How can tire size affect the tractor?

First, let’s talk about gears and how they work.

A gear is a rotating machine part with teeth or cogs that mesh with another gear in order to produce torque. Two or more gears working together can be referred to as a simple machine.

If both gears are exactly the same size, with the same number of teeth, both will rotate at the same speed. If one gear is smaller than the other, the smaller gear must rotate faster, making more revolutions than the larger gear.

For example, if the smaller gear’s overall diameter is half the size of the other gear, the smaller of the two will need to make twice as many revolutions as the larger gear.

So what does that have to do with tires?

For simplicity’s sake, let’s say the transmission is one gear, and the tire is another. If both were the same size, they would revolve at the same speed. Of course, a transmission is connected to the drive axles, because that’s what powers a tractor down the road. Remember, we’re looking at this simply; there are many more components between the two, but you get the general idea.

When you spec a new tractor, the odometer is calibrated so after X number of revolutions of the tire, it will record one mile.

Let’s say you spec a tractor with 295/75R22.5 low profile Bridgestone M726 EL drive radials with an overall diameter of 40.9" and 507 RPM (Revolutions Per Mile). That means, based on the overall diameter, for every mile that tire travels, it will revolve 507 times.

Now you can get an idea of what happens if you change tire size.

Let’s say you buy another fleet and they use 90-series tires. A consultant tells you to minimize inventory costs and streamline parts. Luckily being a truck guy, you know you’d have a big problem on your hands.

How so?

The odometers would be off. The speedometers would be off. And your drivers, who swear they were driving the legal speed limit, would produce a stack of speeding tickets. But only if you switched over to 90-series tires.

The 90-series is a larger tire, so consequently, its RPM is a smaller number. In the case of the 11R22.5 M726 EL, the RPM is 492.

Remember, the transmission is calibrated for 507 RPMs, which is only three percent or so more than 492. So after 507 revolutions, the odometer is clicking over to another mile.

But the problem is, another gear is linked to our gears – the speedometer, which was calibrated at the factory based on the low-profile tire size.  As the old saying goes, garbage in, garbage out, so while the tires are revolving quickly down the road, sprinting in fact, the speedometer is reading less than the actual speed.

But the wheel diameter on both tires is the same.

That’s right, both are 22.5. It’s the overall diameter that affects RPM. 

If you keep an eye on your speed, can you change tire sizes?

There are other factors besides registered mileage and speed to consider. Clearance is just one. It’s even possible that a slightly larger tire could rub against the wheel well when the steering wheel is at either “lock” position.

A second factor to consider is the way you calculate fuel economy. The best way to measure fuel economy is not by using the dashboard computer, but a simple calculation: miles driven divided by gallons of fuel pumped in

Anything else?

We talked earlier about transmissions. If the tire diameter changes, you can lose the “sweet spot” in every gear.

If the speedometer is correct, bigger tires can force engine RPMs to be too low and smaller ones can force it to rev too high for best performance.

Remember: as a tread wears, fuel efficiency increases. You may not see that increase yourself if it’s artificially hidden by change in tire size.

Can wearing down a drive tire affect OD?

Many fleets pull drive tires around 6/32" for retreading, so wearing down a tread a total of 26/32" Isn’t enough to influence your equipment.

Enough to see on the speedometer?

There will be a difference, but very small. You may start having problems in the speedometer and computer-estimated fuel economy, but by then, you’re certainly at the legal limit and it’s time to get off the road and change tires.

If a tire manufacturer ever introduced a drive tire with 35/32" tread depth, the change in overall diameter from brand new to completely worn could very well affect your speedometer and transmission’s sweet spot. But with fuel economy and even wear so important these days, we don’t foresee that happening anytime in the near future.

What about retreads?

Most retreads are about the same diameter as the tires they replace.