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Classic/Antique Car Repair/# of teeth on speedo drive gear


I have a 55 Crown Victoria.  The speedometer is not very accurate.  Shows I'm doing about 70 when I'm actually going 55.  I just recently put a set of 215/75R 15 radials on it, the manufacture shows the tire dia at 27.7".

I have two question:
1.  How do I determine what rear axle ratio the car has.  The 3.31 was standard, but there was an option for a 3.55.  
2. How many teeth did the speedometer drive gear have?  Did the number of teeth change depending on whether you had the 3.31 or 3.55?

I've found charts on the web that allow you to determine how many teeth the driven gear should have based on the rear axle ratio, tire dia and speedo drive gear teeth number.  Just trying to get the speedo close to indicating the actual speed.

Ok the rear ale ratio can only be determined accurately by getting to see and count the teeth on the pinion and the teeth on the ring gear. On your car that means removing the center differential carrier. However you can get a pretty good guestimate by jacking the back end of the car up so that both wheels are off the ground. Make sure to support the car on jack stands. NEVER GET UNDER A CAR WITH JUST THE JACK HOLDING IT UP! Put the transmission in neutral and release the parking brake. With a piece of chalk make a mark pointing straight down on each of the rear tires. Now make a mark on the drive shaft and a mark that lines up with it on the differential housing. Now turn both rear wheels the same direction at the same time watching and counting the number of turns of the drive shaft. If the drive shaft turned 3 1/2 turns then the axle ratio is 3.5:1.
Now to correct speedometer error I will be the first to admit that in my shop I cheat. I put my Garmin GPS in the car and take a test drive. My Garmin has a screen that gives very accurate road speed. I compare this with the speedometer reading and make a note of the two. It does not make any difference what speed you are driving. Lets say you were driving an indicated on the speedometer 70 miles per hour and the Garmin showed 55 then get the old brain working. Divide the smaller number by the larger. 55/70=.78 or 78%. 78 subtracted from 100 means that the speedometer has a 32% error. Next remove the speedometer driven gear and count the number of teeth. Now to get the speedometer to read faster you will need a gear with 32% fewer teeth. To get the speedometer to read slower you will need a gear with 32% more teeth. Let say that the gear that you have in the car has 23 teeth you would multiply the number of teeth by .78, or 23*.78=17.9 teeth or rounding off 18 teeth. Back in the day the manufacturers would settle for 10% speedometer error.  

Classic/Antique Car Repair

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Brad Sears


All automotive including antique and collectible. However if the car has been modified I can only answer in general terms and maybe get you pointed in the right direction.


Automotive tech instructor. Syndicated auto columnist 1970's though the early 1990's. Syndicated auto radio talk show, Ask Brad About cars, CBS Radio 70's through 90's TV Show "Last Chance Garage" 1980's PBS-TV syndicated. Auto instructor for the following companies: Fram Autolyte Holly Carter AMF Ford Motor University Of Conn Blue Hills Technical School Sugar River Technical Center Grew up in a family garage in Needham Mass and turned wrenches from the age of 14.

Manchester Union Leader, Nashua Telegraph, Motor Service Magazine, Yankee Magazine, Popular Mechanics (Saturday Mechanic early 80's), Los Angeles Times, New York Times, and lots more.

More than I care to remember. Basically Franklin Technical Institute in Boston, Northeastern University, Fitchburg State Teachers College, Tufts University, and a lot of factory schools along the way.

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Moto Award winner. And much more.

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