Changing tires to a new size or changing rear-end gears can have an impact on speedometer accuracy. This is because your speedometer is controlled by two small gears that are optimized for a particular combination of tire size, rear-end gear ratio, and transmission type.
The drive gear is attached to the transmission output shaft inside the tail shaft. The driven gear is attached to the end of the speedometer cable that is inserted into the side of your transmission. The two gears are available in a variety of helix spiral direction and "number of teeth" configurations to produce reasonably accurate speedometer readings given the different tire, gear, and transmission combinations.
The idea is to come up with the drive-driven gear combination that produces the smallest margin of error within acceptable tolerances. That's where this calculator comes in! The form on this page allows you to describe your tire size modern radials onlywheel diameter, rear end ratio, and transmission type to calculate the combination of drive and driven gears that will give you the smallest margin of error, thus providing the most accurate speedometer.
Part reference information is taken from the Ford Master Parts Catalog, so it can be considered accurate for Ford parts used up to that point in time. You may need to do your own homework to come up with current part numbers. Please note that this calculator does NOT do extensive error checking of the input values you provide.
Garbage in, garbage out, so please check your numbers carefully if you want to see meaningful results. Finally, note that the values used by the calculator are specific to Ford Mustangs equipped with a cubic inch engine.
The algorithms it uses can be adapted to other applications, but the part references including gear teeth counts are specific to CJ and SCJ Mustangs. In theory you should be able to use these gear combinations without a speedometer gear adapterbut that wouldn't explain why Ford used an adapter in the first place.
More research is clearly needed. Original gears can be identified by tooth count and color. Left helix gears as used in 4-speed manual transmissions are made of whitish nylon with a color-coded tip. Right helix gears as used in automatic and 3-speed manual transmissions are made of colored nylon.
Drive gears are also made of colored nylon. I believe that reproductions of the driven gears are only available in black. Here's a picture of all three types. A tooth, left helix driven gear with orange tip is on the left. A tooth, right helix driven gear in green is on the right. A 6-tooth, pink drive gear is in the middle.Gear Tire Ratio Calculator
Breadcrumb Home. Speedometer Gear Calculator. Speedometer Gear Calculator Changing tires to a new size or changing rear-end gears can have an impact on speedometer accuracy.
On to the calculator. Just enter the data as described below and hit "Calculate".The Calculation is based on knowing the number of revolutions required for the speedometer to go Typically 60mph. From what it looks like most Speedometers use a factor of Revs of the Speedo Cable and equates that to 60mph. Your mileage may vary. I think most electronic speedo's work that way as well. Don't know how this maps to you metric folks, but if you know send me an email and I'll fix it up!
One is a nice cut away of a transmission showing how they work in a GM transmission.
Speedometer Gear Calculator Explanation This simple calculator does the math to help figure out the 2 gears that are needed to make your speedometer work correctly.
Many speedometers have a given RPM they must turn at 60mph to be correct. This calculator allows you to find a close gear set that will make it all happen. What you will need to find is the magic number that your speedometer really needs to turn. For electronic speedometers this seems to be the common case. Other values are said to be stamped on the back of the Speedometer itself. One other thing to watch out for is the size of your tire. So if you have a 'Computed' Diameter of The output from the calculator is a floating point number which is done to give you a feeling of whether to round up More teeth or round down Less teeth.
What is the tire size you plan to use? What is the axle ratio? Do you know the tooth count of the speedometer drive gear in the transmission? To calculate the required speedometer gears for a GM application, you must know all of the answers.
It has an aspect ratio of 60, which is the ratio of sidewall height to section width. The final number, 15, is the actual wheel diameter. Next, you find the sidewall height. You multiply the section width by the aspect ratio.
In our example:. Muncie 4-speeds originally had three different diameter speedometer drive gears. The six-tooth gear no longer available had no driven-gear options except for one steel 20T pencil gear. General Motors discontinued the 1.
Because the 1. AGE makes additional 7- and 9-tooth gears to work with these standardized pencil gears. It is now possible to calibrate your speedometer for a wider combination of rear axle and tire sizes. So, if you have an eight-tooth drive gear and use the above equation with an axle ratio of 3. If you use a large-diameter eight-tooth gear, you may want to switch to a seven-tooth gear.
Because a seven-tooth 1. These are the drive gears. From left to right: 1. This gear has eight teeth and is 1. Notice that the diameter of the blue gear is smaller than that of the yellow gear. Small-diameter pencils work with 1.Speedometer Gear Sales, Click Here. To determine your tires true diameter in relation to determining what speedometer gears you need, measure from the ground up to the center of the axle then multiply by two.
The actual run out of the tire can be as much as 2 to 3 inches less than what the circumference of the tire measures.
Gear Ratio Calculator
In effect your tire has two diameters. To determine your rear end ratio, raise both tires off of the ground, turn both tires at the same time one complete turn, at the same time count the turns of the drive shaft, this is your ratio.
Most cars and trucks with low rear end ratios will take a 15 tooth drive gear. There are two driven gear housings, one for 34 through 39 tooth driven gears and one for 40 through 45 tooth driven gears.
Look a t the bottom of the speedometer face for the numbers,or These numbers are the number of revolutions the speedometer cable must turn in one mile of travel. Your speedometer may or may not show a number. Most speedometer from on use The number on the driven gear housing must match the number of teeth on the driven gear. Reviews There are no reviews yet. Only logged in customers who have purchased this product may leave a review.
Drive Teeth:. Axle Ratio:. Tire Diameter:. Axle Ratio. Tire Height. Driven Gear Teeth.No guessing with this Mustang Speedometer Gear Calculator! Check out our YouTube channel for even more tech tips, installation videos, how-tos, and more.
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Local Will-Call pickup services are temporarily unavailable. Show More Show Less. Share this article! Rear End Ratio 2. Tire Width Tire Series 30 35 40 45 50 55 60 65 70 75 80 Rim Diameter 12 13 14 15 Calculate Speedo Gear. Fetching speedo gear Speedometer Gears.
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Speedometer Gear Calculator
This is the perfect speedometer gear tutorial for your SN and Fox Body Mustang and best of all you don't even have to remember the calculation formula. Read the article in the link above for speedometer correction for all Mustangs. Be sure to bookmark this calculator and share it with your friends! We carry a wide selection of Mustang speed correction parts, We also have gear kits available to save you time and money.
Transcript - If you are changing out the rear gears on your fox body or SN95 Mustang, you are going to have to correct your speedometer. The most popular method for doing so is by swapping out the driven gear on the end of the speedometer cable or speed sensor. You can select your transmission, manual or automatic, your drive gear teeth, 6,7 or 8, pick out which gear ratio you're using and then select your tire size.
Click the calculate now button and we'll tell you exactly what you need to correct your speedometer. These internal drive gears are pretty difficult to swap out requiring you to remove the tail shaft review transmission.The speedometer accuracy is maintained by selecting the correct size driven gear fitted to the gearbox end of the speedo cable - each gearbox has a number of different sized and colour coded gear combinations available to suit nearly any combination wheel and differential variations.
Selecting the correct gear combination can be very difficult, and the calculator below will make this much easier. To use the calculator, you must first gather some relevant information. You will also need to know the ratio of the differential - this is usually stamped on the oil seal cover just behind the pinion where the universal joint bolts on at the front of the diff. You will also need to know what gearbox you are using and what coloured gear is fitted to the output shaft in the gearbox - you should be able to see this by removing the speedo cable from the gearbox and looking in the hole with a torch.
Select the relevant options on the calculator according to the information above, and if known, also select what coloured driven gear you currently have on the speedo cable - the "Speedo Details" section will reflect your speedo's accuracy. Once you have found the gears combination that provides the highest accuracy, the original GM part numbers of the selected gears will be displayed - as most of these are no longer available from Holden, the equivalent Rare Spares part number is also given if this is blank, then they do not have an equivalent.
Tyre Details Width mm : Profile : 85 80 75 70 65 60 55 50 45 40 35 30 Wheel Diameter inches : 23 22 21 20 19 18 17 16 15 14 13 12 10 Differential Details Ratio : 2.Use our speedometer calculator to find the differences in speedometer reading and actual speed when changing to a tire size other than stock.
Speedometer calibration, or knowing how fast you're going, can be pretty important. We all know a ticket is about the biggest buzz kill on earth! So if you're about to get bigger tires than stock or already have them installed, its time to calculate your true MPH. Your speedometer is reading how many revolutions your tires are making not how fast they are spinning. When you go to bigger tires the circumference of the tires is larger, meaning the distance around the tire is longer.
Thus, with each revolution you are traveling further than before. Therefore your actual speed is higher than your speedometer reads. Or lower if you have gone with a smaller tire than stock. Our speedometer calibration calculator will get you the answer you are looking for. Enter your original tire size, your new tire size, and then play with the miles per hour to see how much faster you are going at different speeds.
If you would like to see more speedometer calibration speeds at a time, visit our tire size comparison page to compare different metrics of tires and get multiple speedometer calibration readings at once. Speedometer Calibration Tire Size Chart for Speed Calibration Tire Size Charts by Diameter 21" 22" 23" 24" 25" 26" 27" 28" 29" 30" 31" 32" 33" 34" 35" 36" 37" 38" 39" 40" ". Tire Size Tools. Tire Categories. Popular Sizes.