Tire Diameter Effect on Power (Thoughts...)

[Mooseknuckle]

Right now I have 2 inch body lift with 2 inch BDS suspension lift with Markmc's 1/2 spacer up front & 265/70/17 load range E, which by Pro-Comp standards is 31.9. I'm debating going to 33.0 285/70/17 load range D by the same standards. I keep hearing about the loss of power,is it that significant that I need to switch gearing? I believe right now I have 3.42. Would it be the same issue going to 305/65/17 load range E which is 32.8 ? Anyone have any suggestions? Thanks in advance.

[djthumper]

The bigger your tires the more loss of torque. Your engine still puts out the same power but the lack of torque to the wheels will limit you.

[Mooseknuckle]

ok got it. So do you think that going from 31.9 to 32.8 or 33 will be a huge loss? or is it just better all around to stay with what I have?

[djthumper]

If it were me, I would stay where you are at. Hopefully James will pip in with his experience from his trip. I know some of the guys have run 33s on the 3.42s. When you look at the charts you are already outside of even the highway usage area. The power of our engine helps offset the loss of torque but only to a certain degree.

http://www.4wheelparts.com/aux_incl/gear_ratio.html#

[janesy86]

Power is there, just at highway speeds for long time the tranny temps will go up. Every now and then I'll pop it into 3rd gear to drop the temps.

If you're DD this, I'd say stick with what you got.=, unless planning a regear.

[Trail X]

This is a rather complex subject, a lot of which is opinion and based on your driving style.

I went through this same issue when I got new tires. My final conclusion was that I did not want 33s with my 3.41 gears. But it was largely a personal decision.

I'll try to run down my thought process real quick. A lot of which has to do with the locking of the torque converter lock-up clutch (TCC) (if you don't know what that is, look it up).

I've found that if I control my MAP (manifold air pressure) to about 0.4psi below ambient or less when traveling at highway cruising speeds, the TCC will remain locked. This limits your power output from the engine due to engine RPMs. For me, with 32s, and 3.41s, the engine will be around 1600 RPM at approx 65mph (gps speed, not speedo). This is enough power output to cruise comfortably at 65 with flat roads. However, you're loading the engine a good bit to produce the power required to overcome the wind resistance.

Now, add an incline... engine loading comes to near 100% (14.2 psi MAP at ~ sea level). Depending on the slope, you can maintain speed, but more often than not, you'll find your speed decreasing (if you're controlling the throttle to 0.4psi below ambient). Or, you'll try to get a bit more power out of the engine, and the TCC will unlock, giving you an increase of ~400-500 rpm. At that point in the torque curve, you may be getting enough power to maintain speed or speed up, but at the cost of increased transmission temperatures, and lowered efficiency.

Detailing the efficiency loss, I've found a hill that I can barely maintain speed at 60 mph with the TCC locked. With the TCC locked, I get about 14.5mpg instantaneous. That same hill, same speed, TCC unlocked, efficiency drops to about 10.5mpg instantaneous. (For reference, same speed in 3rd gear, TCC locked, I get approx 10.0mpg).

Personally, I plan to regear to 4.10s when possible, before upping to 33s. However, it's all in how you drive, what efficiency is ok, if you're ok with adding a trans cooler, how attentive you are to the engine and transmission's shifts, etc etc.

Hopefully that answers more questions than it creates.

(Keep in mind that these observations have been made with a RTT/parachute on top of the truck.)

[navigator]

Larry, that chart is really neat.

[djthumper]

Thanks Chris, I think there might be some minor inaccuracies but it illustrates what I was saying fairly well. I have built my fair share of vehicles racing, lowered, off-road. Changing tire size and calibrating the speedo and everything over the years. Changing tire size affect each vehicle differently.

I knew James would have some really good insite to our rigs after his trip out west. I noticed a lot of this going from stock to the 30.8 tires that I have now. It was so noticeable because I had just taken the trip a week prior as well.

[chevycrew]

That's some great info james. On mine with 4.56 gears and 37s, it wants run run with the tcc unlocked all the time while in cruise control. So I'm going to have my tcc unlocking turned off in overdrive.

[Trail X]

Be sure to report back how that works for you. I might be interested in that too. But more than likely I'll just regear in due time.

[rjpoog1989]

I want to regear sooner than later, 3.42's in an EXT blows. There are too many hills around here and even with 30.5 inch tires the transmission temps skyrocket going up and down hills. I do a lot of driving with the thing fully loaded with gear for various trips and I know if I decide on bigger tires next time around I'll have to regear. I need this vehicle to last and I'd prefer not to replace my transmission anytime soon. I also plan on adding a tranny cooler.

[fishsticks]

Be sure to report back how that works for you. I might be interested in that too. But more than likely I'll just regear in due time.

I have a hell of a time keeping the converter locked up.

[djthumper]

One of the things you need to look at with re-gearing as well is if you can tell the computer what gears you have in it after the swap. I know on the 06 and newer there is suppose to be an option to change the gearing and tire size.

[DirtyBacon04]

According to that chart, I need 4.56.

Enough convincing for me

[Mooseknuckle]

James thanks for the title change and all the good info. I think after taking with you guys the best option in order to go to 33's will be to re-gear to 4.11. I don't want to be replacing the transmission anytime soon. I still have plenty of meat left on my Pro-comps. But when I do get the gears anyone wanna help? I will drive to you of course and throw some cash your way.

[djthumper]

Jeff, if I was closer I would definitely be in to help. When you go to do it I would definitely go up to a 8.6 rearend and get a different locker.

[Mooseknuckle]

It's all good. 8.6 from an EXT correct? I need to get lockers.

[djthumper]

yep, I keep looking for them locally to try to keep the cost down.

[Flying Monkey]

I too have 33's with 3.42's. I do have somewhat a temp issue climbing big hills and on long highway runs due to tranny heat. Im goign to be putting a tranny cooler in soon to help with that some. But my real goal is also do a gear swap to 4.10's. Just its a pricey upgrade so will have to be down the road a ways. Mine is my daily driver, but only real ill effects I have for that is my mileage seems to have dropped considerably.

[Mooseknuckle]

Ok will do Larry thanks. This is gonna be down the road some. This is my DD as well so small steps. I plan on putting lockers in,probably a good idea to wait on that since I Will be in there anyway. Right now as James stated I too am ok with 32's and 3.42 gears only difference is he has G80 and I don't. I appreciate all the input. Thing that sucks is as you bigger in tire size so does the cost. I guess we will se how things go I might just stay with what I have and install lockers. After the truck is payed off and this is no longer my DD. Thats when i might do all this other stuff.