My recent drive to Waycross has prompted me to consider an overdrive transmission.  Several club members have them and they enjoy the improved gas mileage.  I decided to calculate how many miles I would need to drive before I would actually save any money. 

 

Here is the formula for calculating the miles.

 

Miles = (Price of transmission change)(new gas mileage)(old gas mileage)/( (new gas mileage - old gas mileage)(price for 1 gallon of gas)

 

In my case I determined or estimated the following:

 

                  Price of transmission change = $1,500 (for parts and labor)

                  new gas mileage = 13.7 mpg (30% overdrive 700R4)

                  old gas mileage = 9.5 mpg (T350 with no overdrive)

                  Price of 1 gallon = $2.45 (04/17/2005 in Richmond Hill)

 

 

 

Miles = ($1,500)(13.7mpg)(9.5mpg)/(13.7mpg-9.5mpg)($2.45)

 

Miles = 18,972

 

old mpg	9.5
new mpg	13.57143
old price	2.4
new price	2.99
trans cost	1500
miles +	19791.67

 

 

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What does this mean?  Assuming that I drive only in overdrive and the 30% overdrive is 100% efficient, then I will need to drive 13,778 miles before the cost of a replacement transmission equals the savings in gasoline.

 

If I drive the car 4,000 miles each year, then in 3.4 years, I will start saving money with the overdrive transmission.

 

If I replace the transmission myself for $1,000, then it would only take five years to reach return on investment.

 

If I pay the $1,100 for someone to install the transmission and the gas price rises to $3.00 per gallon, it will take 11,762 miles or three years to get my money back.

 

If I can replace the transmission for only a $1,000 and gas prices double, then the time to return on investment drops to two and a half years.

 

Conclusion:

 

  Swapping a regular transmission for an overdrive transmission on a car that is not driven many miles per year is only worth the expense if you can do the labor yourself and if gas prices continue to rise.

 

  All of these cases assume an unrealistic situation where you drive only on the highway in overdrive.  While cruising down River Street, around Pooler and Savannah, or idling in traffic, you will not always be using overdrive.  You will not see a 30% increase in gas mileage with a 30% overdrive transmission.

 

  If you are building a car and don't already have a transmission, then you should buy an overdrive transmission.  The little extra that it will cost will be paid back in gas savings.

 

 

 

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Here are the figures for a car getting 18 mpg on $2.00/ gallon gas

 

 

 

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