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Wheel to gearing ratio; size matters
Topic Started: Dec 16 2012, 12:20 PM (721 Views)
Goose
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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.
1963 CJ3B
Roaming the woods of WNC
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oldtime
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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.
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
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So...the taller the tire the worse for the bottom end stalling then.
1963 CJ3B
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oldtime
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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.
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
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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.
1963 CJ3B
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oldtime
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Are you currently satisfied with the 1st gear or "take off" ratio for street use ?
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
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oldtime,Dec 16 2012
03:46 PM
Are you currently satisfied with the 1st gear or "take off" ratio for street use ?

No, that's why I posted this. It chugs and I have to keep the revs high just to get going.
1963 CJ3B
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oldtime
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Your transmission should be a T90 C.

http://www.film.queensu.ca/cj3b/Photos/Tec...SB600Tranny.JPG
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
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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.
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
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Thanks John, that makes it easy to understand.
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For CJ-5 and Dauntless V6 postings, see the Early CJ-5 Bulletin Board.
The Old Willys Forum is dedicated to wagons and pickups.
Surrey Gala and Dispatcher Jeeps are under DJ-3A Dispatcher Posts.
The M38/M38A1/M606 Preservation Group has a forum.
For other CJ and military Jeeps, see the g503 Message Boards.
See the Forward Forum for Forward Control Jeeps.
To post ads for other Willys parts or Jeeps, see the WillysTech Buy/Sell/Swap Message Board.
For tech problems with other Willys models, try the WillysTech e-mail list.
The Bantam Trailer forum covers Jeep trailers.