3.54 gears and 36" tires

[Bill_L]

Before everyone starts I know now that I should have just ordered the 4. 10s when I ordered my truck.



But when I placed the order I got the 3. 54 gears thinking that I was only going to be running 33's or 34's and wanted to get decent mileage.



Well now I am going to 36's and I know that the rpm will be low. I am wondering if it is going to be too low.





The diesel is a low rpm motor but I am wondering if I will be working it too hard to push those tires.



A friend of mine with a gaser has 3. 54's with 35's and it isnt too bad but I am not sure with the diesel.



With that tall of a tire are 4. 56s too much?

 

[SOOT]

A friend of mine has a 99 2500 with 5 in lift and 36" super swampers with the 3. 54. It makes the gear ratio about a 2. 83. So at 75MPH he turns about 1900RPM with a 5 Speed. His does fairly good, except for the shift from 3rd to 4th.



My 99 is identical to his except for the lift and tires, and at 75MPH I turn 2300RPM with a 5 speed.



I got to race him in the 99 (see sig) last night after he installed his Banks Git Kit. And guess what? I WON both times. Not by much but I still beat him!

 

[Craig DeYoung]

Bill,



I'm running 36" Coyotes and 3. 54 gears. They are 20% taller than my stock 245/75s. In terms of the torque applied to the drivetrain, my setup (and yours) should be equivalent to a stock truck with a 20% payload increase over the rear wheels. In my case, my truck is 6400#, about 60/40 weight distribution, so 3840# front and 2560# rear. My current unloaded setup is equivalent to my stock setup with an extra 512# (20%) over the rear axle. Since 6400+512 is less than 8800 (my GVWR), I'm not too worried about overstressing the drivetrain. I just have to realize that my payload capacity is somewhat decreased.



Craig

 

[Walker]

Craig,

Your theory/formula sounds good, but is that how it really works for larger tires? I'm not sure, just asking. It seems like rolling resistance at the wheels would be different than the down force of a bedded load. I hope you're right because I want to run 315's (35's) and would like to keep my 3. 54's hoping they will be enough to move the new set up without robbing a lot of power or mileage.



As for your lift. What kind of 3" lift do you have? Is it the coil spacer or new springs. I want to have enough to clear my 35's with absolutely NO rubbing, and was looking at the Skjacker 2. 5 kit. I also see you have the Skjacker rear springs. This was my idea as well. I want to put the 2. 5 springs in the front, and the 4. 5 leafs in the back so that I can keep my stock "rake. "

 

[Craig DeYoung]

Walker,



I have steel coil spacers in front. My stock springs were 039 and 038, and I swapped to 046 and 039 (stiffer) in anticipation of a heavy winch bumper, but I never got the bumper. I'm now looking at the RanchHand Bullnose which is only 170#, so I put the stock 039 and 038 back in; much more pleasant ride. I was a little disappointed with the skyjacker springs because as you indicate, they should have given me a slight rake, tail high. They were supposed to give 1-1. 5" lift over stock, but instead gave 0. 5" DROP, plus a side-side slant. I had to order another set and pick the best two of four to get level side-to-side, but still needed to use a 1" block to gain any lift (I paid through the nose for all the shipping charges). My truck is now exactly level measured on the pickup bed, but sometimes looks like the nose is high depending where I'm parked. The nice thing is the leaf springs did eliminate my axle hop problem, and the ride is pretty good. I think ProComp is now offering springs for the Dodge, you might look into them...



Regarding my analysis, I figure the torque on the axle shaft is equal to the moment arm (tire radius) times the frictional force at the road. The frictional force in turn is equal to the normal force (the axle load applied downward) times the coefficient of friction (rubber on pavement, say). So torque = radius*payload*coefficient of friction. Assuming coefficient of friction is same between different tires, multiplying the radius by some percent (120%) should give same torque result as multiplying the payload by the same percentage. I think, anyway.



Craig

 

[Walker]

Thanks Craig. You lost me on the science so I'll just believe you .



But, how's your performance with the 36's and 3. 54's? Power (tq. /hp) at certain rpm's, and mileage, handling etc. .



Thanks

 

[Craig DeYoung]

I think my fuel economy is same as stock. I'll have to wail until summer to know for sure. I did notice that inflation pressure makes a big difference in fuel economy with these tires. I wasn't getting full tread width contact in the rear, probably because I'm running a 7" rim instead of 8". When I deflated to increase the contact width, my fuel economy dropped from 19. 5 to 18. I'm running 40 psi front, 30 psi rear now (I think) load range E tires. My best fuel economy was with my 255/85-16 Dunlop Mud Rovers, running at 55-70 mph. When I run constant 70mph, all three tire sizes result in same fuel economy (245/75, 255/85, 305/85).



I don't tow (yet). My only performance upgrade is 275 injectors. I'm happy with the performance at all engine speeds in all gears, it is better than stock. I'll be towing a Jeep on a flatbed in the future, and I've kept my stock tires & wheels for towing in case the 36s end up being too tall for towing.

 

[HEMI®Dart]

I was running the same tires as Craig. Also have a 3. 54 rear gear ratio. If I remember right, I think the tach was at about 1900 rpms @ 65-70 mph. I didn't figure out MPG, but it was about ther same as the stock tire. I didn't find the TALL tire really hurt performance. Maybe a little on the low end. The Cummins has so much low end torque, the tires don't really faze it. I started out in low gear each time to go easy on the clutch. You will give up some handling & less efficent braking with a heavy tire.





Side by side comparison of a LT245/75R16 & LT305/85R16 below.





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[Walker]

HemiD,

What kind of lift did (do) you have for those tires? Did you have ANY clearance problems at all? You said "was" running the same tires. What do you have on now?

 

[HEMI®Dart]

Walker,

All I have is Skyjacker Leveling springs in front. Rear is stock. The springs raised the front maybe 1. 5"-2. 0". The tires come very close the the plastic inner fender liner when the steering is in a certain postion. I ran them for 3 years and never had them rub the fender liner.



When I bought these tires all that they were available in was a load range C, 6 ply. Not an ultra strong tire. I wouldn't tow anything heavy with them.



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