Rolling Precision

How Tire Size Impacts Transmission Performance:

How does tire size affect vehicle performance, and transmission longevity? This has been a hot topic since the first set of aftermarket wheels and tires were installed on an automobile.

The picture below shows the relation between tire height and leverage, picture using two different lengths wrenches and the effect it has on turning effort. With a shorter wrench, more force is needed to turn the nut, than with a longer wrench. The trade-off is the longer wrench needs to travel further to turn the nut the same distance. With that said, when turning the shorter wrench the same distance, the nut will turn more revolutions and require more force to do so.

Our physics example applies in our tire size lesson, but the input force is reversed. The force in the example above was applied to the wrench, now the force is coming from the driveline and being sent to the axle hub to turn the tire. Which means, as tire diameter is increased less of the turning torque from the drive line is delivered to the ground. On the other side of this, the tire will travel further for every revolution.

This is the same for any vehicle, but we will discuss one of the most popular tire modifications we see on the road. That would be just about any ½ ton truck that came stock with 30in tall tires and replaced with 35in tall tires.
We are going to show the effects this change has on two different vehicle combinations of the same model truck.

For simplicity let’s use some round numbers in this example.

Vehicle configuration #1 has V-6 engine, with highway friendly rear gearing.
The engine makes 300 ft lbs. of torque, and a rear differential ratio of 3.23:1. We will say the transmission is using 3rd gear with a 1:1 ratio.
Note if you are not using 1:1 transmission ratio in your example you need to find the ratio of the gear the transmission is in and add that to the rear ratio for the final drive ratio in your calculation.

First step, we need to calculate the torque at the rear axle for vehicle #1.
That formula is: Engine torque x gear ratio = output torque.
(300ft-lbs × 3.23 = 969ft-lbs of Torque)

Second step, we calculate the torque applied to the ground. Like our example shown above, this force is based on the leverage of the tire radius. Radius is half of the diameter, and we know tire diameter is 30inches.

30in diameter / 2 = 15inches
Since we are using FT LBS, we need to divide it by 12.
15in radius / 12in =1.25ft
969ft-lbs (wheel torque) / 1.25ft (Tire radius) = 775 ft lbs. to the ground

Now we need to see what happens to torque when we add a 35in tall tire.

35in dia. / 2 = 17.5in radius / 12in = 1.45ft
969ft-lbs hub torque / 1.45ft = 668 ft lbs. force to the ground.

Vehicle configuration #2 has a V-8 engine, with a tow package that uses a higher numeric rear gear.
The engine makes 400ft lbs. and has a more towing friendly 3.73 to 1 rear gear ratio.

30in diameter tire example:
400 ft lbs. x 3.73 ratio = 1492 ft lbs. to the wheel hub.
1492 ft lbs. / 1.25ft = 1194 ft lbs. of force to the ground
35in diameter tire example:
400ft lbs. x 3.73 ratio = 1492 ft lbs. / 1.45ft = 1029 ft lbs. to the ground

Vehicle 1 started with 775 ft lbs. and dropped to 668 ft lbs.

That is 107ft lbs. lost because the tire diameter increased.

Vehicle 2 started with 1194 ft lbs. and dropped to 1029 ft lbs.

That is 165 ft lbs. lost because vehicle 2 started with more torque and gear ratio it is affected less by thetire size increase.

In fact, vehicle 2 with its bigger 35in tires still puts 254 ft lbs. more torque to the ground than vehicle 1 with its factory 30in tires!

Okay, so why is this important? Let’s say you and a buddy have the same trucks, but his came optioned like vehicle #1 and yours came like vehicle #2.

You both like the look of the truck sitting on 35’s and inevitably one of you will put the larger tires on, which, as the story has proven time, and time again will spark the other to buy 35’s for theirs.

Now your buddy with vehicle #1 is going to have a very tough time towing or hauling with his truck as it was rated from the factory. It will add much higher stress to the engine and transmission causing much higher heat load and ultimately burning up the transmission prematurely and much sooner than you with the combination in vehicle #2. So you can see how one experience may differ greatly from the other even when they are the same year make and model vehicle.

Impact on Performance

When increasing tire size and ignoring the supporting modifications such as changing differential ratio, and adjusting speedometer calibrations you will likely experience issues with:

Acceleration:

Larger tires will lead to slower acceleration. Due to the added rolling resistance and reduced mechanical advantage.

Shorter life expectancy:

The additional heat, stress, and wear to drivetrain components like the engine, transmission, and torque converter, will ultimately shorten their life. The transmission usually takes the brunt of the abuse and fails first.

Fuel Efficiency:

Depending on the driving conditions, larger tires can sometimes lead to improved fuel efficiency on the highway, but the added rolling resistance will likely eliminate any gains you may notice. The added torque/load needed to accelerate will negatively affect MPG in stop-and-go traffic.

Transmission shift points:

If calibration for the new tire size is not adjusted. The speedometer will be off and depending on where the vehicle takes the speed signal for shift scheduling, the transmission shift points will not be correct. Typically, this will cause early shifts further compounding the lack of torque because the transmission will shift into a higher gear sooner.

Shift quality:

Because there is a loss of mechanical advantage in the examples above, additional throttle input from the driver is needed to get the vehicle up to speed. Because all electronically controlled transmissions use some version of torque modeling for line pressure control this results in added fluid pressure during the shift, which causes a firmer shift. Now this may not be a bad thing if you consider the added clamping force needed to hold the clutches with the added load, however, this problem may be compounded with early shifts if not correcting the calibration or gearing. This will likely result in an unfavorable and inconsistent shift feel.
It's also worth mentioning that the shift adaptive tables will likely raise pressure because the shift times will go up with larger tires.

Summary:

You can see how using the wrong size tire can reduce performance and cause additional stress on the driveline components. You can also see the chain reaction that one change can make on the entire drive line of the vehicle.

There is a reason that manufacturers design a vehicle to use certain size tires. When they engineer a vehicle with a range of tire sizes, they take these factors into consideration and adjust accordingly. Information on the required tire sizes is usually placed on a sticker in the door jamb, or in the glove box.

If tires outside this range are used, additional modifications, typically in gearing, must be made to compensate for changes in tire size.

For additional information about performance, gearing, or modifications contact techtraining@moveras.com