HarrierRX300

Do people with 18 inch setup rims/tires lose MPG because it's heavier and the diameter the wheel travel is larger? I'm asking because I'm going 18 inches pretty soon (pics to follow) and was wondering people with RX300 notice their car is a little heavier (meaning pressing on the gas pedal) and have their speedometer been effected in anyway? I'm getting roughly 19 city right now.

thanks

LexRX

Going to an 18" wheel could cause more unsprung weight if the wheels is heavier than what you have on there now. As far as rolling resistance and affecting the speedometer there shouldn't be a problem as long as you "plus-size" you wheel/tire combo.

In other words, you will have a larger wheel but smaller tire sidewall therefore your overall tire+wheel height will be roughtly the same.

Check out this thread for a good reference:
click me!

HarrierRX300

Thanks LexRX,
I'm getting the RX330 rim/tire combo with 235/55/18. What's "unsprung weight"? Should I worry about the unsprung weight or do I need to change the shocks and do an alignment?

LexRX

Didn't mean to worry you about unsprung weight, it's not a problem. Unsprung weight is the weight of those items not supported by the vehicles suspension like the wheels and tires. Instead they come before the suspension.

It's often referred to when talking about high-performance vehicles b/c it affects handling, cornering, and general ride quality. Shocks will be fine - we've had members change to the RX 18's before without any trouble.

HarrierRX300

Thanks LexRX,
Much relief.....

mooretorque

However, you will have greater rotational inertia to deal with (more weight farther out on the radius of the tire/wheel combination), so you'll notice decreased acceleration as well as increased stopping distances (this is why the morons who slap dubs on vehicles with braking systems designed for 15/16/17" wheels often have terrible brake performance unless they upgrade to bigger brakes, too; in which case, they aren't morons).

So you probably will see some decrease in mpg due to heavier pedal pressure to achieve same acceleration as previously experienced.

JRB

I don't think you can say what impact the 18's would have on acceleration and braking until you know how heavy they are relative to the 17's. There are lots of 18 inch wheels that weigh less than many 17 inch wheels, and the two Lexus wheels in question here are of a different style - so it's difficult to say without putting each of them on a scale, with tires. And all other things being equal, the (slightly) wider contact patch of the 18's should result in shorter stopping distances, although here the difference is so slight that I suspect it would have very little effect - less that any difference than could be attributed to the difference in the tires and their tread pattern.

IMO the biggest difference between the two would be in how they look, and the 18's have a definite edge here.

LexRX

So that we're all on the same page, HarrierRX300 has an '01 RX so he will be going from 16's to 18's. At any rate, you're point still holds water.

mooretorque

As Lex pointed out, Harrier is on 16s. And your point is valid but I can tell you from having had both 16s on the '99 RX I did own and 18s on the '04 that I now own, the 18s are indeed heavier. And the larger contact patch of the 18 would give shorter braking distances if all other variables were equal, but even a 3-4 lb difference would translate into significantly longer braking distances for the heavier combination. It's just good ol' Newtonian physics..............

JRB

My guess is that the RX330's braking distance is shorter than the 99 model, despite the heavier weight of the vehicle overall and the wheels/tires (assuming what you say is true). In the end this is all that counts. Until you put the two sets on a scale it's hard to say. I still question your conclusion ("significantly longer") - the tires put on most performance vehicles have gotten much wider over the past few years and their braking performance seems to get better and better. Of course this is due in part to better brakes but in general the more rubber on the road the more there is to grip with when applying braking pressure (snow and ice excepted).

HarrierRX300

Here's the pic I just shot. Stock RX330 18's. When put side by side of the 16's, there is much more tread to the ground. I'm not sure about full turn if it rubs or not yet. I'll find out as soon as the rain stops. Let me know what you guys think of the pics.

Thank you, Mooretorque,LexRX for the useful info:

Daddy-O where are you? Need some input...

HarrierRX300

doesn't stick up like 4 x 4

HarrierRX300

Rims just cleared the brakes by mm???

HarrierRX300

After putting them on, I had a 4 wheel alignment just in case:

mooretorque

JRB, I agree. But we weren't talking about the RX330 vs the 300. It's an RX300 with unmodded (and therefore designed with the performance aspects of the 16" wheel in mind) brakes upsizing to 18s from the 330.

Harrier, those look GREAT on the 300!!!