I have searched the past threads and can't find anything about Mondial speedo calibration. I have a '83 Mondial QV Euro running on 225/55-16 rear wheels which are the same rolling radius as the original metric wheels. I have used the 1km markers we have on our highways to check the speed the car is actually travelling using a tachymetre on my watch. This is more accurate than a GPS for small distances. At all speeds above 50 kph my speedo reads about 10% LESS than the speed the car is actually going. That is, when I am doing 90kmh (i.e. 1km in 40sec) my speedo reads 81kph. Can anyone expain how I can adjust the speedo so that it is a little bit closer to t he actual speed? FF
First off, the 225/55-16 tire is not the same rolling diameter as the original metric. You don't say what brand/model tire you are running, but a quick look at tires that size on Tirerack.com shows that 225/55-16 tires average 25.7 to 25.8" diameter. The original Michelin 240/55VR390 TRX tires have a diameter of 26.2". Now, the number of revolutions per mile depends on the tire as well. The 225/55-16's I looked at averaged 807 to 813 revs. per mile. They did not list the revs. for the TRX, but tires in the 26.2" diameter ranged around 790 to 794 revs. per mile. Unless I'm getting myself mixed up, it seems that the 225/55's should be producing a higher speed per Km travelled than the TRXs, since the wheels must turn faster (more revolutions) to cover the same distance. Was your speedo accurate with the metric tires mounted? AFAIK, the only way to adjust those speedos is to send them out for recalibration. I don't believe that they are owner adjustable. Clyde just sent his 308 speedo out for repair and recalibration, I believe, so maybe he can post the info on who did the work. Hope that helps.
Actually, I calculate the diameter for the metric 240/55-390 tire/wheel combo at 25.74803 inches. I calculate the 225/55-16 at 25.74409 inches. The variance is .0263% which is negligible. I believe all speedometers have some variance which is noticeable at speed. Portable GPS devices will show you the discrepancy between your indicated speed and your actual speed. AFAIK, there is no user calibration availble for the speedometer, but there are aftermarket units that can be used to help calibrate electronic speedometers which require cutting/splicing into the speedo wiring. Bob
Bob, The numbers I got were from Tirerack.com for the 225/55's. Pretty close to what you calculated. The TRX number I got from the Coker Tire website, which shows the mounted dimension according to Michelin Tire. Can't account for the difference between your calculation and Michelin's. Steve
Thanks. Interesting that Coker suggest the rolling radius is 26.2 (I verified this on their site too) versus the 25.74 that I also calculate from the 240/55VR390 dimensions. My tyres are Goodyears F1 GS-D3. By my calculations, if Coker are correct, the difference between the metric tyres and the Goodyears would be that the metric tyres travel 1.83% further with each revolution and consequently for the same car speed, my speedo would reflect a speed of 1.801% less (98.2kph on speedo when car is at 100kph). So this seems to be in the wrong direction to explain my speedo problem given I am on Goodyear 225/55-16. Seems like the best solution is to go for the 245/45-16 rubber on the back which will increase my final drive giving higher revs for same speed (manageable) and a more accurate speedo reading, whilst letting me lower the car 1.5cm which will make a reasonable difference to handling and stance. It looks a little too high on these tyres. (I am changing my fronts back to 205/50-16 from the 215/55-16 on now - to give stiffer wall angle for each tyre pressure and more instant steering response) Thanks for your input.
If your speedometer, odometer, tor tachometer needs repair or recalibration I suggest Palo alto speedometer. http://www.paspeedo.com/ best regards, Jim
If the Mondial speedo is anything like the 3x8 series: http://www.ferrarichat.com/forum/showthread.php?p=135711782#post135711782
Most cars of 1980's era read 10% high, not 10% low as your report, confirmed by several threads over the years. The standard fix is Palo Alto Speedometer Inc in the south bay area of California. Their web site, http://www.paspeedo.com/, gives a procedure to obtaining calibration numbers before removing your speedometer and sending it to them. This insures accuracy to the tires you are using. There are several threads on removal of the steering wheel (hardened metric allen), meter panel (dental floss) etc etc.
I sent my speedo to Palo Alto, who I hold in high regard, for a cleaning/calibration. It worked well for about 1000 miles, but now it says 80 when the gps says 100. Differential does not begin to appear until about 40. Beats me, other than I guess it's getting tired and it thinks I drive too slow. By the way: a friends 308 fuel sender quit and apparently they are unavailable. PAS said no problem so I sent it. Cheers, Tom
I had read all of the other threads on speedos before I posted here. It surprised me as well that all others seem to read high, not low like mine. However, I have checked it both against GPS and against holding continuous speed and timing it using a tachymetre over a measured kilometre, over and over and over, which is the most accurate way to get an accurate reading. It reads under by about 10%. Speedo 90/speed 98; Speedo 100/speed 110; speedo 120/speed 132; and so on as you go up the speed range. Thanks for all of your feedback. It is tough for me to send to PAS as I live in Asia - cheaper to keep buying new speedos till I get one that works.
