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| Wheel to gearing ratio; size matters | |
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| Tweet Topic Started: Dec 16 2012, 12:20 PM (721 Views) | |
| Goose | Dec 16 2012, 12:20 PM Post #1 |
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If I was an engineer I'd know this but; do larger diameter tires lower your bottom end or increase it? With my 4.27 rear end I get good highway speeds but stall too easily in first on a grade. I have no intention of changing gear ratios in my differentials but will consider a larger or smaller tire next time to accomodate this issue. |
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1963 CJ3B Roaming the woods of WNC | |
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| oldtime | Dec 16 2012, 02:43 PM Post #2 |
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Increasing the tire diameter increases both the ground clearance and the speed of the jeep. Standard 6.00 x 16" tires are 28.5" tall and the smallest that one should ever consider. I never recommend tires over 31" actual inflated diameter beacause that is the size of the rear wheel house opening. The difference between 28.5" and 31" is about 5 mph at top speed. |
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Currently building my final F-134 powered 3B . Rock Crawler using factory parts from the Willys Motors era (1953-1963) http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new | |
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| Goose | Dec 16 2012, 02:54 PM Post #3 |
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So...the taller the tire the worse for the bottom end stalling then. |
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1963 CJ3B Roaming the woods of WNC | |
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| oldtime | Dec 16 2012, 03:00 PM Post #4 |
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Taller tires has the same effect as higher gears. With taller tires your 4.27 final drive technically remains a 4.27 final drive... but it yeilds the same effect as a 4.09 final drive with standard tires. |
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Currently building my final F-134 powered 3B . Rock Crawler using factory parts from the Willys Motors era (1953-1963) http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new | |
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| Goose | Dec 16 2012, 03:39 PM Post #5 |
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I'm currently running 31/10.50/15's and they in actuality measure the same diameter as the stock NDCC's that were on it before. Since I'm already bogging at that size I'll probably go with 30/9.50's next time. Thanks oldtime. |
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1963 CJ3B Roaming the woods of WNC | |
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| oldtime | Dec 16 2012, 03:46 PM Post #6 |
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Are you currently satisfied with the 1st gear or "take off" ratio for street use ? |
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Currently building my final F-134 powered 3B . Rock Crawler using factory parts from the Willys Motors era (1953-1963) http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new | |
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| Goose | Dec 16 2012, 05:39 PM Post #7 |
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No, that's why I posted this. It chugs and I have to keep the revs high just to get going. |
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1963 CJ3B Roaming the woods of WNC | |
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| oldtime | Dec 16 2012, 05:46 PM Post #8 |
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Your transmission should be a T90 C. http://www.film.queensu.ca/cj3b/Photos/Tec...SB600Tranny.JPG |
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Currently building my final F-134 powered 3B . Rock Crawler using factory parts from the Willys Motors era (1953-1963) http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new | |
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| johnrb | Dec 17 2012, 12:10 PM Post #9 |
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Yup, the bigger the tires, the lower your first gear needs to be for easy start-up from a dead stop. As Ken (oldtime) posted above, when Willys changed from standard 5.38:1 axles to 4.27:1, they had to lower the first gear ratio of the T90 transmission from 2.8:1 (T90A) down to 3.34:1 (T90C). The math is simple, just multiply 1st gear ratio by axle ratio: 2.8 x 5.38 = 15.05:1 3.34 x 4.27 = 14.26:1 Almost the same overall ratio. This is why Ken says the ideal setup is the T90A (2.8:1 first gear), 5.38 axles and a 25% overdrive. You get the best of both worlds. With the 4.27 axles, big tires and a T90A your overall first gear ratio is not low enough for easy start-up. If you don't want to re-gear your axles to 5.38, then you need to go to smaller tires for easy start-up. Then do overdrive to gain back your top speed. Just remember that the F-head will run out of torque if your tires are too much bigger than stock. Lower gearing is the friend of the little CJ. |
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John Burch - west burbs of Chicago 1946 2A - now I have one older than me 1949 - me - no longer older than all my vehicles 1954 3B - mostly stock - fun driver and snow plowing 1957 - 6 cyl Station Wagon - hope for the future past Willys 1954 3B - no tub | 1959 3B - engine in pieces in a box past addiction - VW 1969 Bus | 1970 Westfalia Camper | 1984 Westfalia Camper | |
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| Goose | Dec 18 2012, 07:11 AM Post #10 |
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Thanks John, that makes it easy to understand. |
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1963 CJ3B Roaming the woods of WNC | |
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3:46 AM Jul 11