Zoom!

If you change your differential gears or even the size of your drive tires, your speedometer and odometer won't be accurate anymore.  All those careful calculations the factory did will get out of wack, and you won't know how far you've gone or how fast you're going.

A numerically higher final drive ratio means for a given engine speed you'll be going slower and your speedometer will read too fast.  And tires with a larger diameter mean you'll be going faster and your speedometer will read slow.

How Speedometers Work

The transmission output shaft has a drive gear that always turns at a rate proportional to the vehicle's speed.  Every time the transmission output shaft rotates once around, the rear axles rotate 1/FinalDriveRatio around.  For example, if your final drive ration is 3.00, then every revolution of the transmission output shaft will turn the drive tires 1/3 of a turn.  This will drive the car forward a few inches, proportional to the size of the tires.

While all of this is happening, a speedometer driven gear is being turned by the drive gear.  The driven gear is in the transmission at the end of a cable that runs up to the speedometer.  And if everything is correct, that cable will spin EXACTLY 1000 times for every mile the car travels.  1000 is just a magic number used by the engineers when they designed the speedometer itself.

The driven gear is easy to change.  It hangs on the end of the speedometer cable and one little bolt lets you pop it out of the transmission.  Then you can pop it off the cable and replace it with another gear with a different number of teeth.  

• Replacing your driven gear with one with more teeth will slow your speedometer down.
• Replacing your driven gear with one with fewer teeth will speed it up.

Estimating the Driven Gear You Need Based on the Driven Gear You Have

There are a couple of ways to figure out what driven gear you need to compensate for a change in final drive ratio or tire size.  If you are certain your speedometer was correct before the change, you can just divide the new final drive ration by the old one and multiple that by the number of teeth on your old speedo driven gear to get the size of driven gear you need.  

NewDrivenGear = OldDrivenGear x ( NewFinalDriveRatio / OldFinalDriveRatio )

For example, I had a 19 tooth driven gear and I went from a 3.00 to a 3.50 final drive ratio.  

NewDrivenGear = 19 x ( 3.50 / 3.00 )

22.16 = 19 x ( 3.50 / 3.00 )

So I must need a new 22.16 tooth gear, right?  Just round the number off, since there isn't such thing as 1/2 a tooth.  Maybe I should order a 22 tooth gear.  Probably, but what if my speedo wasn't perfect before, or what if have slightly different tires on my car than the factory intended.

Estimating the Driven Gear You Need Based on MPH

If I wasn't sure my speedometer was accurate before I started, the previous technique won't work well.  Another way to estimate what driven gear you need is to figure out how far off your speedometer is.  The best way to do this is to find an accurate measured mile and drive through it at a constant speed.  Remember back in 7th grade we learned that

Distance = Rate * Time

Rate = Distance / Time

For example, if I drive at a steady indicted 55 MPH and cover an accurate measured mile in 74 seconds, I can calculate my true speed, using D/T = R.  

Rate = 1 Mile / ( 74 Second / 3600 SecondsPerHour)

48.6 MPH = 1 Mile / ( 74 Second / 3600 SecondsPerHour)

So my speedo said 55 MPH but really I was only going 48.6.   Then you can calculate how far off your speedometer is using 

Error = (ActualMPH - ObservedMPH ) / ActualMPH

Continuing the example, 

Error = (48.6 - 55 ) / 48.6

-.131 = (48.6 - 55 ) / 48.6

So my speedometer must 13.1% fast. (A negative result means my speedometer is fast.  A positive result means my speedometer is slow.)  Put another way, original 19 tooth driven gear must really be 13.1% too small.  To calculate the new driven gear use

NewDrivenGear = OldDrivenGear x (1 - Error)

So in my example

NewDrivenGear = 19 x (1 - -0.131)

21.5 = 19 x (1 - -0.131)

(Algebra note:  And subtracting a negative number is like adding a positive.)

So this calculation tells me I need a 21.5 tooth driven gear!

If the measured mile is accurate and the stop watch is accurate, this is a great way to check your speedometer.  But how can find an accurate measured mile?  Don't be too sure those mile-markers beside the road are really accurate. Sometimes they just plop them into the ground where they feel like it.

But the great thing about this approach is it can compensate for a speedometer instrument that is a little off.  One thing you can do is repeat the experiment a half dozen times using different measured miles.  If you always get the same answer, you're in business.

Calculating Driven Gear Size By The Numbers

It isn't much harder to just do the whole calculation from scratch, just like the factory did, and figure out EXACTLY what size driven gear to use.

First, let's collect some data.  You will need to know the RPM - that's tire revolutions per mile - for your tires.  Most manufactures web sites have this data.  And even if yours doesn't, if you find the exact same size of tires on a web site that does, that RPM will be OK.  This is exactly the number of revolutions required for this tire to cover one mile.  And before you go off assuming you could just use the diameter to calculate RPM, it isn't that easy if you want to be exact.  It turns out that a diameter-based calculation would give a slightly wrong value, since the tire has a static loaded radius less than 1/2  the diameter.

Of course, you'll need to know your final drive ratio.  You probably know this since I bet you just changed it.

You must also know the number of teeth on the drive gear.  For your Ford automatic this is probably 8, but might be 7 or conceivable 9.  You could just disassemble your transmission and count the teeth, but that seems hardly worth it.  Instead, we'll figure it out using what we know.

The formula to compute my factory drive gear is

DriveGear = ( DrivenGear * 1000 ) / (TireRevsPerMile * FactoryFinalDriveRatio )

will tell us the factory drive gear, assuming we put in the original factory values for TireRevsPerMile, FactoryFinalDriveRatio, and DrivenGear.

For example, in my case the original driven gear was 19, the factory final drive ration was 3.00, and the original 14" 70 series tires has an RPM of 780.  

DriveGear = ( 19 * 1000 ) / (780 * 3.00)  

8.11 = ( 19 * 1000 ) / (780 * 3.00 )  

So  my factory driven gear calculates out to 8.11 so it must have really be 8.  

The formula for driven gear is an easy one.

DrivenGear = (TireRevsPerMile * FinalDriveRatio * DriveGear) / 1000

Now I'm ready to go!  I know my TireRevsPerMile, for example 801 for my 275/40R17 tires, my final drive ratio, for example, 3.50, my and my drive gear, for example 8.  Just plug them into this formula:

DrivenGear = (801 * 3.50 * 8) / 1000

22.4 = (801 * 3.50 * 8) / 1000

So if there was such a thing as a 22.4 tooth driven gear, my speedometer would be dead-on accurate.  It turns out there isn't such a thing as a 22 tooth driven gear for Ford automatic transmissions.  I have to pick either 21 teeth or 23 teeth.  I picked 23 since it is a little closer to the ideal 22.4 I need.

The nice thing about knowing the exact ratio you need, is then it is possible to predict about how far off your speedometer will be.  The formula that tells me is

Error = (CalculatedDrivenGear - ActualDrivenGear ) / CalculatedDrivenGear

So in my example case,  

Error = (22.4 - 23)/22.4

-.027 =  (22.4 - 23)/22.4

so I expect my speedometer to read about 2.7% slow.  Just a warning - speedometers are mechanical instruments that are never perfect, even if you had exactly the right ratio for your speedometer gears expect your speedometer to be off a percent or two itself, independent of the ratio.

A final note.  It never hurts to check your calculations by using more than just one of these different methods.  I ran the numbers every different way I could.  And when they all told me to get the same 23 tooth gear I felt pretty confident I knew what I was doing.

C6 / C4 / AOD / AODE Speedometer Driven Gears

You can get these gears from any Ford dealer or most aftermarket vendors

You can also get a 23 tooth gear from some aftermarket suppliers.  I got mine from Steeda.  A lot of people say the teeth on these 23 tooth gears are so thin the don't last long.  The one in my A0DE has 30,000 miles on it.  The one in my C6 has 10,000 miles on it.  I can certainly live with replacing the 23 tooth gear every 30,000 miles if that is all it takes.  I can replace it much quicker than I can change oil and filter and it is inexpensive.

There is a rumor that Chrysler makes a 22 tooth driven gear that fits these transmissions.  But I haven't been able to substantiate it.  Probably it is an urban legend, but if you know any part numbers please let me know.

If you're off this chart you can either replace the output shaft of your transmission with one with fewer teeth, or you can have a ratio adapter built for $100 or $200 that will compensate for any combination you might have.

Search Our Site