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Discussion Starter · #1 ·
To those of you with 2nd gen auto rigs that have 285/75/16s...did you change your gear ratio? If not, did you notice a drop in power or rise fuel economy when switching to taller tires?
 

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To those of you with 2nd gen auto rigs that have 285/75/16s...did you change your gear ratio? If not, did you notice a drop in power or rise fuel economy when switching to taller tires?
I certainly noticed a drop of about 1mpg with the install of the tires. As for power, a little less acceleration on the road, but nothing so bad where I regretted getting the tires.

Another thing to take into account, is that if you get larger tires and calculate your gas mileage by the odometer mileage divided by the gallons input, you will seem like you're getting way less than you actually are. Because if you don't get your ECU recalibrated, your tires are actually spinning slower at the same speed due to the larger circumferance, therefore tricking the ECU. I found that I am off ~6% with mileage and speed. So at 70 on my speedometer, I'm really going ~74. And for every 1000 miles (actual), you only register 940. So make sure you take that into account if you calculate your gas MPG dropping.

I only thought about that when I got stopped for speeding, even though I had my cruise set at 74, I got popped for 79 in a 65 after I got my tires. I then set my cruise to 60MPH then timed how long a mile took to get my error in my speedometer, and I found it to be off by roughly 6% or 56seconds/per mile at a 60MPH gauge reading.

And one more thing, it also messes with the ABS computer sometimes when braking while making a turn. It thinks that the wheel is slipping due to the larger difference of wheel speed per degree of steering angle. I might be wrong about this last point, but it makes sense to me. I've felt the ABS kick in more than a couple of times when it shouldn't have.
 

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Discussion Starter · #3 ·
I certainly noticed a drop of about 1mpg with the install of the tires. As for power, a little less acceleration on the road, but nothing so bad where I regretted getting the tires.

Another thing to take into account, is that if you get larger tires and calculate your gas mileage by the odometer mileage divided by the gallons input, you will seem like you're getting way less than you actually are. Because if you don't get your ECU recalibrated, your tires are actually spinning slower at the same speed due to the larger circumferance, therefore tricking the ECU. I found that I am off ~6% with mileage and speed. So at 70 on my speedometer, I'm really going ~74. And for every 1000 miles (actual), you only register 940. So make sure you take that into account if you calculate your gas MPG dropping.

I only thought about that when I got stopped for speeding, even though I had my cruise set at 74, I got popped for 79 in a 65 after I got my tires. I then set my cruise to 60MPH then timed how long a mile took to get my error in my speedometer, and I found it to be off by roughly 6% or 56seconds/per mile at a 60MPH gauge reading.

And one more thing, it also messes with the ABS computer sometimes when braking while making a turn. It thinks that the wheel is slipping due to the larger difference of wheel speed per degree of steering angle. I might be wrong about this last point, but it makes sense to me. I've felt the ABS kick in more than a couple of times when it shouldn't have.
Why would you experience a drop in MPG? In theory, because more ground is covered in a single rotation, your fuel economy should go up (additional weight of tires not withstanding)
 

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Why would you experience a drop in MPG? In theory, because more ground is covered in a single rotation, your fuel economy should go up (additional weight of tires not withstanding)
Rotating mass is a MPG killer, and my tires weigh about 60/tire. I believe my stocks were about 40/tire about a 33% increase . The weight will depend on a lot of things, but the more ply's they have = more weight = less MPG.

Also, and I am just speculating here, that tires with more aggressive tread are less aerodynamic and cause MPG to drop. THat is probably an infinitesimally small contributor, but I figured I would add it :salute:
 

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And also, if that were the case, don't you think all those econoboxes with high MPG would be stuffing the biggest damn tires they could in the wheel well?
 

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Discussion Starter · #6 ·
I'm just saying that if you don't regear your diff, the math says your mpgs should increase and power decrease. Of course, that's in a vacuum.
 

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There are 3 weights that you need to contend with and consider in a car design and they are:

#1. Sprung weight
#2. Unsprung weight
#3. Rotational mass

They all have an effect on overall handling and performance. However unsprung weight will have more of an effect than sprung weight. Rotational mass will have a bigger effect on the acceleration or your car and its overall quickness. Hence is why some cars opt for reduced mass flywheels and lighter driveshafts and axles.

To get more weight moving, you need more power, plain and simple. That power comes in the form of gasoline :)

However......driving at a constant speed, if you have 30lbs on each wheel or 300lbs, your gas mileage should be the same, because the mass is moving. However city driving (which I do about %30 of) accelerating that mass takes a lot more energy with more weight.


Take look here, I found this online http://www.physicsforums.com/showthread.php?t=488123
 

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I'm just saying that if you don't regear your diff, the math says your mpgs should increase and power decrease. Of course, that's in a vacuum.
MPG's will decrease by roughly 2 to 3 MPG w/ the 285's due in part to weight of the tires.. I haven't noticed any difference in power, your speedo will also be off roughly 3 to 4mph at 65
 

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Discussion Starter · #10 ·
...and I know this has been asked time and time again, but I need to be 100% sure. Nothing more than a melt mod is necessary to clear 285/75/16s on stock S rims with a 2" daystar lift (front spacer/rear shackle) Also, I have the stock spring packs and shocks on there. Is that going to be a problem?
 

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Correct. I ran a 2" revtek lift with 285/75/16 Goodyear MTRs w/ Kevlar on stock OR rims and I just had to melt a little on the driver side.

From what I can tell, the MTRs are on the larger side of the 285/75/16 size too because the BFG/AT I compared them too was at least 1/4 in smaller.

Ideally you would want longer shocks in the back to accomodate the shackle but It wont hurt to run the stock ones. It will just limit your articulation a little. Which wont matter unless you wheel fairly hard on a regular basis.
 

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...and I know this has been asked time and time again, but I need to be 100% sure. Nothing more than a melt mod is necessary to clear 285/75/16s on stock S rims with a 2" daystar lift (front spacer/rear shackle) Also, I have the stock spring packs and shocks on there. Is that going to be a problem?
Nope, that was the first thing I did to her, was buy her new shoes. So even with a stock ride height, only thing I had to do was a melt mod.
 

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...and I know this has been asked time and time again, but I need to be 100% sure. Nothing more than a melt mod is necessary to clear 285/75/16s on stock S rims with a 2" daystar lift (front spacer/rear shackle) Also, I have the stock spring packs and shocks on there. Is that going to be a problem?
Correct.. No it won't be a problem
 

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There are 3 weights that you need to contend with and consider in a car design and they are:

#1. Sprung weight
#2. Unsprung weight
#3. Rotational mass

They all have an effect on overall handling and performance. However unsprung weight will have more of an effect than sprung weight. Rotational mass will have a bigger effect on the acceleration or your car and its overall quickness. Hence is why some cars opt for reduced mass flywheels and lighter driveshafts and axles.

To get more weight moving, you need more power, plain and simple. That power comes in the form of gasoline :)

However......driving at a constant speed, if you have 30lbs on each wheel or 300lbs, your gas mileage should be the same, because the mass is moving. However city driving (which I do about %30 of) accelerating that mass takes a lot more energy with more weight.


Take look here, I found this online http://www.physicsforums.com/showthread.php?t=488123
Yup,At highway speeds, once your speed is stabalized... the tires are also allowing the engine operate at a lower RPM (taller tire, numerically higher gearing), also with the extra rotational mass, and the momentum it has... its a small trade off. Either way... some notice a difference , some dont at all.
I wouldnt worry about it. Even more thought, it depends on the type of tire. I'd expect a M/T to eat more MPG.. Yet again, my truck is a 05 and the speedo read a little high from the get go, so it was right on with the 33's.
 

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For a while I was running only the daystar lift with 285 MTs and never did the melt mod and never had any rubbing. Also - I don't think anyone mentioned this but sitting up higher also increases drag on your X. The higher off the ground, the more air is able to travel under your truck and cause some more drag. Maybe someone should come up with an Xterra Diffuser mod lol
 

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Couple more factors that were poorly ignored for fuel economy.
Wind drag. Sitting a little taller punches a little larger hole in the wind. Well known to the rest of the world that a lift alone (with stock tires) will decrease gas mileage. Rule of thumb that works pretty good is 2" will loose 2 MPG.
Rolling resistance. The more aggressive the tire pattern is the harder it is to roll. The smoother the pattern the better it rolls. And the typical thing that happens is to put large aggressive tire in place of small smooth tires.

Since this is a re-gearing thread. Ever notice how little of a change there is between gear ratios? 3.13 for the standard tire and the larger off-road tire get a little bump to 3.36 for the final drive. That just about matches the tire size increase. Final result is nearly the same cruising RPM. Look at those ratios, they are not common. Figure that they measured the engine, factored in load and figured the exact best RPM for speed. Factor in tire size and you get the ratio. Start changing the tire size and you get outside that perfect happy spot for gest economy. But it is a moot point anyway. There is no ratio available that could fix it, the next possible ratio is the 3.54 from the early non-off-road manual transmission truck which is going too far and nearly impossible to find. The bigger one is the cost of regearing would be several times the fuel savings over the life of the vehicle.
 

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Im curious as to running full skids under the truck would cut down on wind drag under the truck as your creating a flat surface ? Hence the reason some race cars and new vehicles have plastic plates covering the whole underside .

Sorry to jump off topic.
 

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Im curious as to running full skids under the truck would cut down on wind drag under the truck as your creating a flat surface ? Hence the reason some race cars and new vehicles have plastic plates covering the whole underside .

Sorry to jump off topic.
Aerodynamics get very complicated very fast when you start getting into the turbulence area. They may have gotten around it by now, but I remember when the best super computers could only handle the aerodynamics of the space shuttle without the external fuel tank and booster rockets. Add those parts and nothing could calculate the aerodynamics. Just putting random underbody shields in place could make things better, or worse, or do nothing. It would take a little (expensive) time in a wind tunnel to know.
 

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Gearing calculators can make it easy to get a similar final drive ratio as the factory gearing while having larger tires and such.
To me, I'd love to have 4.10s to go with my 35s... but gear swap prices for these things are absurd and it's one of the few things I can't do myself. Even the master rebuild kits are stupid expensive.... darn niche vehicle!
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