by A. Wcjtowicz

If you have considered changing the rear end gears in your Tiger for faster starts and more acceleration, you might be interested to know one of the other possibilities.

The standard ratio Tiger rear is a tall 2.88:1. With 13″ wheels this ratio gives a nice cruising speed with relatively low engine RPM. But there is a difference between the Mark I and Mark II transmissions.

The Mark II Tiger came with a wide ratio transmission (2.78 first gear) that, for most driving conditions is the preferred unit. The close ratio (2.32 first gear) transmission found in stock Mark I Tigers is better for racing where a smaller “drop” in RPM between gears allows better use of engine power.

As we all know, the 2.32 close ratio box (Mark 1) with a tall 2.88 rear gear gives us a low speed start. To improve get away acceleration for street driving with the 2.32 transmission a good compromise rear end ratio is 3.73:1. Engine RPM at highway speed is considerably higher but acceleration is much improved. Try driving around in 3rd gear all day, (yes all day). If you can tolerate the compromise you must make, then install a 3.73:1 rear end.

However, before you pull the rear and start changing gears, let’s look into it a little further. If the close ratio box is a 2.32 first gear and the rear end is a 2.88:1, the total gear multiplication in first gear, or the “break away” ratio is 2.32 x 2.88 which equals 6.68.

If you changed the rear end to a 3.73 the break away ratio would increase to 8.65. For comparison sake, let’s use 8.65 for our “ideal” compromise breakaway ratio. The wide ratio transmission offered in the Mark 11 Tiger has 2.78 first gear. 2.78 x 2.88 = 8.00. This is much closer to the ideal ratio than 6.68. By changing to the wide ratio transmission gears instead of changing the rear, you will have, in effect, the equivalent of a 3.73 rear (the same first gear acceleration) without the high RPM at cruising speed. You can continue to enjoy the “economy” of a 2.88 rear end ratio.

The spacing (%RPM drop) between gears in the wide ratio transmission is a little greater than the close ratio transmission, and the drop between third and fourth in the wide ratio transmission is 4% more than in the close ratio, but the 2.78 first gear is low enough to give great first gear starts.

In looking for an ideal street/strip Tiger, I started out with a 3.73 posi-traction unit plus the 2.32 close ratio transmission. The car took off really well, but in fourth gear I wanted to shift one more time. I changed the rear end ratio to 3.07, because it was the closest ratio I could get to a 2.88 and still retain the posi-unit. At the same time, I installed a wide-ratio transmission with the 2.78 first gear. 2.78 x 3.07 = 8.53 ? very close to what I had before (8.65). The RPM difference between a 2.88 rear and a 3.07 rear at a given speed is about 6% (almost negligible). I am well satisfied with the combination of the 3.07 posi-traction rear end with the wide ratio trans.

Comments (1)

Where can I find a posi unit for my MK1 and a 3.07 ring gear? Are the bolts that hold the 2.88 ring gear the same as the 3.07? Is the posi a 19 spline axle unit? Do you have change the bearings or flange ?
Thanks. Jeff

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