rpm to car speed data wanted.

[sujitroy]

I have a mk1 Stag with the mk1 tach, but with Pertronix points. The tach bounces a bit and I'm aware of the issue with electronic points. Also my speedo is off somewhat as well.
I'm looking for a chart of engine speed to car speed with and without J type OD engaged. Is there such a post? I just looked in the ROM and didn't see anything
I took the car for a drive at around 60 mph (sat nav. confirmed speed) but speedo says 70 mph with OD engaged, the car seemed to vibrate. Without OD at this speed it is much smoother.

I'm interested in engine speeds at around: 50, 60, 70 and 80 without OD and the same with OD. If anyone has this data, please share.
What I've observed with my bouncing tacho, is that the car is not happy with me engaging OD when the rpm sits around 2500-3k and I feel that I should only engage the OD when the RPM's are above 3k. I'm not planning to change anything on the car, but will use the data as a reference.

Sujit

[Phil S]

Cause and effect, tyre size / speed/sat nav reading
diff ratio/ engine speed/ rpm/sat nav speed

Better to sort the problems than try to work out what you are asking.
Just my opinion

[trunt]

It kinda does tell you in the ROM under general specifications.. gives road speed at 1000rpm for A,J and non OD. you can calculate from that.

At 1000 rpm with 185 tyres it says 19.8 mph that is 50.5 revs per mph The J type 25mph = 40 revs per mph.


But to be accurate Use a tyre calculator for rotations (turns) per mile, but factor in an additional 3% for the tyre distortion caused by the weight then multiply by the diff ratio.. divide by 60 and that’s revs per mph.

Multiply by speed and that’s the rpm for a particular speed no overdrive.

eg 185/80/14,

810(787+3%) x3.7/60= 49.95

50 mph - 49.95 x 50 = 2497rpm

multiply by 25% j type overdrive ratio
0.8 x 2497 1997rpm
(I think— better maths people check that!)


Also you say it’s not happy below 3000 rpm. Seems high, mine engages fine lower than that.. the OD pressure is rpm independent, it’s down to road speed as the pump is driven by the output shaft, so how is it in 3rd gear?

Terry

[stag8manod]

My man od j type, 3.45 diff with a 2.5 PI speedo drive gear,195/70 x 14 tyres reads 2450 rpm at 100k. Speedo and TomTom agree. The speedo and rev counter are smooth and steady. the od engages and disengages smoothly at any speed.
Cheers Ian A

[NeilR]

The overdrive, A type or J type, are not RPM dependant. The pump is driven of the gearbox mainshaft, so is speed dependant.

Here's what I used for my electronic speedo calcs.
Based on 185/65/15 - tyre height 23.44" , which is approx the same as 185/70/14
3.7:1 diff
4th gear with O/D not engaged.

MPH / RPM
100 5304
90 4773
80 4243
70 3713
60 3182
50 2652
40 2122
30 1591
20 1061
10 530
05 265

If you want with O/D engaged, either 22% for A Type, 25% or 28% for J Type.
X 0.78 for 22%
X 0.75 for 25%
X 0.72 for 28%

Mine is a 28% O/D, which will engage @ 15-20 MPH.

[sujitroy]

Thanks this will be a starting point as mentioned the the dances around. My tire are not the correct radius either. I will do some more experiments and report back. At lower RPM's that 3000 rpm my OD engages but the car vibrates more like I need to down shift to a lower gear. This if not mistaken behaves the same if I'm in 3rd OD or 4 OD.
At 70 my speedo reads close to 80. With smaller radius tires, is the speedo meant to read higher than lower? Have I got this wrong?

Sujit

[trunt]

That was my math dilemma.. is it 0.75 or 0.8 to convert a 25% ratio, is it a ratio of 4:5 or 5:4?

1000in gives 1250 out of the engaged OD ie 5/4 or 1.25. But I suspect we are looking the other way through the ratio when we calculate the new rpm with overdrive engaged at the same speed.

So now driving at 1250 out with no od, engaging it makes the revs go down to 1000 at the same speed. Now we have multiplied the rpm by 4/5 or 0.8

The 0.8 also jives better with the numbers in the ROM

Again.. I need a mathematician to check that, If it wasn’t so bloody hot I would go out for a drive and test that theory!