Auto transmission slip

[PierreC]

I noticed a strange situation after fitting new tyres this morning. When new, my 2002 XC did 2900 rpm on the rev counter at an indicated 140 km/h. Now, at 110.000 kms and with the new tyres, the indicated revolutions were just shy of 3000 rpm at the same speed.

Is there a logical explanation? I have recently flushed the transmission fluid so possible degradation of the ATF can be ruled out.

[JRL]

I noticed a strange situation after fitting new tyres this morning. When new, my 2002 XC did 2900 rpm on the rev counter at an indicated 140 km/h. Now, at 110.000 kms and with the new tyres, the indicated revolutions were just shy of 3000 rpm at the same speed.

Is there a logical explanation? I have recently flushed the transmission fluid so possible degradation of the ATF can be ruled out.

Look at your tach while driving.
1st thing is to count the shifts, make sure it's going into all gears including overdrive, four shifts from start

If it is, then while in 5th SLIGHTLY lay your foot on the gas, not enough to drop it into 4th but enough for the lock up to engage
You should see higher RPMs, now let off the gas completely, it should drop back down 5-600 rpms, that's into lock up.
If it doesn't change, you have a problem

[PierreC]

Look at your tach while driving.
1st thing is to count the shifts, make sure it's going into all gears including overdrive, four shifts from start

If it is, then while in 5th SLIGHTLY lay your foot on the gas, not enough to drop it into 4th but enough for the lock up to engage
You should see higher RPMs, now let off the gas completely, it should drop back down 5-600 rpms, that's into lock up.
If it doesn't change, you have a problem

JRL

It does all this perfectly ... correct number of shifts (quite smoothly now, after my recent tranny flush) and the lock up is perfect, with revs dropping as you say etc. [happy] The lock-up accurs when I am in full auto mode or in the "manual" (Geartronibc) mode. It even engages the lock in 4th if I use the Geartronic to hold it in the lower gear (at a lower speed, naturally) ...

Its just that I noticed [and you only get the opportunity to note this when you fit new tyres and then only within the first few thousand miles!] that the revs are now about 100 rpm higher at constant INDICATED speed of 140 km/h than it was when the car was new (with and without speed contol, and when in full AT mode or in "manual" Geartronic mode). I know, because I drove long stretches through semi-desert with speed control set at exactly 140 km/h indicated, and I recall mentioning to a number of people that the engine was "ticking over at diesel engine revs" of 2900 rpm at that speed. Now, it is 3000 rpm. :confused:

Maybe the problem is in the speedometer ... or the rev counter? I just wonder if anybody else noticed this "phenomenon" ... and whether there is an explanation for it:eek:

[JRL]

JRL



Maybe the problem is in the speedometer ... or the rev counter? I just wonder if anybody else noticed this "phenomenon" ... and whether there is an explanation for it:eek:
Well, you said it yourself.
You just installed new tires, they have a larger diameter than the ones you removed so the answer may be just that.
100 rpm's is not a lot here and basically immeasurable.
Just wear and tear and mileage on the wquipment could be the reason. I really wouldn't be concerned, go out, have fun

[budrichard]

Tire size is directly linked to revolutions and speed. Your new tires are undoutably smaller causing the wheels to revolve faster to obtain the same speed as before with the old larger tires.
What were the tires in size?
When something like this is encountered always ask yourself "what was the last thing that was done and what was the change?, invariably that is the cause.
By was of training, I am a Root Cause Expert and Probablistic Risk Assessment Expert for Nuclear Power. In this case this is what I call a 'no-brainer'.-Dick

[PierreC]

budrichard:

I am not talking about the revolutions just before I swapped the tyres - I am talking about the RPM with the same size tyres (standard size, 215 65 16), when both sets were (are) new.

The original tyres were the factory Pirelli STR - the new set is Michelin Latitude. I don't believe that there is a meaningful difference in rolling circumference between the Pirellis and the Michelins that can cause this difference in revs. In both instances, the tyres were (are) inflated to the same pressure (2.2 bar).

I am of the opinion that even if I fit NEW Pirelli STR's, the revs will still be higher (3000 vs. 2900 when the car/transmission was new) at the same speed of 140 km/h indicated.

Unfortunately, there is about 110.000 km that has lapsed since my first "observation" and my current observation, and many things could have changed.

[JRL]

budrichard:

I am not talking about the revolutions just before I swapped the tyres - I am talking about the RPM with the same size tyres (standard size, 215 65 16), when both sets were (are) new.


.
Tires of the same size but DIFFERENT brands are all over the map size wise.
They can have a 1" wider or narrower tread section and most definitely be slightly taller or shorter which can easily make a 100 rpm difference

[bwik]

Even if the tires were twice as large, and you were really going 280 km/h now, your RPM should still be 2900 and your speedometer should still read 140 km/h.

That is, unless the Volvo has an optical speed measurement device, which I doubt. So, tire size would not affect the relationship between RPM and indicated speed one bit.

RPM directly causes wheel rotation speed, which gives you indicated speed (which may or may not be accurate). At all times, indicated speed and RPM will share the same relationship by virtue of the mechanical transmission.

Still, I do agree, 100 rpm is not a sign of any problem.

[JRL]

Even if the tires were twice as large, and you were really going 280 km/h now, your RPM should still be 2900 and your speedometer should still read 140 km/h.

That is, unless the Volvo has an optical speed measurement device, which I doubt. So, tire size would not affect the relationship between RPM and indicated speed one bit.

.
Incorrecto.
Tire size, speedometer and actual speed are 100% related

[Its the wifes]

Tire pressures have more to do with overall diameter than new v old tires.

[dhayner]

Incorrecto.
Tire size, speedometer and actual speed are 100% related

I think the only way the speedometer "knows" how fast the car is going is by how fast the driveshaft is turning. If the tires are of a different size than the speedometer is calibrated for, the speed will be inaccurate, but the tach/speedometer relationship will be unchanged.