What is the stock tire and rim size for a 1990 F-250 4x4, 7.5 liter, 3:55 gears.

Would I get better performance if I took out the plastic air ventures in dual air intake lines into the throttle body intake ( it looks very restrictive to the air flow)? I know I would have make something to connect the intake hoses back together. Then there is a a short hose from this plastic piece then goes into the throttle body intake can I just plug this off?

 

Stock tire size is 235/85R16LT on either 6.5" or 7" wide rims. A set of this size tires is sometimes called "pizza cutters" because they are so tall and skinny (but will have lower rolling resistance in general over larger and/or wider tires). Many people prefer going up a size or two for looks and load handling. I myself prefer stock because I almost never go off-pavement and want maximum fuel mileage.

You can't completely remove the plastic piece that you are talking about, as it has a hose connected to the bottom of it that supplies air to the Idle Air Control (IAC) valve. You can certainly cut out the insides to get a straight-through air path, many folks here do just that. It is supposed to improve the throttle response a bit.

I just took one of the hoses off of mine to see if I had that restrictive plastic inside and I don't (1990 F350 with the 460) - if somebody already removed it before I bought the truck (which I suspect), they did such a good job that it doesn't appear that anything was ever in there.