Tractor tire size change

Help Support CattleToday:


Well-known member
Jun 27, 2014
Reaction score
Shenandoah County VA
I have a Ford 8240 4WD with 23.1x34 rear tires and 14.9x28 front tires. They are well worn and I would also like to change out the rears to 18x38 or 20x38 as I have some 16x38 rims setting around. I have read all about keeping the front-rear ratio the same, so with either change I would be off some. Now I have never seen another 8240/8340 with rears like this, which makes me wonder if a previous owner swapped them out them earlier. My local Ford/NH dealer (Forrester) also claims there was no such factory configuration. Any thoughts on this?

Also would there be any signs when running in 4WD if a tractors front-rear ratio is off?
Last edited:
the wrong ratio will put a lot of stress on the drive train. especially in working conditions where you have good traction. you will also notice a rapid wear on the tires that are slipping more. you should be able to use the rear tires and find tires that will use the same rims in front. most owner manuals list compatible tire combinations or call a ford/new holland dealer.
I don't have physical copy of the owners manual but found this online and I can't find any tire combination specs listed in it. It does have the 23.1x34 tire listed but just states for four wheel drive tractors to "always install tires of the same make model and size as those removed". As I mentioned above, the dealer does not know either.

The tractor has about 3800 hours, front are about 5-10% and rears are 30%. Manufacturers do NOT match from front-to-rear. Seems excessive to me too. I haven't used 4WD much, as I subscribe to the though that if its wet enough to need 4WD I shouldn't be out there with heavy equipment. One other observation - I left it in 4WD once coming from field to the road, it made a fairly loud whine.
I'm not going to be much help, but I did some quick online googling. Seems like 8210s,8240s,and 8340s all are basically 24" fronts with 34"rears, or 28"fronts with 38"rears. As for the loud whine, that's normal in my experience. I did have a friend that had a jd 4755 mfwd. Previous owner had replaced front wheels/tires with a different size. He used tractor primarily for tillage. Said it worked fine going forward, but it seemed like it would bind up if he had mfwd engaged in reverse. He found the correct ones and put them back on.
Appreciate the responses. I've heard horror stories of tearing out rears and trans. I just dumped 6k into this 8240 because it needed a main hydraulic pump and worse the hydraulic line for the clutch blew and it needed to be split. We had a series of expenses of late a my wife is telling me to sell it and I just might.

Another question: could the front rear ratio be determined by jacking tractor up and spinning wheels in 4wd w clutch in?
IF? 23.1-34 was the factory tire size with 14.9x28 fronts. Then it's running with a 1.33:1 lead to the front.

A 20.8-38 rear with 14.9-28 rear would give you a 1.34:1 lead.

A 20.8-38 and 23.1-34 are interchangeable.

That assumes the 23.1-34 was the original tire size.

Also 20.8-38 on a 16" rim isn't Ideal. Won't always wear the best, pull the best, and changes the advertised height.
Rainy weather and I am getting back to this. I found out the fwd ratio 1.358.

Using formula for front tire lead (front rolling circumference * ratio)/rear rolling circumference, and I picked BKT Alliance radials for comparison

23.1x34 is 0.1%
18.4x38 is 5.8%
20.8-38s is -1.1%.

All of these options are out of the recommended range of 1-5%. It seems 18.4x38s is best option, but does anyone else have an opinion on this?
Last edited:
You need to compare/measure the rolling circumference of your tires to the specs of new tires. Rolling circumference is the number you need to look at.

Just a quick glance and a 20.8-38 ranges from 213" to 218".

A 23.1-34 ranges from 209" to 213"

And 18.4r38 for 201" to 208".

None match up exactly. But a short or worn 20.8 is about as close as you'll get in my mind.
Personally I would just stick with the current 23.1-34 rear tires. I don't row crop anything so wider is better for me. So if anything I would be swapping from a 20.8 to a 23.1.
Dealer thinks that, based on serial number, it might be, or someone pieced it together as it points to a 2WD 7740.

However I see the "FWD Factor" on the tag, so that doesnt make sense.

Latest posts