How do you calibrate a jeep speedometer for bigger tires?

Speedometer calibration is a method used to compare the speedometer reading against the speed at which you are driving. Speedometer calibration is done on all motorized vehicles but in this case, we’re going to calibrate a jeep speedometer for bigger tires.

What most wrangler owners don’t put into consideration is recalibrating the speedometers after undertaking modifications on their Wranglers and the speedometer is calibrated according to the wrangler’s factory weight and the stock components of the jeep wrangler 

You need to recalibrate your wrangler after you do any cosmetic modifications that add weight to the vehicle as speedometer calibration takes into account the gearing ratio in the drive cable, tire size, and the driving ratio in differential all of which have an impact on the accuracy in the speedometer.

The accuracy decreases especially if you change the tire size since with one complete axle turn, the stock tires will make one complete revolution but with larger aftermarket tires one complete rotation of the axle will result in less distance covered.

When you notice this, you feel the urge to accelerate even more and your speedometer reads high speeds and yet you are driving at lower speeds. Speed is a value of distance traveled divided by time and speedometers measure distance traveled by tire rotation and due to the alterations caused by the bigger tires the speedometer reading will be lower. 

The driving ratio in the differential refers to how the throttle response is while accelerating or braking. The added weight on your wrangler will lead to a slower throttle response. The weight change will have an impact on the speedometer calculation of speed due to the incorrect driving ratio. 

What will happen if I drive with uncalibrated speedometer

• You might get an over speeding ticket on highways that have a speed limit. When your speedometer is inaccurate and you’re driving at a greater speed than what the speedometer is indicating and mostly likely above the indicated speed limit for that highway thus you end up getting an over speeding ticket. 
• Losing control when driving. This mostly happens to new drivers who need to look at the speedometer to know what speed they are driving at.
• When driving, you need to consider specific speeds, especially when cornering or when driving in bad weather conditions.
• Driving at greater speeds in such conditions will lead to the driver losing control of the vehicle especially in larger vehicles such as the jeep wrangler. Even though your speedometer is uncalibrated ensure that you are driving at low speeds. 

How to check if my speedometer is working fine

• Find a highway with markers or a stretch of road with markers, take a friend with a stopwatch, to measure time for distance traveled, with you.
• The markers indicate the distance between them. Drive at a constant speed and ensure that the speedometer is reading is constant for the entire drive.
• After the drive, divide the time taken to cover the distance indicated by the markers to get the average speed. If the resulting average speed is different from the speed that the speedometer was showing when you were driving, then you need your speedometer calibrated.
• If the result is off by a little bit there is nothing to worry about as long as you keep that discrepancy in mind when driving so as not to over speed. 

How to calibrate the speedometer in jeep?

Bigger tires are the main influencing factor leading to speedometer inaccuracy In jeep wranglers as this is the most common modification done by wrangler owners. Other additional weight added by the tires or lift kits does not affect speedometer reading as much as the tire size does.

All stock jeep wrangler speedometers are mechanical speedometers but in case your wrangler has an aftermarket digital speedometer, instructions will be provided on how to calibrate it too.

How to calibrate speedometer using quickcal recalibration device

A quickcal speedometer calibration device is a device that helps you recalibrate your speedometer easily and quickly to match with your larger tires.

• A quickcal recalibration device comes with a cable that you plug into the obd2 port (onboard diagnostics) port that is located on the lower part of the dashboard, near the door on the driver’s side.
• Once you plug in the cable, the quickcal recalibration device will turn on then it will turn off.
• Turn your wrangler ignition key to halfway but do not start your engine. This is for the quickcal device to detect the type of vehicle you have
• Once it has done so the next screen that appears will prompt you either to fix the speedometer, show GPS speed, clear DTCs and restore
• Select the fix speedometer option, turn on your engine and drive at the speed that is indicated on the quickcal device
• When driving you will notice that GPS speed is different from the vehicle speed indicated on your speedometer and the vehicle speed indicated on the quickcal is the same speed as your speedometer reading. The GPS reading should be what your speedometer reads
• Once done driving at the speed that the quickcal device had indicated, the device will prompt you to choose whether you want to apply the new settings on your wrangler
• Before selecting any option make sure your wrangler is completely stationary and select “yes”
• Turn your ignition off and press “ok” and then turn your ignition on and press “ok”. Turn ignition on again and press “ok” then turn ignition off and press “ok”
• Wait for the time indicated on the quickcal device for it to upload the new settings then turn the ignition on and press “ok”
• A prompt will appear “clearing DTCs”, after it is done, turn ignition off and press “ok”, a prompt will appear informing you that your speedometer has been recalibrated and you are clear to unplug the device.

NOTE: Calibration Device only works with 1 vehicle. It can not be used with any other vehicle if it’s already tuned.

Calibrating a mechanical speedometer in jeep wrangler

• Counting the number of teeth in the drive gear.
• Locate your transmission. To easily locate it, put your wrangler in parking gear and ensure it’s parked in a safe place, jack it up insert jack stands under the frame and slowly lower your wrangler to rest on the jack stands.
• From the bottom of your car, locate the speedometer cable. It is normally bolted on the tail shaft of the transmission twist it or unscrew it off. Remove the gear housing by removing the screw that holds it in place using an adjustable spanner as this makes it easier.
• The drive gear is the one that is attached to the transmission and the driven gear is attached to the speedometer cable. Count the number of teeth in the drive gear and note down somewhere so as not to forget. 
• Calculating the number of revolutions per mile of the tires.
• Measure the diameter of your wheel and divide 20,168 by the diameter of the tire to get the tire’s revolutions per mile. 
• Replacing the old driven gear with a more suitable one for the tire size you have.

Multiply the tire revolutions per mile by the number of teeth in the drive gear. Multiply the value you get with the axle ratio which is normally found in the owner’s manual.

Then divide the value you get from multiplying the results from the two calculations by 1001. The number you get is the number of teeth that the driven gear needs so that the speedometer indicates accurate speed.  

Calibrating a digital speedometer 

When buying new driven gear, you should put the following into consideration 

1. Number of teeth that you need in your new driven gear. 
2. Make of your wrangler. 
3. Model of your wrangler. 
4. Year of manufacture of your wrangler.

Once you get it replace the old driven gear with the new one and return everything as it was. To measure the accuracy of your speedometer, use the method described earlier. 

How to calibrate a digital speedometer in jeep?

This is easier to calibrate compared to analog speedometers. 

• To calibrate your speedometer park your jeep wrangler in a flat spot and turn the engine off. 
• Digital speedometers have a speedometer calibration button or function in the speed display panel push the button or engage the feature.
• Drive the recommended distance and after driving the speedometer will automatically calibrate itself. 

Digital speedometers are better than analog speedometers, not only in that they are easier to calibrate, they also don’t get damaged easily and are more accurate than analog speedometers.

Upgrading to an aftermarket digital speedometer can be a costly affair as you also need to undertake other modifications to your wrangler but it’s worth it. 

Other issues that lead to inaccuracy or failure of your speedometer

• Faulty speed sensor – The speed sensor accounts for some functions, measuring speed being one of them. Any slight damage to the speed sensor will cause inaccuracy in the speedometer. When the speed sensor is faulty the “check engine” light comes on when accelerating. If the check engine light does not come on, the following are some of the signs you should pay attention to. 

1. The transmission is slow to get into top gear. 
2. The overdrive light turns on and off. 
3. High revs before changing gear.

If you notice the above issues you need to take your car for a computer diagnostic before directly going to a repair shop to get to know the main cause of the issue.

• Damaged electrical components powering the speedometer- the speedometer is just like any other electricity-powered component in your wrangler and when the wiring or the fuse powering the speedometer gets damaged it eventually leads to the malfunctioning of the speedometer.  
• These are the gears that drive the speedometer’s needles. If these are damaged there’s no way you’ll know the speed you’re traveling at. This situation is ignored as most drivers tend to think it’s another issue and end up spending more trying to figure out what is the issue.
• It is recommended that you first check the speedometer for physical damages before other things, that way you’ll save cash. 
• Faulty engine control unit- the engine control unit controls various engine functions one of them being acceleration. You may notice that your engine revs slowly and a slow rise in the speedometer needle. If this happens get a complete computerized check of all the sensors and replace the faulty sensors. 

Conclusion 

You should calibrate your wrangler’s speedometer the moment you notice there is an inaccuracy in the speed being indicated by your speedometer but before doing this check other factors that might lead to the inefficiency or failure of the speedometer.

If you are caught going past the speed limit but your speedometer is faulty you should get a faulty speedometer ticket which is applicable in some states.

The faulty speedometer certificate indicates that you were not driving above the speed limit and you do not need to pay for the ticket but this is only a one-time solution. Getting your wrangler’s speedometer calibrated is the only permanent solution.