Gearing

[guitarterry]

bigger 4 wheel drive trucks pick their tires first. then use the gearing they need to get the rpm of the engine where they want. what i was wanting to know why use 155/80/13 instead of 145/82/12 tires with even taller gears.

remember in all examples we want the engine to be turning at the same rpm versus mph, (for example 2000 rpm at 55 mph)

This is the deal, the 12 inch car turning 2000 rpm at 55 mph, would set lower to the ground, weight less, reciporcating mass would be less, tire width less.
the 13 inch car turning 2000 rpm at 55 mph would be higher and weight more. is it only because nobody makes gears? some of these guys go to extra ordinary lengths for just a very little economy. this seems to be alot bigger payoff and nothing.

a way to maybe test this would be get a car with the 12's in fifth gear and compare it to 13's in fourth, once we fiddle with making the rpms the same at 55 for both tires, we could see the payoff in fuel economy for the smaller tire. This would justify even a special order gear for a tranny. We are doing alot more for alot less

[wumpus]

Would you believe I just got new tires before even thinking of this

I think going past a 1" lowering is starting to get into areas where hot-rodders start counting the cost, so if you do that (2" smaller diameter) I'm guessing a 15% taller gear. If you are lucky, there exists a car model with plenty of cast-off wheels in that size (not always easy if you are starting with an eco-friendly car).

[Red]

Like I said in the other thread, brakes and however your steering knuckles (added this part) are designed come into play with rim size. You do need some minimum clearances to clear all the mount points attached to the hub. You could negate this some what using wheel spacers but you'd have to run an ungodly large spacer to clear the calipers.

Now if you inboarded your brakes like on this '69 Jag E-Type, you could probably run that 5" rim of yours


But you can basically run as small of a rim as you can clear.

[some_other_dave]

Originally Posted by guitarterry

...is it only because nobody makes gears?

IMHO, yes. That, and the fact that it's a whole lot easier to swap out wheels and tires than to swap gears.

-soD

[aerohead]

A larger rolling circumference typically reduces rolling resistance.By going to the smaller diameter tire/wheel combo,you could increase tire friction which could cut into your gains from lowering and more narrow tire.Hard to know,and an expensive mistake should results not pan out.

[Big Dave]

On pickups, bigger diameter wheels/tires ALWAYS result in LOWER MPG.

To repeat big tires = lower MPG.

Reason: Rotational inertia. When you accelerate away from a stop, you must not only translationally accelerate the vehicle but also those four "flywheels" attached to it. The rotational moment of inertia goes up with the square of the outside diameter. On a pickup, going from 235-85x16 to 305-75x16 tires costs the owner 2 MPG.

[extragoode]

Originally Posted by Big Dave

On pickups, bigger diameter wheels/tires ALWAYS result in LOWER MPG.

To repeat big tires = lower MPG.

Reason: Rotational inertia. When you accelerate away from a stop, you must not only translationally accelerate the vehicle but also those four "flywheels" attached to it. The rotational moment of inertia goes up with the square of the outside diameter. On a pickup, going from 235-85x16 to 305-75x16 tires costs the owner 2 MPG.

But you're also the one who is always preaching gearing. Rotational inertia means a lot in city stop and go, but I would think gearing would trump inertia on the highway. Choose the tires for the driving you do most.

[IndyIan]

Dave,
I think in practice you are right, bigger diameter tires on trucks leads to worse mpg.
But I think its because; the bigger tires are almost always wider, have more rolling resistance due to off road tread patterns, and are heavier which hurts city mpg.

I have 205/75R15's(minivan tires) on my Tracker, and if I could find something like 185/95R15's in an all season tread pattern I think I would do better on the highway with the 6% reduction in rpm. It would be about a 200rpm difference which would get me closer to 2500 rpm at 60 mph.

Last Sunday I had sticky high performance 195/45R14's on the back of my Neon for autocross which lowered the back end about an inch, but I think even just 2 increased the rolling resistence compared to the normal 185/65R14's. I drove with the normal tires on the front and put my R compound tires on the front at the track. I wasn't getting my normal coast distances and the tires were noiser than my all-season tires. Also I think any misalignment creates more drag with low profile tires than high profile tires.

Probably the best bet for increased FE is go with the narrowest, highest profile tires you can find that fit on your stock rims. Usually this won't be very much bigger in diameter for most cars.

Terry,
Aren't 13" tires half as much as 12" tires? and there is way more selection? I would go with 13's just for those reasons, plus 1800 rpms is better than 2000 rpms at 55 mph and you don't have to change gearing at all.

Ian