it's easy to find speedometer differences between tire sizes. how about trying to get that extra diameter and mass moving? i'm trying to decide if moving from a 23575r15 up to a 265 (31) will negate all the torque advantages gained with my present mods (i spend a lot of time pulling the pulling the passes in colorado during the winter).
probably be purely subjective, but i'd appreciate what you've learned.

 

it will effect it... shouldnt be too bad though....

 

the extra torque required to accelerate at the same rate as with your old tires is the exact same ratio as the speedometer will be off.

 

It will affect it. Smaller tires are easier to accelerate, even if you have regeared. This is because they are lighter.

 

Man that's a good question. The torque reduction is easy. However, the larger tires have a larger moment of intertia and so are harder to get rotating. At higher speeds, the increase cross sectional area of the truck will come into play. The added rolling resistance will be a larger percentage of the total losses more at lower speeds The problem is that the inertia goes up a lot because you moved the weight outwards where it makes the most difference.

I'm trying to think of a way to calculate this without actually having to different wheels set up but am not doing a very well.

Here's one thing, engine mods almost aways increase torque higher up the rpm band. Larger tires make the engine run at lower rpms. As a result, the mods are usually ineffective in compensating for larger tires. This is the reason we regear as opposed to build up engines when we go up in tire size.

Frank

 

which i intend to do, but can't afford to right now. i will, however, need tires shortly. trying to preplan a bit. thanks.

 

I moved from a 235/75R15 (or was it /65?) to a 31 and it did make it a bit slower, but nothing really major. Kinda hard to keep it at 75 though, but I think that is more due to the 3.0 than the tires. I was expecting it to be worse than it really was. I'm running a 3.90 rear for now too. What's your gear ration now?

Have fun all.
Daniel

 

it's stock.

 

stock is 4:10s

 

For a quick idea of the nature of the problem, just look at the mass moment of inertia for a thick cylindrical shell (approximation of the tire) and cylidrical disc (wheel, assume doesn't change)-

For Thick Cylindrical Shell:



So it can be seen that increasing the outer diameter of the "tire" will have a squared effect on the moment of inertia. So changing a tire's outer diameter by 6% (31" to 33" for example) will change the tire's moment of inertia by 10% (and this doesn't take into account the added weight.)

Then, say the bigger tire weighs 6% more (could be more or less, depends on the brand of course...) that means the moment of inertia would change by 17%!

This is a big deal when it comes to acceleration and braking, not to mention the brakes have to work harder because of the larger overall diameter.

Granted this approximates the tire as a solid when it is not, but the approximation can give you an idea of what's going on.

Ok, lecture's over

 

By the way, the effect is similar when going to bigger rims for example, which is why trucks on 20's with stock brakes are a BIG problem IMO...

 

Not to mention we can regear 35% for $600. Try to find 100 more horsepower for that....

 

We use a similar formula to calculate rolling moment and righting moment on sailboats. People really don't understand the effect of mass/distance on rotating bodies (me included.) But a neat example is to change out all the wire halyards on a typical 30 foot mast to modern hightech rope, saving only 20 pounds up the mast - it's the equivalent to adding 75 more pounds of lead to the keel, or putting another 200 pound person on the rail! HUGE benefits for weight savings the farther from the center of rotation you can get. So yeah, heavier Or larger diameter tires (and certainly Both!) are harder to get rolling and harder to stop.

 

IN simplest terms - changing the tire diameter changes the torque multiplication directly, so a 15% increase in tire diameter will result in a 15% loss of power.
BUT, if you change the final gearing far enough to drop down the engine torque curve to an rpm where less horsepower and torque are available, that simple 15% might actually net a much bigger loss. You have to re-figure the entire drivetrain math again to find the real effect.

Here's one of the neatest dissertations I've seen yet:
http://www.mustangsandmore.com/ubb/D...orqueVsHP.html

 

just being nit-picky, but i think it's important: 15% increase in tire diameter will result in 15% less force applied to the road, but the power output will stay the same. see the proof in this thread:

https://www.yotatech.com/forums/f116/few-questions-about-my-90-4wd-3-0l-69025/

 

You guys are beginning to worry me with upgrading to 285's from 265's .

 

started following the thread, thanks, and read this:

"you will not necessarily reach a lower top speed. You will be in a different torque requrement with different tires at speed. I lost 20% in mpg going from stock to 31's on an '84 pu. You won't be able to redline in high gear in either case."

now THERE'S another issue!

 

285s are nothing to turn...especially on a 3.4L. If you want a race car street mobile, then stay w/ a street stock tire size. I turn 37s on my 4banger 5 speed and run up and down the interstate 80 mph+ w/ NO problem. It is all about your gears to maintain the proper relative speedo reading...mine is off by 5 mph...which ain't bad considering I'm like 4.2 million sizes over the stock tire size.

 

That is true. But the ability to accellerate will NOT remain the same. 15% less force applied to the road... Semantics. It still feels exactly the same as having less power, that's all that really matters.