Tires

[luvindalex]

I own an 95 sc400. I have chosen to move up to 18" rims. KMC 127Dimes 18's. I know the rims will fit, and the offset, bolt pattern etc is fine. My question is, how big can I go on the rubber?

225/45/18 is what is recomended, but the wheel well is so big, it looks kind of small within it. Would it be possible to go to like 245/55 or 50's? I would like to improve on the smoother ride, rather than feeling every bump on the road, as well as fill in the look of the wheel well. However, I've been told it could affect the ABS. Is it only drastic changes in the wheel diameter when the ABS would be affected, or should you be carefull either way?

The difference in the speedo doesn't concern me.

[Brendon Bosy]

Stock overall tire diameter is 25.74". With 245/50/18 tires, it would increase to 27.65" and 245/55/18 would increase to 28.61". That's an increase of almost 2 and 3 inches respectively! Those definately wont fit. A general rule of thumb is not to increase or decrease the overall diameter by more than 3% unless you do some heavy mods to the suspension and body. The reason why your wheel well has so much room is to prevent your wheels from having any chance of scrapping, even if the springs gave out. Most cars' wells are designed like this.

[sc4sooner]

as far as the ABS goes as long as you keep the overall tire and rim weight around the same the ABS should not be effected. remember that rotational weight has a much greater effect then stationary weight. the lighter the wheel and tire combo the better off you are.

[xaine]

as far as the ABS goes as long as you keep the overall tire and rim weight around the same the ABS should not be effected. remember that rotational weight has a much greater effect then stationary weight. the lighter the wheel and tire combo the better off you are.

i would go with a 235/45/zr18 or 245/45/zr18

[luvindalex]

Just wanted to say thanks to the guys who replied.

The guy I'm buying the tires from, said the 245 45's would rub. I think I'll end up with 245 40's.

Thanks again guys.

[Brendon Bosy]

Here's a little extra info for anyone else interested in the effects of increasing your tires...

If you increase your tire diameter a lot, your acceleration will suffer significantly since your reducing the drivetrain's mechanical advantage. Here's a quick example:

1995 SC400 stock:

Curb weight: 3625 lbs

Tire: Goodyear GT+4 all-season 225/55/16

Tire Circumference: 81.05"

Revolutions per mile: 781.74

0-60: 6.9 seconds (Lexus.com)

1/4 Mile: 15.3 seconds (Lexus.com)

With 245/55/18:

Tire: Yokohama ADVAN S.4.

Tire Diameter: 87.34" (7.76% increase)

Revolutions per mile: 725.44 (7.20% decrease)

0-60: 7.44 seconds

1/4 Mile: 15.73 seconds

A stock 95' SC400 has a power:weight ratio of 14.5 lbs per HP. The affects of installing 245/50/18 tires would cause the equivalent of raising your ratio to 15.76 lbs per HP. In layman's term that would be the same as:

1) dropping your engine's power from 250 HP to 230 HP (a drop of 20 HP, or 8% powerloss)

2) or increasing your car's curb weight from 3625 lbs to 3940 lbs (an increase of 315 lbs)

Now another thing you got to remember which I didn't take into consideration (much more complex math) is rotational inertia. A bigger wheel and more importantly a bigger tire is going to increase the amount of rotational weight your engine has to overcome. The weight furthest from the center (i.e. the tire) affects rotational inertia the most, so even if you have an ultralight 18" rim, its not going to offset the damage a big heavy tire will cause. Increasing rotational inertia hurts your acceleration, cornering ability, braking distance, and MPG. Basically its bad in every respect.