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.

Doug T

The Highlands, Louisville, Ky.

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.

Frank/Rebop

Bristol, In ( by Elkhart) 

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!!

A.J.

South Jersey

SMSgt USAF Retired 
1955 Ford Ranch Wagon & a couple of old guitars...Life's Good!

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.

Royal Oak, Michigan (Four miles north of Detroit, and 12 miles NORTH of Windsor, Canada).  That's right, we're north of Canada.

Ford 292 Y-Block major overhaul by simplyconnected

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.

  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. 

Frank/Rebop

Bristol, In ( by Elkhart) 

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?

John - "The Hoosier Hurricane"

Too much sugar, or old age. One of those!!

Frank/Rebop

Bristol, In ( by Elkhart) 

mad cow disease

moz. geelong victoria australia.

graduate 1980, bus, truck, car, hot rod, boat, submarine, hovercraft, hydrafoil, firetruck, mobile home, jet, helicopter, cruise ship, motorcycle, bicycle, santa's sleigh, clock, alloy bullbar, alloy fuel tank, lens, dr who's tardis, matter - anti matter warp drive buffer & y-block lover

GPS in a 55 Ford-Kind of like back to the future if you think about it.