road speed, 18.4-38 vs 20.8-38
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Category: Allis Chalmers
Forum Name: Farm Equipment
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=143123
Printed Date: 19 Aug 2025 at 4:36am
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Topic: road speed, 18.4-38 vs 20.8-38
Posted By: nick121
Subject: road speed, 18.4-38 vs 20.8-38
Date Posted: 04 Oct 2017 at 6:21pm
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just wondering if the tire size makes much difference in travel speed? were 18.4-38s factory standard on a 7040 and 20.8 an option? Thanks
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Replies:
Posted By: Gerald J.
Date Posted: 04 Oct 2017 at 7:20pm
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According to my Titan/Goodyear Ag Tire Handbook which I downloaded from the Titan ag tire page a couple years ago, the rolling circumference of the 18.4-38 is 207 inches and the 20.8-38 is 215 inches. So the 20.8 will be 3.86% faster than the 18.4 or conversely the 18.4 speed will be 96.3% of the 20.8 speed. Gerald J. |
Posted By: AC7060IL
Date Posted: 05 Oct 2017 at 3:37am
| Traveling speeds change. Also remember that the 20.8s will raise the 7040's drawbar 2-3 inches taller than the 18.4s. This drawbar height change can really play into leveling/setup of certain trailing implements (ie AC1300 field cultivator). |
Posted By: nick121
Date Posted: 05 Oct 2017 at 7:10am
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thanks Gerald good chart for future reference. Ac7060IL It is on a tractor i'm going to look at.. current one I own is on 18.4-38, should not make a difference for my equipment accepct might have to adjust hitch on the 2500 model disc. Thanks for mentioning that though.
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Posted By: Tbone95
Date Posted: 05 Oct 2017 at 7:12am
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Won't it depend a little on what rims you put the tires on too? I make this statement because of a situation I just ran into. I have 18.4 38's on my tractor, and never paid much attention to the rims to be honest, as everything was left as I bought it. One of my rims rotted out and got one from a neighbor who had a junk 7020 with 18.4's on it. Come to find out, I had 18 inch rims and he had a 16 inch rim. So now my tractor sits a tad uneven, the tire mounted on the 16 got a little taller than it was. |
Posted By: Gerald J.
Date Posted: 05 Oct 2017 at 9:19am
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According to the handbook I referenced above, the 18.4-38 preferred rim is 16" but a 15" is acceptable. 18 is too wide but is the rated width for 20.8 -38 tires. The wide rim will make the 18.4 squat. Gerald J. |
Posted By: Tbone95
Date Posted: 05 Oct 2017 at 9:28am
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Yes, now that one of the rims is a 16", it "looks right". When I get rich, i'll get another 16" for the other side.  Or I'll get even richer, and get a new 18"rim and 2 new 20.8 tires!! Yeah, that's it!
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