How to Find the Gear Ratio of Vehicles

by Contributing Writer; Updated June 12, 2017

The Vehicles has been equipped with a number of different gear ratios in its long history. Calculating the gear ratio is important to determine whether the original gears have been changed. It is also important to determine whether to increase or decrease the existing ratio by changing to different sized gears. The larger the gear, meaning the larger the ratio, the better the gas mileage Vehicles high speeds because the engine runs slower. A smaller gear will increase acceleration, but gas mileage will suffer Vehicles high speeds as the engine runs faster.

Under The Hood:

 How to Find the Gear Ratio of a 1994 GMC 1500

Examine the inside of the vehicle’s glove compartment and locate the large square-shaped RPO label. The label contains several three-digit codes that indicate the trim options installed on your truck, as well as information relating to the drivetrain.

Search the label carefully for codes beginning with the letter “M” and “G”. Write the codes down on a piece of paper. The RPO code beginning with “M” relates to the transmission installed on your vehicle, while “G” codes represent the drive axle(s).

Open your computer’s Web browser and navigate to the GM-Trucks.com “RPO Code Lookup” Web page (See Resources). Enter the RPO code beginning with “G” into the “Search for:” box displayed on the Web page and click on the “Search” button.

Examine the results displayed on the Web page relating to your vehicle’s axle ratio.

Navigate to the VibrateSoftware.com “General Motors Corporation Transmission Gear Ratios” Web page. Examine the left-hand columns marked “RPO” to locate the matching “M” RPO code relating to your vehicle. The columns positioned next to the appropriate RPO code will list the gear ratios for the vehicle’s transmission.

 How to Find Out Your Gear Ratio on a Ford Mustang

Measure the diameter of either rear tire with a measuring tape. Record the diameter in inches.

Drive the Mustang at a steady speed and record the speed of the engine as indicated by the tachometer and the speed of the car as indicated by the speedometer. Record the car's speed in miles per hour rather than kilometers per hour.

Multiply the speed of the engine by the tire's diameter. For example, if the speed of the engine was 2,500 rpm and the diameter of the tire is 26 inches, the result is 65,000.

Multiply the speed of the car by 336. For example, if the Mustang was driven at a speed of 50 miles per hour, the result would be 16,800.

Divide the result found in Step 3 by the result found in Step 4 to determine the gear ratio. Here, 65,000 divided by 16,800 equals 3.869. Thus, rounded up, the Mustang's gear ratio is 3.87:1.

Items you will need

  • Measuring tape

 How to Find the Gear Ratio on a GM Rear End

Raise and support the rear of the vehicle with jack stands. Read the metal tag attached to one of the differential cover bolts. This tag, which is always present (if not removed by a previous owner), will indicate the gear ratio that came with the vehicle originally. If the vehicle was purchased secondhand, this tag may not be valid if the previous owner has changed the ratio, and you will have to remove the cover to find the gear ratio.

Place a drip pan under the differential cover. Remove the differential cover (it simply unbolts) so you can determine the exact gear ratio and whether the differential is a limited slip or an open differential.

Count the teeth on the small gear in the forward part of the axle housing, called the pinion gear. Count the number of the teeth in the big ring gear attached to the differential. Divide the number of teeth on the pinion into the number of teeth on the ring gear. This is the gear ratio.

Clean the gasket material from the differential cover (use the gasket scraper). Wipe the mounting surface of the cover with a cloth. Apply a 1/8-inch bead of RTV sealant around the circumference of the cover. Allow the sealant to skin-over for 10 minutes before installing the cover.

Install the cover and the bolts. Tighten the bolts with the 3/8-inch socket and ratchet. Remove the filler plug on the side of the axle housing above and to the rear of the pinion. Fill the differential with the two bottles of differential oil.

Items you will need

  • Floor jack

  • Jack stands

  • 3/8-inch drive ratchet

  • 13mm 3/8-inch drive socket

  • Tube of RTV silicone sealant

  • Gasket scraper

  • Drip pan

  • 2 bottles of differential oil

  • Common screwdriver

 How to Find Out the Gear Ratio With the VIN Number

Find the VIN of your vehicle. It can be found on the sticker on the inside of the driver's side door or on a stamped metal plate embedded in the dashboard above the steering wheel. It also is on your vehicle's title.

Identify the VDS section, the fourth through eighth digits of the VIN. This section typically includes information such as the vehicle model, engine size and body type. While not every vehicle manufacturer includes gearbox ratio information in the VDS, this will be where the information is found if it was included.

Find a guide online, such as Edmunds.com or Decode This, or contact an automobile dealer to help interpret the VDS. The order of meaning in the digits and the type of information included varies by manufacturer. Edmunds.com and Decode This are linked in the Resources section.

 How to Find a Gear Ratio for Racing Cars

Setting the right gear ratios for a specific track

Step one the Control Lap:

Take your car out as is for the first lap. It is important to run as hard a lap as possible so that you will know exactly what needs changing in relation to each turn. When running check to see what gear you are using for each corner and straight trying to almost memorize the track. The most important thing is to look at engine RPM at each corner. This can be done either by checking your tachometer while racing or more accurately if you have the ability to record lap telemetry.

Adjusting the Gears:

When going over the telemetry or doing it by memory, remember how many RPM your engine was at at each corner. The idea is to make sure you reach maximum RPM just as you are about to brake into a corner. This allows for the maximum acceleration on the straight before having to let off for the upcoming corner allowing the car to make as fast a lap as possible. This will also help set up a braking point once your ratios are set correctly. Once you hit maximum RPM you know it's time to let off and begin braking for the upcoming corner.

When adjusting the ratios, a bigger ratio gear will allow you to stay in the gear longer and achieve a higher speed in that gear. If you find that you are having to up-shift near a corner or hitting the rev limiter a few times, you will want to adjust to a bigger ratio for that gear.

If you find that you are not at maximum RPM before entering a corner the solution is to set a smaller gear ratio. Although a smaller ratio means that the car can not achieve as high of speed in that gear, it will allow the engine to reach higher RPM quicker thus making the car accelerate quicker. Setting a smaller ratio is usually a little trickier than a larger because although a smaller gear will help accelerate if it is set too small it might reach top speed before you are at the braking point. The trick is to decrease by small increments and to balance the top speed with the ability of the car to accelerate to that speed.

Repeat:

Go out for another lap with you gears adjusted and see what has changed. You might need to go back and vary them again if it is not set up correctly. Change ratios in small increments to make it as accurate as possible. On this lap, you might notice that while one set up is good for one corner but it might not be the best for another. This is a common occurrence in racing especially when planning out which racing line to take when driving. Try setting it up both ways and also try a medium or "compromise" setting and then decide which is better to use based on other corners and the over all lap times produced.

Items you will need

  • Race car

  • Adjustable gear ratios

  • Race tack

  • Ability to record lap telemetry is possible

 How to Find a Gear Ratio for a Chevy Truck

Drive the truck at a steady speed and write down the engine’s revolutions per minute or “RPMs” as noted by the tachometer, as well as the speed the truck is traveling, as measured by the speedometer.

Measure the diameter of one of the rear tires with a tape measure.

Multiply the number of RPMs found in Step 1 by the tire’s diameter. For example, if the tire’s diameter is 28 inches, and if the RPMs noted in Step 1 was 2,600, multiply 28 by 2,600 to reach a result of 72,800.

Multiply the speed of the truck noted in Step 1 by 336. For example, if the number of RPMs was 2,600 when the truck was driven at 60 miles per hour, multiply 60 by 336 to reach a result of 20,160.

Divide the result found in Step 3 by the result found in Step 4 to determine the gear ratio. In this example, 72,800 divided by 20,160 equals 3.61. Thus, thus truck’s gear ratio is 3.61:1.

Items you will need

  • Tape measure