm at 60 mph.

     Engine rpm at 60 mph can be determined by simply multiplying the tire rpm by the rear end ratio. For a 3.89 rear end
     the engine rpm at 60 for the 27.95" tire is 722.1 x 3.89 or 2808 rpm, for the 26.1" tire engine rpm at 60 mph is 3006
     rpm. In this example changing you car's tire diameter from 27.95" to 26.1" raises engine rpm the same amount as
     changing the rear end ratio from 3.89:1 to 4.16!!!!!

     When the speedometer reads 60 mph, your car is actually going 56 mph.

     Notice that we only went from 70 series to 60 series tires with all else equal.

CONCLUSION: Tire size plays a very big role in determining the overall gearing and thus the performance of a car.


                     
     
                                       

[57AGIN]

lalessi1:

Amen to that bro.  It just doesn't explain why a "tall" gear has a low numerical rating and a "low" gear ratio has a larger numerical rating. lol

Bob
57 AGIN

[lalessi1]

lalessi1:

Amen to that bro.  It just doesn't explain why a "tall" gear has a low numerical rating and a "low" gear ratio has a larger numerical rating. lol

Bob
57 AGIN

I thought long and hard about this.... If a low (numerically) gear was a "low" gear in car speak and it made a car go faster on the "top" end, we would be discussing why does a low gear increase top end?