Mechanics

DODGE VAN TIRE PROBLEM

1990 Dodge Van • 176,000 miles

I have 1990 dodge B250 3/4 ton and it currently has 15" rims and tires on it 235/75 R15. Someone gave me 235/75 16's. Currious if I can get some 16" rims and put these tires on my van? Will this damage anything? Is it even possible? Thanks for your time!
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Helpmepls1990dodgeB250
January 26, 2013.




As a suspension and alignment expert, I would never do anything that alters where the tire tread sits on the road in relation to the van. If the wheel offset is different, the tire will move left or right. That will have a big affect on braking and steering response. If the diameter of the tire is larger than what the van was designed for, that will also affect those things. The secondary alignment angle that no one talks about or is aware of is called "scrub radius". If you look from in front of the van back to the two ball joints, and draw an imaginary line between them, that line must intersect the road surface exactly in the middle of the tire tread. That causes the inner half of the tire tread to want to pull toward the center of the vehicle, and the outer half to pull outward. The two forces counteract each other and make that tire want to go straight. When you alter scrub radius, each tire will respond to every bump in the road and make driving the van very tiring.

If you buy new wheels, be sure they're designed specifically for your van to maintain the same offset. If the new tires are taller, you'll have to worry about them rubbing on the fenders not only when standing still and turning the steering wheel, but when turning and going up and down over bumps.
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Caradiodoc
Jan 27, 2013.
I guess I assumed the tires would ultimately be the same size accept on the middle of the tire itself if they are both 235/75-(15 or 16) 15 or 16 being the inside diameter of the tire. This is not true? Forgive my ignorance as I am not a suspension or alignment expert. Thank you for your response.

I've always stuck with original size tires while working at a Chrysler dealership. As the owner of many Dodge muscle cars from the '70s, I know what looks better than the original sizes, but when it comes to handling, those cars were pretty primitive in their engineering compared to newer stuff, so you couldn't really degrade their handling too much.

As an instructor I spend a lot more time on correct ride height. Altering that has an even bigger affect than changing tire sizes and is the type of modifications lawyers and insurance investigators love to find. I can't stop my kids from from making those changes to their cars and trucks, but they won't do it in my shop.

The "235" in your tire sizes refers to 235 millimeters across the tread, so that is the same for both tires. The "75" means the sidewalls are 75 percent the size of the tread, or in this case, 176.25 millimeters. Since the 16" tire is 1" larger in center diameter, and the sidewalls are the same, the outer diameter also has to be 1" larger. That's pretty significant. The circumference will be over 3" larger. That will make your speedometer read lower than your actual speed. I can't say for sure, but I would suspect that even if the tires don't rub on the fenders, they are likely to rub on the strut rods that go from the lower control arms to the frame brackets under the seats.

Caradiodoc
Jan 27, 2013.

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