Picking The Right Transmission
When you are re-building a muscle car, the transmission choice should suit your use. The first question I ask customers is, “What is your intention for this car?”
Drag racing requires a close ratio transmission, but you’ll guzzle gas and have trouble driving on the street.
M21 close ratio (2.20 1st gear) transmissions were mostly used for street or drag racing applications, and compatible with 4.11 and 4.56 axle gear ratios. However, most of these were designed when gasoline was cheap (I’m so old, I remember gas wars getting down to 12 cents a gallon!)
Many folks are choosing the M20 transmission (2.52 1st gear) which are more compatible with 3.73 and lower numeric axle gear ratios. The wide ratio will be easier to drive on the street and be more economical to drive.
Auto gear has a wide ratio kit available, and they have an article explaining your choices as well: click here to read the article.