Speed v Revs V GPS

[kiwi]

Nice and technical and complex so stay awake while reading twice

The situation!

BX19TZS 1991 v BX19TRS 1990 both have the same tyre brand and depth size etc except one has an odometer/speedometer the wrong side of accurate. ie TZS shows 106kph and TRS shows 97kph when the GPS reads 100kph! This flows through to the odometer obviously.

Now you would think that two BXs of the same specs just differant trim levels would then read the same on the REV counter in 5th at 100kph on the GPS...yes???

Nope! The 19TRS Revs at 3200 at GPS 100kph the 19TZS reads 3100rpm ok not to much a differance.
But when the 19TZS is doing its indicated speed of 100kph true 106kph the revs sit at 3200rpm where as the 19TRS indicates 110kph to read 106kph GPS and the revs are 3400rpm.

Hope you guys understood that?

So by using this method of establishing true speed with indicted and revs would the differance of 200rpm@106kph and 100rpm@100kph point towards a differance in gear ratios at the gearbox?

This 19TZS is a mystery car having sat 2 years in storage before being registered on NZ roads. Yes both cars are whats called NZ new meaning they were never driven overseas. If only cars could talk!

[MULLEY]

Perhaps the rev counters are out as well, does the higher revving car actually sound like its revving more than the other?

If it does, then it must be the gear ratios, perhaps factory spec was changed at a point in time, or somone has changed the box or fiddled with the ratios.

Are there any reference guides that detail what the ratios should be on a 1.9 carb model for those particular years?

[kiwi]

Perhaps the rev counters are out as well, does the higher revving car actually sound like its revving more than the other?

Couple hundred revs you going to hear that?

If it does, then it must be the gear ratios, perhaps factory spec was changed at a point in time, or somone has changed the box or fiddled with the ratios.

Thats what I am thinking hence if Cars could talk!

Are there any reference guides that detail what the ratios should be on a 1.9 carb model for those particular years?

Hence asking the question here.

[MULLEY]

Yup, i can hear the difference between 3000 & 3200, quite a bit louder, perhaps not in a petrol?

[docchevron]

Rev counters are not all that accurate, I'd suggest trying a calibarted tacho on both cars maybe?

I dont have all the gearing specs to hand but it's entirely plausable that they differ.

It would also be useful to know the long number on the top of the speedo face usually hidden when it's fitted in the binnicle.
I suspect they'll be different.

[mat_fenwick]

I'd suggest trying a calibarted tacho

Or failing that, an automotive multimeter with a tacho function. It won't necessarily be any more accurate, but you can easily swap it between cars and as you are looking for differences rather than absolute values, any errors present don't matter.

[citronut]

so did you carry out these trial's on exactly the same road and from same point to the same point,

and the same time of day,

i dont think you can judge this by sattalite,

regards malcolm

[Way2go]

Why don't you do the simple test first and see what the odometers actually read compared over a measured/calibrated distance between fixed points? This is a direct link (via gearwheels) to your road wheels as opposed to the speedo which relies on magnetism/eddy-currents coupling.

If the measured distances are out on the odometer then it stands to reason that the speedo will also be out by a similar error in each case anyway!

[kiwi]

so did you carry out these trial's on exactly the same road and from same point to the same point,

and the same time of day,

i dont think you can judge this by sattalite,

regards malcolm

Unfortuantely I do not have a lab with rolling road to conduct these tests.

However of a long enough period of driving the said vehicles it is something I have become aware of quite notably. Conducting the trip on the same journey which is open road all the way. GIves enough data to be able to reach conclussions mentioned.

[kiwi]

Why don't you do the simple test first and see what the odometers actually read compared over a measured/calibrated distance between fixed points? This is a direct link (via gearwheels) to your road wheels as opposed to the speedo which relies on magnetism/eddy-currents coupling.

If the measured distances are out on the odometer then it stands to reason that the speedo will also be out by a similar error in each case anyway!

DONE!!!! Even had a Dyno road check done on the TZS!

[kiwi]

Rev counters are not all that accurate, I'd suggest trying a calibarted tacho on both cars maybe?

I dont have all the gearing specs to hand but it's entirely plausable that they differ.

It would also be useful to know the long number on the top of the speedo face usually hidden when it's fitted in the binnicle.
I suspect they'll be different.

Have switched the speedo heads between vehcilces the Tachometers returned the same result. However the speedometers which have identical Binnacle numbers (I checked) were wildly out when swapped between vehcilces indicating that the speedometer head on the TZS has at some stage been calibrated by a third party.

The dyno road check I had done also concurs with the GPS reading!

Whatever the issue (not really a problem lies) it is most certainly in the Gearbox area.

Hence the question on Gear Ratios!!!

This post on trademe got me thinking more about the ratio effect.
http://www.trademe.co.nz/Community/Mess ... 70&topic=1

[Vanny]

Going by the dealer manuals, there seem to be some 30+ gearboxes in UK Mk2 vehicles alone, i suspect the simple answer is ratio's.

Though a rolling road might give the ultimate answer.

[kiwi]

Though a rolling road might give the ultimate answer.

I have had a dyno road check done on the TZS thats why I also know the inaccuracy of the speedometer. Although this wont help because I was told then that it maybe the head in the gearbox thats differant and what I have found and mentioned already is the speedometer head has been recalibrated by the evidence.

However at $80 a pop and 600kms worth of petrol getting the tow BXs down to a dyno road just to do this "test" is a bit excessive.

What I also found which is obvious given the discrepancy in the mileage reading is that fuel consumption is heavier in the TZS than the TRS. Not much once the error is taken into consideration and the carb differance and the fact one has aircon (although have not noted much differance the aircon has to fuel use)

Now after reading the other post about the 1400cc on a motorway being heavy on fuel, it does beg the question what is the optimum efficant revs area.

Of course given the rev differance that could also affect fuel....hmmm mysterys

btw, it was reading this that had me asking the original question

http://www.trademe.co.nz/Community/Mess ... 70&topic=1