Tire Size Question

If he has a X, it is the 4.0. The 31's won't hurt it that bad, he just won't be using 5th gear much. When I put 31's on my 2002 X, i found I was running 70 in 4th at 2000 rpms. It still had get up and go. That was with the 3.07 gears which are stock in the X.

Patrick ?Ô

Proably not a lot. It will depend more on exactly what speeds he runs on his commute. The tire change will move him over about half a gear (old effective gear ratio is about in the middle between two new effective gears). If that means the engine is overreving or lugging, mileage will sink rapidly. It also means that 5th gear is not going to be very useful most of the time.

A TJ do> Yep another one,

I read somewhere that every lb of unsprung weight is worth 10 lb of sprung weight ; don't know if that's true or not.

Dave Milne, Scotland '91 Grand Wagoneer, '99 TJ

When I tossed the stock tires on my 2.5l, I was surprised at just how fast I could move.

Once I get these 4.88s in, I'm hoping to get back to that point. Not that I need to be darting everywhere lifted this high... :-)

Eric

Daughter's 95 YJ with 2.5 sucked on gas stock, sucked on gas with 31"s, and still sucks on gas with 33"s and 4.88s.

That is why I asked the question it makes sense. Also, it was an SE TJ I looked at it yesterday. He is a young kid at work and his dad gave him the TJ because his car died. It gave his dad a new excuse to get a Rubicon. He see me with my TJ and has a ton of questions I told him I do not know some of the answers but I know where to ask.

My '95 YJ with the 4 cyl. Had 30x9.50's on it and now has 235-70's. On a measured 135 mile round trip commute it averages 20 mpg...with either tire on it. Just has a bit more pick up with the 235's

My wife's '94 YJ with a 6 cyl. Had 215-70's on it when we got it. Now has 30x 9.50's. On the same 135 mile commmute, it also averages around

The only way tire size seems to have affected the gas mileage was by the change in the speedo reading. Over an actual measured course, not using the odometer as a reference, the mileage didn't hardly change.

-- Old Crow '82 FLTC-P "Pearl" '95 Jeep YJ Rio Grande SENS, TOMKAT, BS#133, DOF#51, MAMBM "There's only *one* RE"

Bill,

E=mc² is not a formula from classical mechanics, which is what you are talking about here. The energy to get a wheel, or any other object, spinning depends on its "inertial mass", which is usually called "moment of inertia", and the effective radius. Calculus gives the formula E=1/2 IR², which is similar to the formula for a moving object, E=1/2 mv², where m is the mass of the object and v is the velocity.

Now, once you get an object moving or spinning, there is NO energy required to keep it doing that, unless there are losses. In a vehicle, losses are friction, air resistance, and that hundred pound girlfriend's credit card usage. The wheels on Lance's bike have to be light because of the need to accelerate, decelerate, and turn. Big heavy mountain bike wheels would slow him down, but not for the reasons you state.

Earle

I didn't invent perpetual motion. Newton did. The wheel would keep spinning forever, except for friction from the bearings, road and air. What keeps the Earth spinning? What happened to Stephen Cowell, anyway? I think that we need his expert opinion here.

Earle