So what really is the 5th gear ratio on the BA 10?

I have a BA 10/5 speed tranny (1988) and am going to swap an AW4 in soon. I already have some 4.88 gears, and I'm concerned about getting my OD RPMs right, meaning I need to decide on getting an AW4 with .70 od or .75 od.

Eagle said very specifically that the OD 5th on the BA 10 is .72, and I tend to believe him. BUT, the other day I put the speedo at 65 in 4th and it was 2600 rpm, then I shifted to 5th and matched 65mph again and it was 2050. According to my calculations that is a .79 ratio, which is what I thought it was before, and is what the AX15 is.

If it is .72 then I can go ahead and get a .70 AW4 (cheaper) and my RPMs will be fine.

If it is in fact .79 then I need to find a .75 AW4 to keep my RPMs in the right spot.




arrrghghhh

thanks guys
brett

Quote from the FSM B.V. BA 10/5, publication # 8980 010 302:
The Model BA 10/5 is a 5-speed, synchromesh manual transmission. Fifth gear is an 0.72:1 ratio overdrive range. All forward gears feature synchronizer controlled engagement.

Click to expand...

That's all I know. I have heard rumors that the first BA 10/5s, in the '87s, had a different overdrive ratio. In fact, my '87 MJ "feels" different from my '88 XJ, but I have never gone through the exercise of rigorously comparing speeds vs RPMs for both to confirm the OD ratio.

Frankly, if you run the math for a .72 OD and a .79 OD, I think you'll find that the difference is so slight that it shouldn't affect your axle gear selection.

Eagle said:

Frankly, if you run the math for a .72 OD and a .79 OD, I think you'll find that the difference is so slight that it shouldn't affect your axle gear selection.

Click to expand...

okay I just ran the math (I have a spreadsheet I made) and at 70 mph the difference is 2590 vs. 2360. That's 230 rpm and I consider that a big difference. It's nearly the exact same difference between 4.10s and 4.56s.

My RPMs feel just a little on the low side now with 4.10, BA10, and 33s. I need to make sure that with 4.88, AW4, and 35s the rpms are higher, which they won't be if my BA10 really is .79 and I get a .70 AW4.

I hear you. I think 4.10s are better suited to 31" tires, maybe 32s. Not enough gear for 33s.

However, I don't see it as an issue so much for highway as off-road, and you're aiming your selection more based on highway gearing. A bone stock 2000 automatic turns 2200 RPM at 65 MPH. You might use that as a benchmark.

My own personal benchmark, being the olde pharte that I am, goes back to my pre-overdrive days with earlier incarnations of this engine. In the "good old days" they were generally geared 24 MPH per 1000 RPM, which meant that 60 MPH was an even 2500 RPM.

I'd say you'll likely be happier if you get an auto with the 0.75 OD ratio.

Eagle said:

I'd say you'll likely be happier if you get an auto with the 0.75 OD ratio.

Click to expand...

I know I will be!

but I'm hoping I would be at least pretty happy with a .70 cause the thing is I can easily pick up a .70 OD AW4 for free, and no modifications to my t-case. I have not been able to find a .75 OD AW4 for less than a couple hundred bucks and would require changing the input gear on my t-case, or maybe a D300 while I'm at it

Here are the ratios I have for the Peugeot:

1st - 3.39:1,
2nd - 2.33:1,
3rd - 1.4:1,
4th - 1.00:1,
5th - 0.79:1,
Rev - 3.39:1


When I regeared, and calculated speed versus RPM in the different gears, this seemed to be correct. I got the info from an old magazine that I think is no longer published - "4x4 Power").