RPMs versus MPH Mystery

#1
Hello

In what might be my first post since the early days of the Obama presidency, I have managed to dust off the cobwebs but one mystery remains as it was then. I have a 2000TC "ornament" that I bought with 31k miles on it. Now, I am starting to rack up the miles as I am storming up on 32k miles - over 10 years.

I used to have a 2200TC back in the early nineties as my first car and as the speedo was like a trampoline artist on steroids, the figure of 19.5mph/1000rpm was burned into my brain. Fast forward 10 years and I get a 2000TC and immediately I think "too many revs". 4000rpm and I am at 60. I just think that maybe I am getting used to new cars and I forgot how noisy the old ones were and the tachometer was knackered. However, finally tested with a meter on the distributor now I have got the car on the move again and it says the same as the tach.

Does anyone have any ideas what might be at the root of this? No, I have not put mickey mouse mini wheels on it. Previous owner says it was unmolested. Hopefully my next post will not make me feel as much of an idiot for asking in case I have overlooked something very silly.

Thanks
Adrian
 

Demetris

Well-Known Member
#2
Did you check your speedo indicated speed against a GPS?
The chances are that your speedo is somewhat lazy from the lack of action through the years.
Definitely at 4000 rpm you are doing quite a bit more than 60 mph.
 
#4
Hi

The speedo is not a problem - the speeds are definitely accurate. The issue is the number of RPMs needed to get there and the harsh environment that creates in the occupant section
 

quattro

Well-Known Member
#5
I assume it's a manual, as it's a TC, so there's not many things to check.

The wheels are too small or the clutch is slipping. What size are your tyres?

Richard
 
#6
There's quite a big discrepancy there. At 4000 rpm you should be doing 78 mph, so the wheels would have to be a lot smaller and in any case the speedo would still show 78 as it would be calibrated for the correct size wheels. If the clutch were slipping it would be obvious by the revs rising under acceleration or when climbing hills and you would soon smell the burning.
 

corazon

Well-Known Member
#7
It sounds a lot like you haven’t got top gear whichever box it is. That or the diff ratio is incorrect.
What’s the rpm at idle?

Just another thought. Couldn’t binding brakes be causing this scenario, the engine having to work harder to overcome them/having to produce more revs for indicated road speed

Jim
 

sdibbers

Well-Known Member
#8
It sounds a lot like you haven’t got top gear whichever box it is. That or the diff ratio is incorrect.
What’s the rpm at idle?

Just another thought. Couldn’t binding brakes be causing this scenario, the engine having to work harder to overcome them/having to produce more revs for indicated road speed

Jim
Revs would remain the same vs MPH, its a direct gear relationship. It could cause more noise though for sure as the engine is under more load.
 

sdibbers

Well-Known Member
#10
the only way you'd see a difference between the indicated RPM and MPH would be if the gearbox had been modified. My bet is on the speedo cable hanging up from not being used (remove and clean/lubricate) or the tacho needs to be recalibrated. You can recalibrate th etacho by using the adjustment screw on the back while comparing engine speeds via a modern tachometer.
 
#11
All this time I just assumed I was over-reacting to the noise and the RPMs were plain wrong and the tachometer was way out of cal, even though my immediate thought when I drove it was something was wrong. The "revelation" on Sunday was my sticking a multimeter on the distributor and getting the same or higher RPM as indicated on the tachometer when free revving in the garage, implying that the revs are real and the speed is certainly real so something weird is happening. And if I want to go over 40mph I am revving like a bugger. I know the previous owner did have it apart on the front half but the final drive / rear half looks completely stock. Although I am not sure there are even any changes there that would create this much of a difference the "wrong" way in terms of mph per rpm. When the car is in it's usual ornament state this is less of a problem.

The attached pic from last night (sorry its a bit dark) shows I have got "normal" wheels on there and I have not forgotten about some lowrider ridiculous rims I had slapped on.........
 

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mrtask

Well-Known Member
#13
Very pretty car, very poor parking! ;) Sorry I can't contribute any practical advice regarding your issue, but I wish you luck sorting it out, and look forward to some better pictures of your yellow P6 at some point.
 

Demetris

Well-Known Member
#14
If everything is as you describe, then your clutch is slipping.
The top gear is a direct drive in the gearbox (so you cannot change it) and there was no option for a lower final drive.
Are you able to detect a slipping clutch?
 

PeterZRH

Well-Known Member
#15
A slipping clutch wouldn't be constant though would it? It would feel like an automatic under load, slipping more.

And that amount of slippage would be easy to detect via the nose!
 
#16
Not too helpful I'm afraid, but my '67 TC does the same as regards the tach/speed readings.
As I know the transmission is standard spec it's definitely due to a faulty tach, which I ignore.
I realise you've checked it and would suspect the clutch, but your nose would soon tell you that was at fault as 'suffolkpete' wrote.
The tach dial glass does make the perfect to mount the sat-nav sucker .
A tach is just wasted on me, I change gear when it sounds or feels right,. Mind you, how else would I know that my MX5 has a rev-limiter that cuts in at 7600?:rolleyes:
 
#17
Thanks for considering my situation. I will do some more checking at the weekend on all the various inputs to the equation so I have some exact numbers.

On the parking. Rule #1: Never park in a parking space as someone might come along and park next to you and that's not appropriate. I will post more pics if/when I am out and about before being deafened by the trips. OK, so its not that bad.
 

Demetris

Well-Known Member
#19
I was trying to say, that if the speedo and tacho readings are correct, and the wheels/tyres are correct, the only link that would allow for this discrepancy is the clutch.
However, as it was correctly pointed out, this is not really possible in practice.
Which takes us back to square 1. At least one of the inputs are wrong.
As far i remember from my TC when it had its original final drive, it wasn't that bad at 60 mph. Beyond that, yes, there was some considerable drone, until it smoothed out again towards 5000 rpm.
So my money is on the speedo reading low for some reason.
 
#20
Well, if there is an obvious answer then its probably that (note to self AGAIN). If it's not that - triple check the results that ruled out the obvious answer. Or not, if you are a muppet like me.
So, the tachometer was wrong. The first independent RPM measurement was also wrong. The nifty add on Tachometer I bought today (only 8 years too late) unsurprisingly was correct.
Adjustment screw located and now the RPMs are normal. Bonus repair: now the clock and tacho lights illuminate.

Psychologically, the car is much quieter now the RPM are so much lower :)

Attached the marvelous proper RPMs versus speed on the coast highway San Diego, and a pic during the run to burn off the fuel that went into the car in the Jurassic period.

I celebrated on the Department of Motor Vehicles web site by ordering a "ROVER TC" custom Cali license plate.


Eventually_SM.jpg

CoastSM.jpg
 
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