View Full Version : TR2/3/3A Final drive ratio for TR 3A

11-20-2009, 04:53 PM
From what I have read the TR2 through 3A had a final drive ratio of 3.7:1 and the TR4 through 4A had either 3.7:1 or 4.1:1 short of counting the amount of teeth on the crown wheel and the pinion gear, how do I determine the final drive ratio for my car?
The diff was rebuilt but I don't remember that we replaced the pinion or the crown wheel. We just replaced the bearings and seals. Had to do some machining on the differential casing. Since we didn't replace the crown wheel or the pinion gear it should be the same ratio, Right?

Any thoughts would be appreciated.


11-20-2009, 05:16 PM
The ratio represents how many turns of the input shaft it takes to turn the output shaft one revolution.

In other words, how many revolutions of the driveshaft it takes to make the rear wheel go around once.

You most likely have the original 3.7 that went in most of the TR3s. BTW, you can't change just the pinion or the crown wheel, as they come as a matched set.

11-20-2009, 07:29 PM
Even the later TR2-3B were available with different ratios from the factory. The 4.1 was listed as an option; but others could be had by special order.

However, it appears that very few cars were so equipped by the factory. TS39781LO is the only one I've ever seen that had the 4.1 from the factory (as evidenced by the speedometer calibration); although it's more common as a later (even dealer installed) modification. TS13571L had 4.1 gears when it came to me, as did the first TR3A I ever saw (which my Dad now owns).

There are various ways to check; the easiest method depends a bit on circumstances. If the car is mobile, you can remove the plug in the driveshaft tunnel and make marks on both the U-joint under one edge of the hole and one wheel next to the ground (so the mark is vertical). Drive forward slowly while watching the mark on the U-joint until it makes exactly 4 (or 8) turns; then look at the wheel to see how far it turned. If the wheel went more than 1 (or 2) full revolution, you've got a 3.7 (or perhaps 3.45); if it went less than 1 full turn, you've got a 4.1 (or possibly 4.55).

Personally, I think the 4.1 gears are a waste on the street behind a stock engine, even with the (mandatory) overdrive. In first gear, the engine wastes a significant portion of it's torque just accelerating the crankshaft and flywheel. In fact, Steve Hedke reported that Scrappy's acceleration times actually improved on switching from 3.7 to 3.45 gears (which I also plan to do with TS13581L).

11-20-2009, 08:11 PM
Thanks Randall, as usual you cleared that up for me. I will check that tomorrow. I suspect that it is a 3.7:1 bt wanted to check.

Thanks, Tinkerman

11-20-2009, 08:17 PM
TS 74568 came stock with the 4:11 and a OD.
Enjoy it just as it sits.

11-20-2009, 08:49 PM
TS 69891LO Came Stock with the 4:11 as well along with competition springs.

M. Pied Lourd

Geo Hahn
11-20-2009, 08:58 PM
Remember that if you do the 'count the turns' method with the car immobile and one wheel on the ground, one off -- then the number of turns the moving wheel makes is double what it would be with the car on the ground.

At least I think that's true.

11-20-2009, 09:22 PM
At least I think that's true. It is. Unless of course you've got a limited slip or welded differential <<GRAEMLIN_URL>>/grin.gif

11-21-2009, 03:03 PM
Randall I checked it with wheel rotation and it comes out to 3.7 which is very good because the speedo I have is calibrated for a 3.7.

Another little chunk in place.