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The Starting Line Ratio (SLR), also known as Launch Ratio, is very important to consider when selecting a transmission and/or a rear-end gear ratio.

Starting Line Ratio = 1st gear ratio * rear gear ratio

First Gear Ratio:

Rear End Gear Ratio:



Note: If the SLR number is real big, like >12,  then your launch will resemble a dump truck; lots of noise but doesn't go anywhere. The reverse is true if you have a ratio in the < 8  range, the car will likely go 100 mph in first gear but the clutch may have shortened life spa due to the amount of slippage needed to get the car rolling.
A 9-10-11 range SLR is desireable.

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However, the above calculation alone isn’t the whole story.  You must also consider your rear tire diameter.  For example, a 25” tire will result in a higher overall SLR compared to a 31” tire. Try out ‘distance travelled in 1st gear” calculator here.

Near future possible enhancements to this page:

  • a chart to show range of choices with pros/cons, and an indicator to show where on the chart the calculated value is at.  Colored with recommended range (green), red for areas not recommended.
  • add discussion about tire rollout also! 
  • Show # of feet travelled for first shift at given RPM
  • link back to calculator



13+  shift before leaving the garage!
12 - shift before the crosswalk
11 - shift comes quickly, but not un-usable
9-10  ideal
8 - pretty long in 1st, hard on clutch
<7  very hard on the clutch

talk about personal preference, tire spin, clutch slippage, etc.  estimated distance til first shift (on calc page)



Trans Gear Ratio:

Rear End Gear Ratio:

Tire Dia.

Engine RPMs to travel 100 feet

Scenario 1

Scenario 2

Scenario 3

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