Still have that trani lag.

[SinCityFX4]

It is nice. And, a very important issue, especially when you hear of all these guys suggesting the different gears I wonder if they realize the full extent of the change on the vehicle.

[Jackpine]

Originally Posted by SinCityFX4

It is nice. And, a very important issue, especially when you hear of all these guys suggesting the different gears I wonder if they realize the full extent of the change on the vehicle.

I seriously doubt it.

I was amused by one (or maybe more than one) poster on the f150 forum that thought the reason you lost power by going to bigger tires was due to the "increased inertia" of the larger ones, making it harder to start them "spinning". Totally missed the real effect of the larger tire acting like a higher gear, turning slower at a given engine RPM, and giving you less mechanical advantage during acceleration.

- Jack

[SinCityFX4]

Originally Posted by JackandJanet

I seriously doubt it.

I was amused by one (or maybe more than one) poster on the f150 forum that thought the reason you lost power by going to bigger tires was due to the "increased inertia" of the larger ones, making it harder to start them "spinning". Totally missed the real effect of the larger tire acting like a higher gear, turning slower at a given engine RPM, and giving you less mechanical advantage during acceleration.

- Jack

Inertia....good lord.

[Power Hungry]

Well, inertia does account for some loss in acceleration as there is some power lost in trying to spin twice the weight (or more) of larger tires and wheels. The biggest loss though, as you pointed out, comes from the resulting change in effective gear ratio. That is much more brutal on performance.

[Jackpine]

Originally Posted by Power Hungry

Well, inertia does account for some loss in acceleration as there is some power lost in trying to spin twice the weight (or more) of larger tires and wheels. The biggest loss though, as you pointed out, comes from the resulting change in effective gear ratio. That is much more brutal on performance.

Yup, I know there IS some increase in inertia, but, once you get it spinning, it's like a flywheel - it wants to keep spinning. And, the radius of gyration is not twice what it is for smaller tires, so, I don't really think inertia is much of a factor, right?

- Jack

[Power Hungry]

I was merely reflecting on the affects of acceleration, not sustained movement. You are correct that once it is moving it takes much less effort to keep it in motion.


For the sake of discussion, here's one thing to ponder; Let's assume you go from a 31.5" tire (average stock height) to a 35" tire (most common upgrade) while keeping the same 20" rim.

Surface area of the wheel = 314.159 square inches.
Surface area of a 31.5" tire assy. = 779.311 square inches.
Surface area of a 35" tire assy. = 962.224 square inches.

31.5" tire alone = 465.152 square inches.
35" tire alone = 647.954 square inches.

This is means at a minimum, the 35" tire has roughly 39.3% more surface area and presumably 39.3% more mass (or weight, whichever you prefer).

I know this doesn't seem like much, but when you figure in the fact that a 35" tire would have a thicker sidewall to allow for the taller sidewall height, is usually wider than the average stock tire, and often has an aggressive tread pattern which may add to the total mass, the average 35" tire may be more in the area of 60% greater mass (weight) then the average stock tire. Throw in some "bling" wheels and the ante just went up.

I can't remember the exact figures, but when we swapped out the stocker tires on the '06 F-150 for a set of 35"s and new wheels, the 35" wheel and tire assy. was almost TWICE the weight of the stockers. Of course, the stock wheels were the standard Ford 6 spoke variety and the replacements where solid "western" type with bead-locks, which I'm sure added a bit of weight.

One thing to note as well... After spending years as a mechanic and changing tires of all sizes, a 35" tire assy. will often take up to twice as long to spin up to speed (150 RPM) for balancing when compared to a standard stock tire assy. will. This nice thing about this comparison is that the HP of the electric motor (usually around 2HP) doesn't change and there's no transmission or differential to factor in. The ONLY difference is the wheel assy.

Of course, I'm not even getting into the torque factors due to the larger radius of the tire and the energy needed to move the mass at the edge of the new tire radius as those calculations are frankly a little more involved than I care to even think about right now.

Anyway, just food for thought.

[Jackpine]

Your point is well taken, and I'm possibly wasting everyone's time - you could even be writing tunes! (But, you need a break from work now and then).

Good observation about the spin up time using a balancing machine. That DOES clearly show the effect of getting the greater mass into motion. I grant that there IS that effect, but as a percentage of the total power delivered by the truck's engine, I think the tire mass effect is greatly overshadowed by the loss in mechanical advantage due to the greater distance between the axle and the road. And, that was what I thought I was saying.

Doesn't an aggressive, wider tread contribute more to "rolling resistance" than anything else? That's certainly the effect I feel between a mountain bike and a road bike when pedaling or coasting.

There clearly IS some optimum gear ratio and tire size combination for each of the following though:

1. Acceleration (0 - 60)
2. 1/4 mile speed
3. Fuel economy at cruise
4. Max top end speed (accelerator floored)

And, unless I'm totally out to lunch here, I think each one of these goals requires a somewhat different combination.

I suppose I'm reacting to the "one size fits all" mentality I've observed by some posters in other forums where they seem to think they can mod their trucks in such a way as to get all four of the goals I cited above at once. I'm especially amused when they complain about economy after putting in mods to improve acceleration.

- Jack

[dave-m]

Originally Posted by JackandJanet

I seriously doubt it.

I was amused by one (or maybe more than one) poster on the f150 forum that thought the reason you lost power by going to bigger tires was due to the "increased inertia" of the larger ones, making it harder to start them "spinning". Totally missed the real effect of the larger tire acting like a higher gear, turning slower at a given engine RPM, and giving you less mechanical advantage during acceleration.

- Jack

sorry to drag up an older thread, but this issue is something I'm wondering about presently. I'm planning a Gryphon purchase, and larger tires as well. With my stock 3.55 gear ratio some people are telling me 35's are too tall and my tranny life will be reduced along with a loss of power. Can a custom tune accommodate this, as in more torque and possibly firmer shifts/less slippage? I'd be interested to hear your thoughts on this.

[88Racing]

Originally Posted by dave-m

sorry to drag up an older thread, but this issue is something I'm wondering about presently. I'm planning a Gryphon purchase, and larger tires as well. With my stock 3.55 gear ratio some people are telling me 35's are too tall and my tranny life will be reduced along with a loss of power. Can a custom tune accommodate this, as in more torque and possibly firmer shifts/less slippage? I'd be interested to hear your thoughts on this.

Custo tunes can help alot with firmness and shifting but in certain circumstances such as larger wheels the first step I'd take is regearing to 4.10 or 4.56. The other step I'd take is doing this valve body modification. Like these:http://www.troyerperformance.com/cgi...on%3BAutomatic
Then I would proceed to a custom tune.
JMHO

[Jackpine]

Lars is right on, Dave. A programmer is not going to give you much at all with 35" tires and 3.55 gears. I really think 4.10 gears are the way to go with those tires. The shift kit would be fine, but I'd get the gears first. (And, if you have 4WD, you need gears for both the front and back). It may get a bit expensive.

- Jack