metalgraver 12 Report post Posted May 14, 2015 As some of you know I am rebuilding a 69 Mach 1 that was pretty much a total rust bucket. ( See Rags to Riches ) So while I am going thru the car I am trying to make good choices for the reconstruction. The rear end that came with the car is a 9" 28 spline open differential with 3:00/1 gear ratio. Now I know that I could run this car just fine with this set up, but while I am rebuilding this car I was thinking that I should put a Posi-traction in it while I had the rear end out. I am going to have to take it apart and clean it up anyway. I am thinking about 17" tires. I have a 351W with a mild cam and Edlebrock RPM manifold with a T5 transmission. Not over 400 HP. It has been suggested that I put a Detroit Tru-Trac differential for the type of driving I will be doing, but Currie told me to go to the calculator to see what gear ratio I would want. Problem is I do not know enough to put the right RPM numbers in for the calculation. Here is the numbers they want to be able to calculate the gear ratio. What RPM should I be running at 70 MPH ? 1- RPM ? 2- Tire size- 17" 3- MPH- 70 1 Miltonlob reacted to this Quote Share this post Link to post Share on other sites
barnett468 418 Report post Posted May 14, 2015 . no, the calculator needs tire diameter and final gear ratio of the trans which is 1.0 if it is not an overdrive and the rear gear ratio you will use which will probably be 3.50. http://www.wallaceracing.com/gear-speed.php . Quote Share this post Link to post Share on other sites
shaun071 62 Report post Posted May 15, 2015 Not a Mustang, but gearing is gearing. My XR8 had 5.0, T5 5speed, 3.45 rear and 245/45/17 tires and ran 2100rpm @ 62mph so 70mph would be around 2350rpm. Quote Share this post Link to post Share on other sites
SA69mach 39 Report post Posted May 15, 2015 Good plan. I would recommend the tru-trac for sure. I have a variation of positraction rears and the tru-trac is quiet, strong, and works perfectly. Unless you simply must have a 60's era clutch style, get a tru-trac. Save some time calculating wheel sizes and go with your choice of 3.5 or 3.73. Either will work perfectly well with your car and a T5. With overdrive you will be running around 2000 revs in 5th gear at 70 mph. Give or take a hundred revs, but it will be a breeze at highway speeds, no problem. You can go to 4.11 if you really want to , but you will be changing gears a lot around town. Once you are moving it feels a lot like a close ratio gearbox. 3.50 is 'generally' regarded as ideal for automatic, and 3.73 is often considered the ideal ratio for 5 speed (or 4 speed) manual transmissions. Quote Share this post Link to post Share on other sites
metalgraver 12 Report post Posted May 18, 2015 Thank you. Quote Share this post Link to post Share on other sites
