calibrating the speedometer and its´ tolerance
Discussion
@ one of my first test-drives few months ago, I noticed, that with the suggested (from Ultimasports) 6.400-Pulses-per-Mile-Setting of the speedometer, it showed a 80km/h, when I was going 110km/h, so 30 difference. Measured the circumferemce of my tires: approx. 2.02metres. Then calculated: 1 mile = 1.609 kilometres. 1609 metres / 2.02 metres = 796,53 tire-rotations per mile. 6 "teeth" on the speedometer "wheel" means 796,53 * 6 = 4779 pulses per mile. Took 4780 and re-calibrated the speedometer. Compared with the car of a friend and it was much closer then(showing 140 km/h, as friends Porsche 911 showed 137 km/h)... Today did few times 170mph-190mph...Speedo displayed 320km/h once (199mph), but the speedometer of my Pioneer Avic-X1 Navigation System showed only 275km/h (170mph)(it measures the speed via GPS signal)...Questions to you guys now are:
1. 4780 pulses per Mile Setting sounds familiar to anyone of you using the manual original ultimasports speedo ?
2. Experienced an increasing discrepancy coming along with increasing speed (the more speed, the more discr...)?
3. Better to believe in the values given by GPS or by mech. Speedometer ?
4. What would be a precise and affordable Method of measuring real speed ?
1. 4780 pulses per Mile Setting sounds familiar to anyone of you using the manual original ultimasports speedo ?
2. Experienced an increasing discrepancy coming along with increasing speed (the more speed, the more discr...)?
3. Better to believe in the values given by GPS or by mech. Speedometer ?
4. What would be a precise and affordable Method of measuring real speed ?
cmoody: The speedo is calibrated via the little button on the green cable. Keep it pressed when turning on the iginition. Then figures will appear in the display where usually stands the miles and day-drive...with pushing the button, you can increase the single figures...when you got the one you need (in case of 4780 first figure to set is 4) just wait a few secs, then it will go automatically to the next figure...and so on...
steve: you may be right, but what I know for sure is, that I once had 5800 rpm on tacho. with my formula, I calculated, it must have been a 296 km/h. (5800*2.02)/(16,67*0,6875*3,444) 5800rpm is quite sure, because tacho is directly fed by the ECU. 2.02metres tire circumf. is also quite sure, as we measured very precisely and several times. 0,6875 is very sure, as it is a 22:32 gear ratio which is very determined as it is the 3.444 (final ratio) which is a porsche standard... So even if you say air resistance / slight expansion of the tires at high speeds due to centrifugal forces....this should be quite close...(this idea came after I wrote this thread yesterday night...). But now I got three figures for one speed: Speedo:320 km/h, GPS: 275 km/h, Calcuated: 296 km/h..... Again my questions:
1. 4780 pulses per Mile Setting sounds familiar to anyone of you using the manual original ultimasports speedo ?
2. Experienced an increasing discrepancy coming along with increasing speed (the more speed, the more discr...)?
3. What would be a precise and affordable Method of measuring real speed ?
Thanks
Lutz
steve: you may be right, but what I know for sure is, that I once had 5800 rpm on tacho. with my formula, I calculated, it must have been a 296 km/h. (5800*2.02)/(16,67*0,6875*3,444) 5800rpm is quite sure, because tacho is directly fed by the ECU. 2.02metres tire circumf. is also quite sure, as we measured very precisely and several times. 0,6875 is very sure, as it is a 22:32 gear ratio which is very determined as it is the 3.444 (final ratio) which is a porsche standard... So even if you say air resistance / slight expansion of the tires at high speeds due to centrifugal forces....this should be quite close...(this idea came after I wrote this thread yesterday night...). But now I got three figures for one speed: Speedo:320 km/h, GPS: 275 km/h, Calcuated: 296 km/h..... Again my questions:
1. 4780 pulses per Mile Setting sounds familiar to anyone of you using the manual original ultimasports speedo ?
2. Experienced an increasing discrepancy coming along with increasing speed (the more speed, the more discr...)?
3. What would be a precise and affordable Method of measuring real speed ?
Thanks
Lutz
Moin Lutz,
1. 47.. sounds familiar concerning speedo pulses
2. As you know, my speedo is only accurate up to a speed of 160km/h (100mph), the rest is left to my imagination. From there I have to concentrate more on traffic than on speedo figures....
3. GPS is always 99,9% accurate, thus it`s used when measuring e.g. world record runs. So if your GPS says 275km/h it will be in the worst case 274,725.
In "normal" cars the discrepancy between the speedo reading and the true road speed will always rise with your speed. In my Audi for instance the true (GPS) speed is 220 when the speedo says 245.
Btw, with the Ultima I`d rather use "GS" and "TAS" (Ground Speed and True Air Speed)...
1. 47.. sounds familiar concerning speedo pulses
2. As you know, my speedo is only accurate up to a speed of 160km/h (100mph), the rest is left to my imagination. From there I have to concentrate more on traffic than on speedo figures....
3. GPS is always 99,9% accurate, thus it`s used when measuring e.g. world record runs. So if your GPS says 275km/h it will be in the worst case 274,725.
In "normal" cars the discrepancy between the speedo reading and the true road speed will always rise with your speed. In my Audi for instance the true (GPS) speed is 220 when the speedo says 245.
Btw, with the Ultima I`d rather use "GS" and "TAS" (Ground Speed and True Air Speed)...
You may be right for professional GPS used for measuring... But I don´t think neither Ultimasports (for their records) nor militarys use Pioneer-Consumer-Market-Leisure-Time-GPS-Equipment for serious and precise measurements...I think after all I stick to the calculating method as this must be quite exact as it has hardly any unknown parameters...
lvjost said:
You may be right for professional GPS used for measuring... But I don´t think neither Ultimasports (for their records) nor militarys use Pioneer-Consumer-Market-Leisure-Time-GPS-Equipment for serious and precise measurements...I think after all I stick to the calculating method as this must be quite exact as it has hardly any unknown parameters...
Your calculations are not taking into account slipage of the tyre on the road surface.
Steve
Bit on the expensive side but for the ultimate in speed measurement you probably want a SPEEDBOX from these guys:
www.race-technology.com/content.php?pcat=2&cat=865
It looks to be considerably smarter than the VBOX from: www.racelogic.co.uk/?show=VBOX
If you wanted to make a super accurate speedo for reasonable money it should be possible. What I'd do is start with a GPS receiver for the long term stable measurement of speed. I'd use this to accurately callibrate and correct the values obtained from one or more wheel speed sensors to give you an instantaneous value for speed. To actually display the output I'd synthesise a pulse stream and feed that into the back of a standard Ultima speedo (well one that had been corrected to show the correct mileage/speed).
All you need to do this is an off the shelf GPS module and a simple microcontroller. Total parts cost of under £100. Bit of a pipedream for me at the moment as I'm still tuning my EFI system and the next project is traction control...
www.race-technology.com/content.php?pcat=2&cat=865
It looks to be considerably smarter than the VBOX from: www.racelogic.co.uk/?show=VBOX
If you wanted to make a super accurate speedo for reasonable money it should be possible. What I'd do is start with a GPS receiver for the long term stable measurement of speed. I'd use this to accurately callibrate and correct the values obtained from one or more wheel speed sensors to give you an instantaneous value for speed. To actually display the output I'd synthesise a pulse stream and feed that into the back of a standard Ultima speedo (well one that had been corrected to show the correct mileage/speed).
All you need to do this is an off the shelf GPS module and a simple microcontroller. Total parts cost of under £100. Bit of a pipedream for me at the moment as I'm still tuning my EFI system and the next project is traction control...
lvjost said:
Wanted to reanimate this topic, as it isn´t resolved yet IMO. What other methods of measuring speed than the mechanical Utimasports and GPS are available /do you guys use ? I´m really not satisfied with the result of the both mentioned...
In the UK our motorways have mile markers so it is possible to drive at a set speed and have an assistant time the passing of the markers.
Steve
The only problem with measuring the circumference and using pulses per mile is that the Ultima Sports speedo is (at least was in 2001) a 160MPH speedo but with a replacement 200MPH face. Things might have changed since then but I was advised to use 6370 pulses per mile, and my speedo is accurate to the mph as far as I have checked it.
LuckyP was advised to set his speedo to 4???, which means that either Ultima are now using a proper 200MPH speedo or they have changed the number of pins on the speedo pickup wheel.
At a steady speed consumer GPS units are very accurate.
LuckyP was advised to set his speedo to 4???, which means that either Ultima are now using a proper 200MPH speedo or they have changed the number of pins on the speedo pickup wheel.
At a steady speed consumer GPS units are very accurate.
Yihaa: Got 6 Pins on the speed wheel...And as I had 63XX Pulses set up, my speedo showed 80km/h while driving 110!!! So we calculated and got 4780 as quite right. It is actually accurate up to 65-75mph..But after that (if I believe in the figures showed by my Pioneer Avic X1R Navi-Sys there are tolerances up to 50 km/h...)
lvjost said:
It is actually accurate up to 65-75mph..But after that
My be a problem with the sensor or its adjustment, might be too far away. Gives a clear enough signal at lower speeds but then missing pulses at higher speeds. Or does it overread at higher speeds?
Is the sender cable shielded? It is in the standard harness, but I don't know how many modifications you have made to the loom.
Yihaa: Sender is calibrated and positioned quite tight. I think it overreads at higher speeds..Otherwise (missing signals) would mean it shows less speed than driven in fact, but mine does the other way....Didn´t change anything on the original loom...but don´t know, if the sender cable is really shielded..
I have taken a look at one of Ultima's demo pics on their Website. I've notice on their GTR720 demo, the speedo shows 240mph. I think before, they have a 200mph. On their price list, it shows 200mph, no info on the 240mph speedo.
What is the process of setting your car so that it can actually show the speed of the car when
the gear ratio is extended on the gearbox?
What is the process of setting your car so that it can actually show the speed of the car when
the gear ratio is extended on the gearbox?
Thank you. One last question. Can the speedometer be removed by placing a small covered screw driver around edge and prying it out, or does the factory use a mounting bracket or threaded sleeve to set the speedometer? I would rather know this before I start prying and then damage the gauge.
Edited by cmoody32 on Wednesday 25th October 20:09
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