Still have that trani lag.

[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

[SinCityFX4]

Well, I guess the increased surface area and type (MT/AT) would have a huge impact on the co efficient of friction as well...hmmmm. Food for thought as I ponder tire choices...

[Jackpine]

Originally Posted by SinCityFX4

Well, I guess the increased surface area and type (MT/AT) would have a huge impact on the co efficient of friction as well...hmmmm. Food for thought as I ponder tier choices...

There's a reason racing cars run "slicks", isn't there? The tread is for improved traction on wet, slick or "loose" surfaces, but it hurts you on dry pavement. And, if you never had to turn corners, like running on the salt flats, you'd want the narrowest tires you could mount.

- Jack

[pullmeoverred]

Andddd now I'm going to totally ruin the quality of the coversation here...

I've got a 06' 5.4L 4x4 with the 3.55LS, and now it's got 275/60/20R cooper zeons to deal with (32.99") which has robbed it of a decent amount of performance. I've been searching for a few days now on this forum and other f150 forums for what my best option would be. So far the vast majority of responses I've gotten have reccomended I buy a grypon (which I've been considering already) and rolling the dice on my extended warranty of 5 yrs 75k (I'm at 25k). Only two people have suggested regearing and my Ford dealer basically thought I was a mental case for asking them if they could install them for me. I've accepted the fact that my knowledge on this subject is severely lacking. I'm not a total lead foot, but I do enjoy performance. I work with one of your customers (glockwinger) and he giggled like a school kid when he got his custom tunes. I'm intrigued...

Is your Gryphon the answer? Or should I be looking at new gears? Or both?

Thanks, Aaron

[Chris74]

I'd say just go with Gryphon first. That may completely satisfy you........but if not, then I'd throw some deeper gears in it.

With your new tires you now have an effective ratio of 3.34. You would technically need a 3.76 ratio just to get back to the feel of stock. So purchasing 3.73's won't even quite get you there. You would want 4.10's or 4.56's if you decide to do gears as well. That's my opinion anyway
BTW - Don't worry about the Ford dealer looking at you funny!