| Speedometer Calibration Gears | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Since fox bodied Mustangs come with mechanical speedometers which utilize a cable that runs from the transmission to the back of the gauge panel, the speedometer will not read the correct mph after changing to either a numerically higher or lower gear. The only two ways to rectify this situation are to buy an electrical speedometer with sender or replace either the speedometer drive or driven gear, or both. (The driven gear bolts into the transmisstion, while the drive gear is actually inside the transmission. The driven gear is the most commonly replaced gear when changing gear ratios.) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| (8 tooth drive gear) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gear Ratio | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Speedometer Gear | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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2.73 3.08 3.27 3.55 3.73 4.10 |
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18 tooth - Grey 20 tooth - Orange 21 tooth - Purple 23 tooth - White 23 tooth - White 23 tooth - White |
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| Gear Ratio | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Speedometer Gear | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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2.73 3.08 3.27 3.55 3.73 4.10 |
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16 tooth - Blue 18 tooth - Grey 19 tooth - Tan 20 tooth - Orange 21 tooth - Purple 23 tooth - White |
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| Gear Ratio | Speedometer Gear | |||||||||||||||||||||||||||||||||||||||||||||||||||||
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2.73 3.08 3.27 3.55 3.73 4.10 |
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18 tooth - Yellow 20 tooth - Black 21 tooth - Red 23 tooth - White 23 tooth - White 23 tooth - White |
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| Gear Ratio | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Speedometer Gear | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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2.73 3.08 3.27 3.55 3.73 4.10 |
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16 tooth - Maroon 18 tooth - Yellow 19 tooth - Pink 20 tooth - Black 21 tooth - Red 23 tooth - White |
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| NOTE: If you have a transmission with an eight tooth speedometer drive gear and you install a gear ratio numerically higher than 3.55 there is no driven gear that you can install which will ensure your speedometer is 100% accurate. As an estimation, if you install 3.73 gears with the 23 tooth speedometer driven gear your speedometer will read approximately 5% faster than you actual speed. If you install 4.10 gears with the 23 tooth speedometer driven gear your speedometer will read 10% faster than your actual speed. The two solutions to this problem are 1) change the speedometer drive gear, which involves removing the transmission from the vehicle, or 2) installing an electrical speedometer and sender. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Electrical Speedometer and Sender: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Autometer makes electrical speedometers which can be calibrated while driving. All you need to do is select a spot which you know to be exactly two miles. (You can find a spot driving a different car with factory odometer a distance of two miles to get your reference point. What I would do is follow someone o a highway, start at the same point, then have the driver of the lead car tap his brakes when his odometer ticked off exactly two miles.) At that point all you need to do is push a button mounted under the dash and the speedometer is calibrated. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| If you decide to go this route you will need a sender that converts the turning of the mechanical speedometer cable to an electrical signal. Autometer sells this also, shown below. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| This setup would enable you to recalibrate your speedometer not only when you change gear ratios but when and if you use a larger or smaller diameter tire, which also changes the calibration of the speedometer. The problem with this setup is that the speedometer and sender cost about $200 to $250. Mounting the speedometer in the stock location is another issue, but this can be rectified as some companies sell an aluminum guage plate which fits in the dash. However, these cost around $60. The other problem is that most inspection service stations (at least here in PA) will not inspect your vehicle if you have an aftermarket speedometer and don't have the mileage documented by a speedometer shop. I'm quite sure there are ways around that, but they all seem to be a pain in the backside. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Back to Contents | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Speedometer Gear Installation | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
