987687 Posted April 23, 2014 Share Posted April 23, 2014 I'm not really sure how to title this one. But I'm looking for how many rotations the speed cable does per rotation of the output shaft on a 5mt/dr. Context: I want a working speedo on my GL, and I want it to display the correct speed. I also want to be able to calibrate it for different tire sizes, etc. I'm just gonna throw a three wire VSS in the transmission and drive a stepper motor or whatever to spin the gauge head. Since I'm not gonna see the car for a few more weeks, knowing how fast the cable spins will let me chose the proper parts and get most of the circuit and firmware ready so it's an easy process to just plug it into the car. I don't know if I can spin a stepper fast enough to drive the gauge head or not, so knowing how many turns the cable is supposed to do per turn of output shaft will help figure out how best to drive it. Quote Link to comment Share on other sites More sharing options...
skishop69 Posted April 23, 2014 Share Posted April 23, 2014 You're going to have to do some math and you'll need the car. You'll need to measure the circumference of a correct size front tire. This will be the distance covered by the tire per rotation. Jack up one front wheel, put the car in neutral and rotate that wheel one time while someone counts the number of revolutions the speedo cable makes. That will be the number of rotations the cable makes covering that distance. Multiply 5280 by 12 giving you the number of inches in a mile. Divide the number you get by the circumference of the tire and then multiply by the number of rotations the cable made rotating the tire once. This will give you the number of cable rotations for 1MPH. I'm going to have to think about the rest of the math since the actual speed the cable increases with the vehicle speed is an exponential function. Quote Link to comment Share on other sites More sharing options...
987687 Posted April 23, 2014 Author Share Posted April 23, 2014 (edited) It's not exponential. The cable is connected to the output shaft which turns 3.9 times faster than the wheels. That's about it. I'm good on the math front. I need to know how far the car travels per pulse from the speed sensor, have a timer counting accurate time, and just do some simple math on it. If I chose the right units for distance and the correct prescaler it'll be extremely easy to to calibrate for different tire sizes. Just drive one exact measured mile. See how far you went on the odometer, and bammo, there's your scale factor to calibrate the system. I can do math and write the firmware.... I just need to know how many turns the cable would do per turn of the output shaft. By knowing this, I know how fast I need to turn the gauge head for any given speed (roughly), and the main reason I need to know that is so I can chose a motor to run the gauge head. Some steppers might not go fast enough, etc. Of course in a few weeks when I have all my spares and stuff out of storage, yea. I can test it. But for now I have most of the code written, I just need to chose some parts. The size of the tire is actually somewhat irrelevant. All you need to know is how far you traveled, and how far the odometer indicates. This is your scale factor. I guess I also don't really need to keep track of time. But ultimately I also want a digital display for exact mileage (for TSD race purposes), and have some other info like oil temp, pressure, etc. Edited April 23, 2014 by 987687 Quote Link to comment Share on other sites More sharing options...
czny Posted April 23, 2014 Share Posted April 23, 2014 (edited) Wonder if the reed switch pulses in the speedo head could be measured in Hz with a DMM on 5v scale. My BK Precision 2707A DMM has such a setting. Tap into the ECU wiring for your DMM connection. Then test run at say 65 in 4th gear(1.0 ratio?) & a GPS(cell) to determine pulses per mile. Just a suggestion. Edited April 23, 2014 by czny Quote Link to comment Share on other sites More sharing options...
987687 Posted April 23, 2014 Author Share Posted April 23, 2014 Few things: As I mentioned I don't have the car here, also I don't have a speedo cable in the car anyway. I'm driving my horrible outback, I don't even know why. The gl is better. Quote Link to comment Share on other sites More sharing options...
wagonist Posted April 24, 2014 Share Posted April 24, 2014 Go to the wreckers & get the cluster from a car which already has an electronic speedo. late 90's Camry should be good Then you'll have a stepper motor that's capable of driving appropriately. Legacy & WRX turbos (not you're stupid SOHC EJ22T version, the ones with the proper DOHC EJ20T) with a 260km/h speedo were electronic, not cable driven, so the sender should fit directly into other Subaru trans. Or you could just go out and buy an aftermarket speedo kit which should be already set up to do everything you want... Quote Link to comment Share on other sites More sharing options...
987687 Posted April 24, 2014 Author Share Posted April 24, 2014 Yea, I'm probably gonna have to use the guts from the gauge cluster on something else :\ Aftermarket speedo kits are expensive, and I'm cheap/poor.... Quote Link to comment Share on other sites More sharing options...
czny Posted April 24, 2014 Share Posted April 24, 2014 (edited) Near forgot I had this lying around from a 96 Lego. Here's the backside. Edit: Yours if you want it for study, 987686. . Edited April 24, 2014 by czny Quote Link to comment Share on other sites More sharing options...
skishop69 Posted April 24, 2014 Share Posted April 24, 2014 #1 on grabbing another speedo from a fly by wire set up. I'd have to check the circuits on one of those set ups, but I believe one of the driver resistors could be swapped for a pot that would allow you to calibrate it for different tire sizes. You are correct about the cable speed remaining static. The ratio is not the same for all makes though. Manufacturer gearing decides that. The change is also not exponential, but linear. My bad. I was half asleep when I replied. lol Easy example for others: If you're driving 60mph but the speed reads 50mph, then you're slow 1mph per every 6mph. As far as getting distance per pulse, I'd help you there but my XT is under wraps til I get my bathroom done and move on to other projects. I assume you want to be able to correct for tires sizes so you can either lift it or rally it? Quote Link to comment Share on other sites More sharing options...
987687 Posted April 24, 2014 Author Share Posted April 24, 2014 If I'm gonna go through the trouble, I may as well be able to calibrate it exactly, also I can add an LCD for TSD rallys and stuff. Distance per pulse is easy to figure out. I have a rough estimate, so to fully calibrate it I just have to stuff different numbers in the algorithm to work it all out. I could even have settings for each different set of wheels, or whatever.... Your options really are limitless when you start controlling stuff with a computer Quote Link to comment Share on other sites More sharing options...
DrieStone Posted November 15, 2018 Share Posted November 15, 2018 Did you ever figure this out? I'm running bigger tires in the Loyale and I can't seem to find any evidence of anyone doing a VSS or speedo correction on the Loyales. Quote Link to comment Share on other sites More sharing options...
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