Automatic transmission slip

[morrisman]

I've just changed the rear axle gears on my Morris, from 4.9:1 to 3.9:1.
Using the calculator here (www.bgsoflex.com/auto.html )I worked out I should now be getting 58mph at 2700rpm. But, I'm only getting 50mph. My speedo is electronic and set by hand held GPS, so that is dead on. The tacho is electronic too.
Is it possible that the transmission is slipping to the tune of 8mph? I know they are not 100% efficient, but surely mine doesn't lose more than 10% ?

www.bgsoflex.com/auto.html

My baby:

[eliot]

Ive got a lockable torque convertor on autobox, when locked it drops the revs down by nearly 500 rpm - almost like another gear.

[morrisman]

eliot said:

Ive got a lockable torque convertor on autobox, when locked it drops the revs down by nearly 500 rpm - almost like another gear.

500 rpm? That sounds like the amount mine is 'out' by.

I just went and did a 'stall' test on it, where you stand on the brakes, handbrake on, in 'Drive', then floor the throttle for a couple of seconds and see what the maximum revs go to. I've been told it should be about 1800 rpm max on the Rover V8 autobox, but mine went to 2500 immediatly Looks like bad news.

[F.M]

Fill up the Gearbox and Axle with the correct type and volume...It will affect your readings if they are needing attention...

[GreenV8S]

If you go to neutral throttle you should have zero slip, so just see how much the revs vary between gentle acceleration/engine braking and that will tell you how much slip is occuring.

[morrisman]

GreenV8S said:

If you go to neutral throttle you should have zero slip, so just see how much the revs vary between gentle acceleration/engine braking and that will tell you how much slip is occuring.

I had a look at that but I'll try it tomorrow a bit more accurately. And also try gentle deceleration down through 50, which should produce zero slip, I think. If it still reads out it means I have either a bad tacho, bad speed reading, or wrong calculation somewhere.

I'm correct in running the tacho off the negative pole of the coil?

You're the same GreenV8S off the V8 owners forum?

[GreenV8S]

morrisman said:

You're the same GreenV8S off the V8 owners forum?

Yes, unless the other one is spouting b*llocks and then it's somebody else.

[love machine]

That's a rude vehicle. Noticing your Firestone indestructables which I used on my moggies, it must understeer like hell!!!

[eliot]

GreenV8S said:

If you go to neutral throttle you should have zero slip, so just see how much the revs vary between gentle acceleration/engine braking and that will tell you how much slip is occuring.

Yes - that's a good test, under very light throttle you hardly notice the lock-up engaging on mine. Also check the fluid level.

[morrisman]

eliot said:

GreenV8S said: If you go to neutral throttle you should have zero slip, so just see how much the revs vary between gentle acceleration/engine braking and that will tell you how much slip is occuring. Yes - that's a good test, under very light throttle you hardly notice the lock-up engaging on mine. Also check the fluid level.

I did some tests last night. Between just starting to accelerate at 50 and going to light throttle, just starting to lose speed, I get about 250-300rpm change. Apparently that is somewhere near normal.

I also floored it when doing a steady 40mph, and it kicked down and the front wheels damn near came off the ground. It soon got up to 80mph and the Firestones started losing the plot and wandering around the road a little too much for comfort. I think the steering box needs nipping up a little to lose the touch of backlash in it.

You don't realise just how crap the roads are in England until you drive a car with dubious suspension