Dual Tire size ?

Hi
I have a 4020 john deere that I was looking at putting duals on. My grandfather and uncle farmed with this tractor and had some things (implements tires etc.) left that they didn"t sell when they retired that they used with this tractor and others they sold. There are three duals sitting in the barn one is a 16.9x34 which is the same size tire that the 4020 has which was put on when they moldboard plowed. Then there are 2 15.5x38"s that look to also be for the 4020. Is there much tire size difference between 16.9x34 and 15.5x38"s? The reason I say the 38"s look to be for the 4020 is the spacer ring is slide inside the rim and butted against a bevel and welded there so it measures the same diameter as my 16.9x34 dual. Also will it hurt anything to run a different size as duals? the tires on the tractor are new 16.9x34"s and all the duals are fairly worn. Also the only other tractors they had to farm with were a jd 2030 and 4630 and an IH 1466 which would have had completely differnt size tires on them. I have looked at tire size charts and there looks to only be a couple inch difference between the two just lookin for anybody with much experience with type of thing.

Thanks for any help

Grant

Many is the tracror that got a new set of tires, the old ones ended up as duals for temporary use in the spring... As far as different sizes, I'll yield to others more experienced.

The 16.9-34 for plowing would have been in the air unless conditions were soft, extra weight on the landside was a benefit to plowing, 16.9-34 overall dia 62" 15.5-38 overall dia 61", would travel well on the road and be there when needed in soft ground

Grant, for all practical purposes either size will work fine. The overall diameter of each tire is practically the same. If you measure the tread width you will find it is very close as well. Being you have both sizes and the appropriate "step down" dual rims for the 38" to 34", I would use the set that had the best tires on it. Usually you will run a few pounds less air in the duals. Mike

Years ago when duals were just coming into popularity the general thought was to have the duals either one size smaller than the main tires, fully inflated, or duals the same size as the main tires, only with the duals at half the pressure as the main tires. We used to see a few farmers around here with duals that were one size smaller. Jim

I remember when the dual tire deal came along a lot of questions arose and seeing the response to this question here It does not seem to have been resolved,same size? one size smaller? makes no difference? The duals on my D21 were the same size as the tractor tires and I ran them with about 25 Lbs more air than the tractor tire mainly because when plowing with this tractor I ran on the land side of the furrow. The duals on the 190XT were one size smaller because that was what was on them when they were bought at a farm sale.When working ground the 190XT was used to pull a 17" wheeel disc or a field cultivator.Ideally I think one size smaller is the answer but I"ve given two examples and basically I don"t think either is the correct answer but they worked for me.Then again my dad who started farming with mules and horses thought duals were a stupid idea.My friend back in ILL farms several thousand acres and he does it with 2 CAT tractors which are equipped with tracks,so no duals there.Several of his wheel tractors have duals and the 4440 with a loader he keeps duals on it year around.He runs one size smaller on all the wheeled tractors which are all John Deeres.

Thanks everybody for all the suggestions experience and thanks for the picture of the nice A.C. Happy thanksgiving to everyone!