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By Doug T - 15 Years Ago
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The Speedometer in my truck is pretty inaccurate and I am trying to get an idea of how much. My truck now has an electronic tach. So this would be easy if the truck was manual shift but it isn't, it is a Ford-O-Matic. The tires on the rear (235R75-15) have a rolling, loaded, circumference of 90.69" and the revs/mile are 720. So with the rear axle ratio of 3.73:1 the engine speed with no slippage will be 2685 RPM @ a true 60 mph. What would be a reasonable assumption for slippage in the torque converter at normal load and at W.O.T. ? Of course I could run down the road with a second car at the same speed and answer this question for normal load.
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By pegleg - 15 Years Ago
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Doug, All the slippage will take place in the convertor. The speedometer drive is located on the output shaft which is, of course, directly connected to the rear end. Unless you're me or John at Columbus. Then anything's possible. Take it out on I65, set it at 60 mph and time yourself between mile posts. I usually use 3 or more to average the mistakes. You will be able to tell the deviation from the times you record. 
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By YukonCor55 - 15 Years Ago
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I recently replaced my speedo, cable, and drive gear in my Fairlane. I used my portable GPS to check calibration. Even the cheap GPS units will give you a MPH readout that is very accurate. I just mounted the unit on my windshield, got out on a straight back road and got the car up to 60 mph. One I had it steady, I checked the GPS and it showed 59 MPH. Close enough for me! It's kind of surreal to be driving around in a 55 Ford with a GPS in the windshield though!!
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By simplyconnected - 15 Years Ago
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My GPS is a USB, attached to my laptop. I recently flew to Florida from Detroit, and put the puck up to the airplane's window. Sure enough, 550MPH. The map on my laptop showed exact location, but what was real wild was to look out the window and see an arial view of my laptop screen. It was VERY cool to see my map constantly refresh. The 'crumb trail' was funny to see, too. BTW, my GPS shows speed in tenths of a MPH. It's the best for calibrating speedometer/gears.
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By Don Woodruff - 15 Years Ago
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In the T bird I have noted 100-150 RPM from 0 throttle to maintaining speed to 300+ rpm shift from full throttle to 0 throttle. A GPS is the easy way. Calculating using mile posts next best.
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By pegleg - 15 Years Ago
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I recently flew to Florida from Detroit, and put the puck up to the airplane's window. Sure enough, 550MPH. South West must be a little faster, I recently flew from Sacramento to Chicago and my hand held maxed out at a bit over 600. Most of the time it was mid 500's. Whole lot faster than driving.
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By Hoosier Hurricane - 15 Years Ago
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Pegleg: You were flying with a tailwind, Dave was flying in a crosswind. Oh no. Now we've strayed into weather conditions from speedometers. Why do our minds wander so much?
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By pegleg - 15 Years Ago
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Too much sugar, or old age. One of those!!
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By Moz - 15 Years Ago
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mad cow disease
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By oldcarmark - 15 Years Ago
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GPS in a 55 Ford-Kind of like back to the future if you think about it.
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By Hollow Head - 15 Years Ago
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Our Ford-O...or should I say Mile-O slips the whole quartermile  Ain't build to handle over 400 ft lbs of torque
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By Doug T - 15 Years Ago
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I finally got the chance to run the truck down the interstate following another car with a known good speedo. The Speedo in the truck was rebuilt by a known shop a couple years ago. It was sent back once for being wildly too slow. The rebuilder said to send the face of the speedo the second time which I did. It came back and was better but still much too slow. At a known 60 mph the speedo reads 48 mph. pretty crappy but worth knowing. The speedo doesn't start to read until something like 15mph so it may just need to have the anchor of the spring moved if that is possible. Anyhow the tach in the truck was reading 2800+ rpm, with 100 rpm hash marks it is difficult to be more precise. With no slippage the truck would have been going 65 MPH at that engine speed. Therefore the slippage is 65/60 => 8.3%. This is probably not all lost energy, The torque converter would be increasing torque with this much slippage and this torque was required to move the truck.
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By pegleg - 15 Years Ago
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Doug, The commonly accepted number I've heard is 5%. That would be the number I'd use for the end of a quarter @ speed. I would think the amount in your truck would be less than that with a stock (?) converter, but I've been wrong before! I can usually get some idea by finding a cruise speed rpm, then backing off until the car/truck starts to slow down. the difference being slippage. I suppose as aerodynamic as the truck isn't, and given the weight you may well be higher than a car. If you can get it apart, pull the needle and reset zero by letting it just rest aginst the stop on the bench, Blow out the area around the magnets sometimes foriegn material gets in there and affects the readings.
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