overall wheel size/ gearing ratio formula

[CRegal]

Hey folks,



I am interested in finding the formula for overall gear ratio with different overall size wheels.



I found a place out of Texas www.chromewheel.com that makes an adapter to take the hubs from 8 lug to 10 lug. This is similar to the rickson wheels but they only have 8 to the larger 8.



My interests was of course..... how bloody big can i go b 4 I run our of top gear ratio. I have a 4. 10 rear end & was looking at an overall hight increase to 36". (22. 5" alcoa wheels w 245/70/22. 5 tyres)



I am afraid to spend that kind of loot and find out 6th gear is useless & i should of went with the 19. 5" rims. If anyone can lend me a hand in figuring that out... or point me to the right formula, I would be glad to post pics of the MacMassive pics if it will work.



thanks



Cole

 

[MUDDY]

cole,



36 inch tires would be ideal for your 4. 10 gears. it would be like running 3. 47 gears with the 30. 5 inch 245s.



4 lo . com has some good calculators for comparisons on ratios and tire size.



http://www.4lo.com/



mm

 

[CRegal]

SSSSSSSSSSSwwwwwwwwwweeeeeeTTTTTTTTTTTTTT!!!!



spanks mm... ... I knew i could count on a mudder... ... . hehe

 

[Alan Reagan]

Hmmmmm. I assume you want to calculate before you buy.



Lets say that you are currently running 265/75 R16 tires.



265 (width) X . 75 (aspect ratio) = 198. 75 mm (height) divided by 25. 4 (mm per inch) = 7. 83 inches (sidewall height) x 2 (height on each side of rim) + 16 inches (rim diameter) = 31. 65 inches tall.





245/70 R22



245 X . 7 = 171. 5/25. 4 = 6. 75 X 2 = 13. 5 + 22 = 35. 5 inches tall.





New ratio delta equals new diameter divided by old:

35. 5/31. 65 = 1. 12. 4. 10(original gear ratio) / 1. 12 (delta ratio)= 3. 66 effective gearing.



Let's check the work another way.



31. 65 (dia) x 3. 14 (pi) = 99. 38 inches circumference

35. 5 (dia) x 3. 14 (pi) = 111. 47 inches circumference



111. 47 / 99/38 = 1. 12. 4. 10/1. 12 = 3. 66 effective gearing.



Check.



Muddymess is correct on the 36 inch tire if you want to calculate it. The 22 inch tires may be that tall. I have provided theoretical values based on accepted tire sizing standards. Real tires may vary by manufacturer.

 

[MUDDY]

ok, you have a drw rig. you would have tires in the 215/86 size which are approximately 30. 75 inches tall, give or take 1/4 inch.



the optional size was the 235/85 16 at approximately 31 and 3/4 inches tall, to 32 inches tall.



these measurements are taken from an unmounted tire laying on its side measured across with two squares upright.



lets assume you are running a 235 tire at 31and3/4 inch height.

lets say you go with the 245/70 22, at 35. 5 inches tall.



at 65 mph your rpm would be, with an od ratio of . 80 and 4. 10 gears, approximately 2259 rpm, with the 235 tires.



at 65 with the 35. 5 inch tires, it would be approximately 2017 rpm.



a 242 rpm difference. not enough to worry about especially with the 6 speeds close ratio gears and the high torque the cummins gives you at reasonable rpm.



you can calculate this by taking the given speed, times the gear ratio, times the top gear ratio, times the magic number of 336, divided by the tire height.

the way to be precise in the calculation in regards to the tire height is just measure the distance between the ground and the center of the hub, use that number instead of the tire height number and use 168 instead of 336. this would be more precise because it takes into consideration the weight of the truck squashing the tires down.



one other factor to consider is the weight of the new tires and rims compared to the old ones. that would be something to think about too.



anyway, imo, the taller tires should work well with the 4. 10 gears and the good spacing of the 6 speed. it would be nice to have that extra ground clearance and a better load rating with tougher sidewalls that would hold up better in the rocks.



if you do this, dont forget to post pics.







mm

 

[CRegal]

Ok that is alot of info... ... . but I believe mm is correct in saying the 6 speed would fair well with the 4. 10. I notice now compared to my 5 speed w/ a 4. 10 that it is a much closer gear ratio then the 5 speed, making everyday driving more of a chore. So a easy option would be the larger tyres.



I spoke with Burnie from chromewheels out of houston today on overall price. This is what he has offered me.



4 alcoa alumin 22. 5

2 steel (inner) 22. 5

6 Dunlop 245/70/22. 5

all the accessories

8 lug to 10 lug adapters

for a whoping tag of $4300

19. 5' would b $4000



Now I did speak to dual designs and there price on the 19. 5" kit was $4846. I do believe i will go with the larger rims and tyres for less, due to tyre longevity and well simply kewler looks.



Only thing is... ... I am short $500 Smackers... ... .

anyone need a set of 6 brand new rims & tyres w/ 1k miles???

 

[Alan Reagan]

Hey, I agree the taller tires will work better if you desire a higher speed gear. I didn't see where you have a 4X4. When I tried to go to 33 inch diameter tires on my 98, they were not going to clear the fender wells on a turn. Better keep that in mind. MM is correct and thus my disclaimer about theoretical versus actual tire size. My calculations will get you into the ballpark on size and gearing. Muddymess will be almost exact.