Can I use 235/75-15's instead of 230/70-15s on my '93 Buick Roadmaster Wagon?

Yes, but make sure the rear (drive) tire size always remains the same, because different size tires can ruin your differential

The change will cause your speedometer to be off. Drive and check it against another vehicle to find out how much you are off and take that in mind to know what your actual speed is.
 
I'm not surprised that the 235/75-15s are cheaper since there is no such thing as a 230/70-15 tire on the market.

Now; if you are referring to a 235/70-15 tire the rolling diameter difference will be so negligible that it won't throw off your speedo by more than a few MPH at an indicated 60MPH.

And I have absolutely no idea what VENTURA or SEEKER are talking about.

235 is not diameter it's the tread width in millimetres

75 is the pecentage of sidewall height in relationship to tread width.

Nowhere on a tire does it indicate diameter or circumference.
 
Better check on a 230/75-15 instead. The first number relates to tire diameter. If you change it your speedometer will read too slow and you'll get a ticket. The 70 is a wider tire and therefore more expensive.The 15 is the size of the rim diameter. The first number gets bigger as the diameter increases. The second number gets smaller as the tire gets wider. Therefore a 235 is taller and has a bigger diameter than a 230. A 70 is wider than a 75. The difference in the width shouldn't cause you any trouble. It is a small difference.
Good luck
 
Yes, but speedometer will read 4-6mi slow at 60mph
Tire sizes 1st#=profile height/ distance from rim to tread face
2nd#=tire width/70=7.0in/75=7.5in, Ok to go 1/2in wider
3rd#=the rim diameter
A higher profile=a greater total circumference of the tread face. Because you're going 1 profile size higher you're covering a little more distance with each turn of the rim making your speedometer read a little slow since it can't adjust to the new tire. The math for your speedometer was factory set to the original tire size. Good Luck!
 
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