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Speedo Calculations for Diff and Wheel Variations |
This purpose of this page is to help with the various calculations required when changing Differential Ratios, Rim size, and Tyre profile, in regards to Speedometer inacuracies.
This page will allow you to tinker with Common Diff Ratios, and Different Wheel / Tyre Combinations for your Ford Falcon, and Calculate the speedo sender Gear type required to make your speedometer Accurate.
Information on this page includes Standard Ratios and wheel sizes for EA (1987) Falcons to EL (1998) model Falcons, and includes options such as Tickford wheels, XR LSD Diffs, Wagons and sedans and LTD's
Remember, this program should only be used as a GUIDE. I hold no responsibility for the misuse or abuse of this program !
PLEASE NOTE : Changing the Diff Ratio on Models fitted with the intergrated PCM for Automatic Transmission (I.E. EF/EL/AU Inline 6's) will also need to change their respective EEC-V Module, as changing the Speedo Gear Sender will change the Auto Shift Points !
This is due to the fact that the EEC-V combined PCM Module will use the Vehicle speed, NOT ENGINE RPM, to determine when Automatic Gear shifting should occure !
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Required Information |
Obviously, for this program to do its job correctly, you will need to know some information about your car.
This information includes
1) The STANDARD (Factory Fitted) rim size, and Tyre Profile fitted to the Rear of your car, when it left the factory. (This is due to the fact that your speedo was originally calibrated for these wheels when it left the factory.)
2) The standard Differential Ratio in which your car was fitted with from the factory.
3) The standard Speedo Gear used, for the speedo sensor fitted on the transmission.
Don't be too fussed if you do not have all these figures with you, as I have a number of these for different models, which can be used as examples
This program will prompt you for the Standard figures, (As described above), and will also prompt you for New figures, (I.E. Diff Ratio, tyre profile, and Rim size) and will automatically calculate the percentage difference for Tyre Circumferance, and Diff Ratio, and Finally give you a percentage difference for the speedometer reading.
What is then possible with these readings, is to change the Speedo sender gear, to minimise the Speedo Percentage error, from these wheel and Diff Changes
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Standard Factory Figures |
The following information is Standard factory figures for Tyre profiles, Rim sizes, Diff Ratios, and Speedo Gears, for Factory Ford Falcon models
If you do not have all the relevant informaion regarding your model, then simply select the model from the list shown here, that best describes the Tyre Profile, and Diff Ratio Here
Remember, EA to EL Models use the same speedo gear sender unit, regardless of Automatic (3 or 4 speed), or Manual Transmissions. What is most important with this information, is matching the Tyres, Rims, and Diff to the model that most closely resembles your model, as it left the factory !
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| Tyre Profile |
Diff Ratio |
Speedo Gear |
Vehicle options and Description |
| ???-??-R?? |
2.77 |
Purple-19 |
EA Series 6cyl (Auto & Man) |
| ???-??-R?? |
2.92 |
Pink-19 |
EA Series 6cyl (Auto & Man) |
| ???-??-R?? |
3.23 |
Light Green-21 |
EA Series 6cyl (Auto & Man) |
| ???-??-R?? |
2.92 |
Pink-19 |
EB Series 6cyl CEFI (Manual) |
| ???-??-R?? |
3.08 |
Brown-20 |
EB Series 6cyl MPEFI (Auto) |
| ???-??-R?? |
3.27 |
Light Green-21 |
EB Series 8cyl (Auto & Man) |
| 205-65-R15 |
3.08 |
Brown-20 |
EF 6Cyl Sedan except LSD (Auto & Man) |
| 205-65-R15 |
3.23 |
Light Green-21 |
EF 8Cyl, 6Cyl Wagon & LSD (Auto) |
| 205-65-R15 |
3.45 |
Grey-22 |
XR6 (Auto & Man) |
| 205-65-R15 |
3.23 |
Light Green-21 |
EF XR8 (Manual) |
| 205-65-R15 |
3.45 |
Grey-22 |
EF XR (Auto) |
| 225-50-R16 |
3.45 |
Black-23 |
EF Tickford 16" wheel option (Auto & Man) |
| 225-50-R16 |
3.45 |
Black-23 |
EL XR6 & XR8 |
| 215-60-R16 |
3.?? |
???? |
AUII Falcon, Fairmont |
| 225-50-R16 |
3.45 |
???? |
AUII XR6 |
| 235-45-R17 |
3.23 |
???? |
AUII XR8 |
| 235-45-R17 |
3.?? |
???? |
AUII Tickford Wheel Option |
| 225-50-R17 |
3.?? |
???? |
AUII Tickford Wheel option |
| 245-40-R18 |
3.?? |
???? |
AUII Tickford Wheel option (Auto & Man) |
| 215-60-R16 |
3.23 |
???? |
BA XT-Futura/Fairmont (Auto Sedan/Wagon) (Manual Wagon |
| 215-60-R16 |
3.45 |
???? |
BA XT/Futura Manual Sedan |
| 235-45-R17 |
3.45 |
???? |
BA XR6 Models and XR8 Auto (Standard Wheels) |
| 235-45-R17 |
3.23 |
???? |
BA XR8 Manual (Standard Wheels) |
| 225-50-R17 |
3.45 |
???? |
BA Fairmont Ghia |
| 245-40-R18 |
3.45 |
???? |
BA XR6 and BA XR8 Auto (Optional 18" Wheels) |
| 245-40-R18 |
3.23 |
???? |
BA XR8 Manual (Optional 18" Wheels) |
If anyone has any further information, regarding standard specifications, then please let me know, and I will add them to this list
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Tyre, Wheel, Diff and Speedo Sender Calculations |
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Examples of using the above Calculator |
Example 1 - EF/EL Chaser/Police Dash Calibration.
The above method could be used to help calibrate a Chaser/Police Dash optioned vehicle, if the tyre or wheel profile has been changed
Say for example that wider profile tyres are going to be fitted to an otherwise stock standard car.
In this example, we will take a EF Falcon Gli, with 205-65-R15 Profile tyres, fitted at the factory.
On the Original specifications, enter these Values for the Original Factory Tyre profile, as per how the vehicle was built at the factory.
Lets say for this example, we wish to fit 215-60-R15 profile tyres.
To work out the speedo percentage error, we would punch in these figures under the New Specifications.
By clicking on Calculate, we would notice that there is a 1.33% Difference in the tyres, meaning that there would be a -1.31% difference in speedo Error.
At an actual speed of 100Km/H, without any speedo calibration, the speedo reading would be 98.69 Km/H
To correct this error with a Police/Chaser Instrument Cluster, you would enter a calibration figure of -1.5%
Example 2 - Changing to a higher Diff Ratio for improved Acceleration
When changing Diff Ratios, it is important to remember that Increasing a Diff Ratio will improve vehicle Acceleration, but will decrease Top Speed, and in some instances fuel economy.
This is due to the fact that the engine is reving higher for a given speed.
Also Decreasing a Diff Ratio will (If the engine has the power to overcome the increase in Wind Resistance), a Higher top speed, but it will in turn sacrifice Acceleration.
A lower Diff Ratio can improve fuel economy, due to the engine reving at a lower RPM at Highway cruising speeds.
Also, changing Diff Ratios Greatly effects the Speedo Calibration, and as such, your speedo will show a higher, or lower speed than the actual speed that your car is travelling
This is important legally, as you may think that your car is only travelling 60Km/H when in actual Fact, the vehicle could be travelling at well over 70Km/H.
Increasing a Diff ratio will mean that the speedo shows a higher speed than what the vehicle is travelling at, where as
Decreasing a Diff Ratio will mean that the speedo shows a LOWER speed than what the vehicle is travelling at. (Great for speed cameras !)
What is the solution to this problem, you may ask ?
Easy, use the above calculator to determine what the percentage error for the speedo will be, and the fit a new speedo sender gear, to negate this percentage error !
Lets take for example, an EF Fairmont Ghia. The Diff Ratio fitted from the Factory is 3.23
Now say if I wanted faster acceleration to try to keep up with an XR6. The XR6 of the same year carries a 3.45 Ratio Diff.
Simply fitting this diff will give me faster acceleration than standard, but will effect a number of things.
1) My Speedo will be out of calibration by -6.81, meaning that If my speedo says I am travelling at 110 Km/H, I will actually be travelling at 102.59 Km/H
2) The above car is speed limited at 180 Km/H. This is based on the speedo reading. So instead of the vehicle actually being speed limited to 180 Km/H, it will now be limited to 167.72 Km/H
So how do I fix the above problem ????
I know that this model car has a Light Green, 21 Tooth Speedo Sender Gear Fitted as standard.
If I changed that gear, to say a Grey 22 Tooth Speedo Sender Gear, I could reduce the speedo error to -2.26%.
This would mean that at an actual vehicle speed of 110 Km/H, my speedo would read 107.514 Km/H.
This would also mean that my actual Speed Limited top speed would now be 175.932.
Although the above figures are good, I could further reduce the speedo error by using a different Speedo Gear, and Different Profile Tyres.
The Above model came factory fitted with 205-65-R15 Profile tyres.
If I used a wider profile, say 215-60-R15, and the next higher sized speedo Gear sender (Black 23 Tooth), then I could get the Speedo Error down to +0.58%.
This would mean that at an actual vehicle speed of 110 Km/H, the speedo would be reading 110.63 Km/H.
My Top speed limited speed would actually be 181.044 Km/H.
As you can see from the above examples, this program can be very useful !
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Ford Speedo Sender Gear Information |
The ECU, and the speedo receive information regarding the vehicle's speed, from the Speed sender unit
The speed sender unit uses a plastic gear, that can be changed, that alters the speed at which the speed sender unit is driven
This in turn changes the speed given by the Instrument cluster, and the ECU.
A Big Thankyou goes out to Maverick for providing the Ford Part Numbers for these Gears !
The following values are available
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| Colour |
Driving Gears |
Part Number |
| Purple |
19 |
XE17271A |
| Pink |
19 |
XE17271B |
| Brown |
20 |
XE17271C |
| Light Green |
21 |
XE17271D |
| Grey |
22 |
XE17271E |
| Black |
23 |
XE17271F |
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