Tires for JD 790 Help

[Firefly0556]

Gentleman, I need some help. I have a JD 790 2WD with turf tires front 23x8.50-14 / rear 13.6-16. I want to get some new rear tires that are more aggressive. In fact I should have never got the turf tire option to begin with but they have worked out so/so. I just have to becareful not to get in mud. They are pretty much dry rotted now and I need 2 new ones. The fronts seem to be ok. But the rears are bridge stone and the fronts are goodyear. Yeap came from the dealer new like that.

I was in JD today and of course I left pissed at their prices. Not even going to get into what they wanted to change out 2 tires and refill them. I did find out that I can go with a 12.4x16 tire. What I forgot to ask was and I assume it is a different rim for a r4 tire size of 15x19.5. Does anyone know if they use the same rim as the turf tires 13.6x16 are is it different. From what I can find online it looks like they use the same rim. I got pissed at the prices and forgot half of the stuff I needed to ask.

I have found 2 sizes in a tire Id like to try the goodyear R14. The sizes are 15x19.5 which JD says is the true r4 size for the 790 if I dont require a new rim size. The next close one is 12x16.5. Do you think the 12x16.5 will work? I did find a fire stone ag tire in 12.4x16 that will work. BUt that new r14 looks nice.

I do not want to buy new rims.

Thanks for any help.

 

[JD3520Turbo]

If your rear tires currently are 13.6-16, then a 15-19.5 tire will not fit on the same rim. You are going from a 16 inch rim to a 19.5 inch rim, so you will have to get new wheels and tires to make that switch.
The 15-19.5 is larger around than the 13.6-36. by about 1.7 inches. This could create a little more lag between your front and rear tires.
It is basically 2.6% change in relationship between your front and back tires from what you are currently running.
What I mean by lag is that if you use your same front tires, they will be covering 2.6% less distance per revolution on the fronts with the new rear size than they did with the old size. In other words, they are lagging behind the rear tires in distance traveled.
You might be able to get away with that much difference and be okay. This only makes a difference if you are locked in 4wd. You can normally get away with a 2% plus or minus on the lead and lag and still be okay.
The terms lead and lag describe the front tires relationship to the rear tires. If there is a lead, that means that the front tires will cover more distance than the rear tires will in 1 revolution of the rear tire. They use the rear tire revolution because it is the larger of the two. 0 lead/lag would mean they both cover the exact same distance, which never happens. The optimum is to always have a lead, and never have a lag. The reason is because you want front tires pulling about 1 to 2% faster than the backs. If you have a lag, the rear tires will want to push the fronts around, and you won't be able to turn as well and you damage more soil and wear the tires quicker. With a lead, the front tires are leading the pulling effort and will turn better with less soil disturbance and wear.

What front tire matches what rear tire is based on the transfer case ratio for your tractor. Since I don't what that ratio is, I used your current tires overall diameter numbers as a baseline and figured it out from there.

Your 12-16.5 looks like it is to small based on what you have now. The 13.6-16 is 38.2 inches in Overall Diameter. The 12-16.5 is around 32.7. In this case you create way to much lead and the tractor wouldn't perform correctly.

I think if you really want to do it right, I would bite the bullet and buy the rims and tires and go with the 15-19.5s.

 

[arrow]

He must have found something 3 months later. His tractor is 2wd. He can put any size he wants in the back as long as it clears his frame work..

 

[JD3520Turbo]

My bad, I missed the 2wd part.

 

[CoyPatton]

As indicated in later comments the lead/lag discussion in post 2 does not apply to the 2wd tractors. (It is critical to 4wd however).
Typically the the last series of numbers on a tire size indicate the diameter of both the wheel/rim and the open area of the tire (inner circle)
Since we are talking diameters, there will be 3.5” greater diameter on a 19.5” diameter tire compared to a 16”. Or 1.75” between the 19.5” tire and the 16” wheel all the way around if they are centered with the wheel inside the tire.