| How-To
If you’re new to the off-road world or unfamiliar with the way the axles on your truck work, then you might be unfamiliar with why you’d want to bother changing the gear ratios in your truck’s axles.
Modifying the axles’ gear ratios changes the amount of revolutions of the driveshaft per each revolution of the wheel. Going to lower gears (ironically, a numerically bigger ratio) will increase the rpms of an engine (taking for granted the same diameter tire and going the same speed).
When you add taller tires to your truck or SUV, the larger circumference of the tires allows your vehicle to travel a further distance at the same engine rpm. This effect is taking better advantage of the work being done by allowing you to do more work (travel further) at the same engine rpm (theoretically using the same amount of energy).
Unfortunately, the theoretical world does not carry over to reality in some cases. Adding a taller tire puts more space between the ground and the axle center point, which lengthens the leverage arm. This decreases the amount of force applied at the end of the arm (torque to the ground at the contact patch of the tire). Making matters worse, it takes more torque (which uses more fuel) to move a heavier wheel-and-tire combo, and bigger tires are almost always heavier. Therefore, adding larger tires to your truck almost never helps economic or acceleration performance. The small exception this rule may be the diesel truck. Some diesels trucks have so much torque that the effects of adding a mildly taller and heavier tire are negligible and they can actually get better fuel economy and see almost no power loss.
To counteract the effects of larger tires, you can change the gears in your differential(s) to increase the ratio of how many revolutions of the driveshaft it takes to make one full revolution of the wheel (raising the engine rpm at the same speed).
Swapping out to “lower” gears will offset some effects of larger tires and (if you gear appropriately) will put your engine rpm versus speed close to where it was from the factory.
This will usually create the optimum balance for best fuel economy and best power while at the same time correcting the speedometer (in most vehicles), realigning proper shift points in automatic transmissions, and relieving stress on the rest of the powertrain prior to the pinion.
What does all this mean? Well, a happier YOU, really. Having the correct gear ratio for your truck and its tires will give you the best fuel economy, improve acceleration, throttle response, and your ability to maintain highway speeds.
Find What Gears To Run
After you add taller tires to your truck or SUV, It’s fairly simple to figure out what gear ratio you’ll need or what rpm you’ll be cruising at with these two formulas:
rpm= mph x gear ratio x 336 / tire diameter
(new tire / old tire) x original axle ratio = new axle ratio
Don’t Go Too Low
When changing gears for taller tires, you are heading to a numerically higher gear ratio.
The higher the gear ratio, the smaller the pinion gear. You do not want go too low in an effort to increase power to the wheels and/or relieve stress in the rest of the drivetrain. Too small of a pinion, and it will become the weak point in your drivetrain.
For many axles, pinions start to get a little scrawny at the 5.13:1 or 5.29:1 ratios.
Pictured here is a smaller 5.13:1 pinion gear (left) next to a larger 4.56:1 pinion gear (right) for a Super Duty rear axle.
While You’re In There…
If you’re paying to do a gear change, you may want to take advantage of the fact that it will be little or no cost in labor to add a locking or limited slip differential at this point. The old gears have to be removed and new ones set up regardless, and it may actually be easier and less time consuming for your drivetrain specialist to set up a new gearset around a new differential.
This ARB differential added a little more cost to the setup of our WCD gears since it required running an air line out of the axlehousing.
Thinking about changing the size of your tires?
You’re not alone—people run larger wheels and tires for aesthetic purposes.
We’ve also seen guys reduce the overall size of their tires for various reasons. However, there are some serious consequences to any changes to tire diameter.
Tire diameter, along with rear-axle gearing, plays a role in the final drive ratio, which in turn affects acceleration and overall performance. For example, a Camaro that has decent acceleration with a set of 26-inch tall tires and 3.55 rear axle gears becomes a dog with 29-inch tall tires and the same gearing. That’s why it’s important to compare final drive ratios with different tire heights before making any changes to tire diameter. By doing so, you can determine the axle gearing you’ll need to use with your new tire diameter in order to maintain (or enhance) performance. You’ll need three figures to do this: your vehicle’s current rear axle gearing, the diameter of your existing tires, and the diameter of the tires you want to use.
The easiest way to determine tire diameter is to call your tire dealer and ask for it—easy, but not always an option.
The second way is to use the following formulas and figure things out yourself. Please note that these formulas are for metric-rated passenger and light truck tires. Slicks, large off-road, and some larger, Pro Street style tires are already rated by diameter, so you don’t have to do any calculating for them.
We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula:
Tire Diameter = 2 x (section width/25.4) x (aspect ratio/100) + rim diameter
When you plug in the values from our sample tire, the formula looks like this:
Tire Diameter = 2 x (235/25.4) x (60/100) + 15
Now, let’s do the calculations:
Tire Diameter = 2 x (9.25 x .60) + 15
Tire Diameter = 2 x 5.55 + 15
Tire Diameter = 26.1 inches (round down to 26)
Once you determine the diameter of your old tires and potential replacements, you can compare the effect of new tires on the final drive ratio.
For example, if you have 26-inch tall tires with a 3.55 gear, and you want to go to 28-inch tall tires. To calculate the effective drive ratio of the new tires, multiply the diameters together, then multiply that figure by the gear ratio as follows:
26/28 = .92857142857
.92857142857 x 3.55 = 3.2964 or 3.30
As you can see, the larger tires reduce your final drive ratio to 3.30—a measurable difference. You can also do this to determine the effect of smaller tires on your final drive ratio.
Now that you know that 28-inch tall tires reduce your final drive ratio, you can calculate the gearing need to match the performance of your original 26-inch tire/3.55 gear combination. First, divide the new tire diameter by the original tire diameter, then multiply that figure by the original (3.55) gear ratio:
28/26 = 1.0769230769
1.0769230769 x 3.55 = 3.82
With 28-inch tires, you would need a 3.82 rear-axle gear to match vehicle performance with 26-inch tires and 3.
55 gears. Since a 3.82 gear is not offered for most rear axles, you will need to choose a gear between 3.73 and 3.90 to get close to the optimum.
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.08.2021 12:01
Boris Zakharov
It's no secret that the car's handling directly depends on the parameters of rims and rubber, while, if everything is more or less clear with the pros and cons of products of certain tire and brands, then in the question of the merits and demerits of certain sizes there is no such clarity. For example, many drivers cannot decide which tires to choose - standard, high-profile or low-profile. Let's try to clarify this issue.
Mazda Press Service
First, let's look at what tires are considered to be low profile. Take for example one of the most popular sizes 205/70 R16.
The second number 70 is the ratio of the profile height to the width of the rubber, measured as a percentage. If this value is below 60, the tire is considered low profile, if it is above 80, it is considered high profile.
For example, racing cars are fitted with low profile tires with a height to width ratio of 25 - 35%. At the same time, low profile tires will, by definition, be wider than standard and high profile tires. To provide enough air in the tire and, accordingly, the curb weight of the car, the creators of low-profile models are forced to increase their width.
Pros of low profile tires
Beauty
Hardly anyone would argue with the fact that low profile tires are beautiful. As the size of the rim increases, the proportions of the car become more sporty, and the car as a whole begins to look more solid. In addition, since low profile tires are more expensive than standard and high profile tires, it is prestigious to own it.
Acceleration and braking dynamics
/h).
Accordingly, such tires have the best performance in terms of acceleration and dynamics, as well as better response to control actions than analogues. In addition, due to the low profile, the car sways less at high speeds and rolls less when cornering. In addition, the braking distance of low-profile tires is shorter on average by 5-10% than that of standard tires.
Improved grip and handling
In strict accordance with the laws of physics, a wide profile increases the contact patch, which has a better effect on driving safety.
At the same time, these tires help reduce unsprung masses, making the car more responsive and predictable to drive. For example, a car equipped with low-profile tires is more difficult to skid and is more stable in a high-speed straight. Naturally, such rubber is installed as standard on many models with a sports bias.
Fuel economy
As you know, cars equipped with low-profile tires are more economical, which is primarily due to a decrease in rolling resistance and a lower weight of low-profile models relative to high-profile ones.
Durability
Low profile tires are characterized by rigid carcass, durability and endurance in aggressive driving conditions.
Cons of Low Profile Tire
Poor Ride
Since the rubber profile on the rims is much smaller than other tires (because of this, slightly low profile tires are jokingly called duct tape), it absorbs bumps worse.
When driving on bumpy roads, especially at high speeds, the driver and passengers are more likely to experience shaking. In addition, the risk of tire damage up to its complete rupture increases, as well as damage to the rim.
Suspension load
When a wheel with low profile tires hits an uneven surface, such as a deep hole with sharp edges, in addition to the risk of damaging the rim, there is an additional load on the suspension.
Since such a tire is not able to provide a high amount of shock absorption, the forces and impacts from the road are taken over by the running gear.
Hence - its increased wear and reduced intervals between repairs.
Offroad not recommended
For essentially the same reasons, cars with low profile tires are not recommended for long off-road driving.
In particular, such shoes do not like driving in mud, sand and deep snow, firstly, because of the not pronounced tread, and, secondly, when driving through bumps, there is an increased impact on the suspension and rims.
High noise level
Rigid frame and increased profile width create more noise, which is especially noticeable at speeds from 80 km/h, which is aggravated if the asphalt is broken or abrasive.
Expensive
It's no secret that low profile tires are more expensive than high profile tires. Moreover, the prices for tire fitting will also be much higher in their case.
Meanwhile, since such tires are usually installed on powerful and fast cars, increased wear occurs during sharp starts and accelerations.
Low-profile tires can also be sentenced if you regularly drive on bumpy roads and do not monitor tire pressure. Indeed, due to the design features of such rubber, low air pressure can cause deformation of the sidewalls.
Conclusion
Thus, the decision whether or not to take low-profile tires must be made based on the conditions in which the car will be operated.
If you drive mainly on highways and like active driving, these tires are your choice. And vice versa, drive along the country road more often and prioritize ride smoothness, which means it is more logical to take tires with a standard or high profile.
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But what does this number mean? The number obtained as a result of division is, in fact, the number of revolutions of the rear wheel that it will make in this gear in one revolution of the system.
So, for example, with a ratio of 40/10, the wheel will turn 4 times in one circle of the system, for 40/40 - once, for 40/50 - 0.8 times, and so on.
Having this information, let's try to tie it to the diameter of the wheels.
To find out how far your wheel travels in one revolution, remember the formula for the circumference - C \u003d ∏d (circumference \u003d 3.14 * circle diameter). For a conditional wheel of 26 inches, it will be equal to 3.14 * 26 \u003d 81.64 inches. Which is equal to 81.64*25.4=2073.656 mm= 207.3656 cm=2.073656 m. tables for bike computers. But it is easy to measure the circumference of your tire on your own, just a wheel and a tape measure, roll it on the floor and measure from nipple to nipple, for example.
Same stars, same cadence - different diameters.
To begin with, let's take for our calculations a conditional cyclist who rides with a comfortable cadence of 80 rpm, with a 34x11-40 transmission on bicycles with different wheel sizes.
In order not to overload you with numbers, let's take the maximum / minimum, that is, the ratios 34/40=0.85 and 34/11=3.09
| Speed | |||
| 29x2.2 inch (2298mm) | 3.09x2.298 = 7.1 m | 80x7.1Mx60min = 34080M/h = 34km/h | |
| 80x1.9Mx60min = 9120m/h = 9.12km/h | |||
| 26x2.1 inch (2068mm) | 3.09x2.068 = 6.39 m | 86MA 30528m/h= 30.5km/h | |
| 0.85x2.068=1.75 m | 80x1. 75mx60min=6960m/h | 0034 | |
| 24x2.0 inch (1925mm) | 3.09x1.925 = 5.95 m | 80x5.95mx60min = 28552.5km/h80x1.64Mx60min = 7854m/h = 7.8 km/h | |
| 20x1.95 inch (1565mm) | 3.09x1.565 = 4.84 m | 90x ,84mx60min=23232m/h= 23.2km/h||
| 0.85x1.565=1.33m | 80x1.33mx60minh=63840033 6.4 km/h |
As we can see from the table, with the same cadence and the same transmission, the speed of cyclists varies significantly depending on the size of their wheels. And if the 34th star in the front is quite reasonable for large wheels (I have 27.5x2.8, which is essentially the same 29 in circumference), then for folding bikes with 20 inch wheels it is clearly not enough.
Different cadence, different chainrings, same diameter.
But cadence can also be changed! Beginning cyclists ride with a cadence of about 60 rpm, while the cadence of athletes exceeds one and a half hundred! Comfortable values for most range from 70 to 120, but this is highly dependent on the level of training. Behind me, I have noticed that over the years my cadence has become noticeably higher, and instead of power pedaling in high gears, I increasingly choose to ride in lower ones with a high cadence. This reduces the load on the knees, but requires some training.
Consider the change in speed depending on the cadence on the example of a single speed bike that you might like to build yourself, but do not know which stars to choose in order to comfortably ride it around a flat city like St. Petersburg.
So, you have a trendy single/fix, light and fast, on narrow tires 700x25 (2105mm circumference), an 18 tooth chainring in the back, and you want to screw 40 km / h on it, but you are also curious how comfortable a slower one will be riding.
