Tire Size = Tractor Speed

Example. If the 16.9-38 gives you 4mph the 15.5-38 will give you approx.3.75mph. What will your next lower gear put you at. 1/4 mph can make alot of difference in field work.Probably alot of wok unless you can just swap wheeels with tires from one tractor to the other.

steveormary

The ROLLING CIRCUMFERANCE is the distance the time travels in one revolution, and will vary some between manufacturers and tire models within a specific manufacturer, even though the "offical" size is the same.

15.5-38 ... 184"

16.9-38 ... 198"

So... 184/198=.93

So... Changing the tractor over to the smaller tires would reduce the speed to 93% of the speed it was going with the bigger tires.

The overall diameter of the smaller tires is 28.4", versus 30.4" with the larger tires.

Once again, this may vary a bit depending on who made you tires, but this should get you close!

• Ag Tires

From the Firestone website, here's some info.
The ROLLING CIRCUMFERANCE is the distance the time travels in one revolution, and will vary some between manufacturers and tire models within a specific manufacturer, even though the "offical" size is the same.

15.5-38 ... 184"

16.9-38 ... 198"

So... 184/198=.93

So... Changing the tractor over to the smaller tires would reduce the speed to 93% of the speed it was going with the bigger tires.

The overall diameter of the smaller tires is 61.5", versus 66.3" with the larger tires.

Once again, this may vary a bit depending on who made you tires, but this should get you close!

• Ag Tires

So for every revolution of the smaller diameter tire it would travel one foot less in distance than the larger tire.

Assuming the larger tire is maybe about 4.5 feet in diameter, the circumference would be 4.5 x pi (3.1415) or about 14 feet.

So, one revolution of the smaller tire is about 13 feet, or roughly 92 percent of the larger.

The speed (distance/time) of the smaller tire would also be somewhere around 8 percent less than the larger tire, more or less. (Adjust the math for whatever the diameter of the larger tire is.) The larger the bigger tire is in diameter, the less the percentage change is.

In my opinion, it could be significant, and you may be on the right track.

Not enough to matter, drop a gear.

Allan

Ain't no smart about it....just old and have chopped a heck of a lot of corn. :>)

Allan

Maybe the poor fella has one of those "red" tractors that lacks both the power and a low enough gear!

Overall, I agree that a 7% reduction in ground speed isn't going to get you much. If low gear was 2 mph, then with the smaller tire you would be at 1.86 mph. Hardly any difference. I have seen this issue discussed before and the verdict is always that you would need a m-u-c-h smaller tire to make enough difference. Just a guess, but I would think it would take at least a 20% difference in ground speed to justify the change.

I sure hope I did't come off sounding glib or like a smart a$$ and in all fairness to Lance, I really don't know his situation or what he is up against; perhaps his tractor is right on that "invisible' line of horsepower/load.

But, overall, I've found that swapping out tires and wheels to gain a little speed advantage is a huge amount of work and also is very time comsuming for the small result gained.

The biggest thing a guy can to, to my mind, is to keep those knives sharpened up and that cutter bar tucked way up close. This act alone will usually get you up to that next gear.

As usual, just my opinion and stated as such; thanks for the hollar back,

Allan