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The 245/50 is also going to be at least 1/2 less in diameter and that will make a small difference in the speedometer and odometer readings. The 245/50 is suitable more for a 7" to 8.5" wide wheel. If you have 16x9.5" wheels on all four corners, I would stay with a 255/50-16 tire as that size is correct for the 9.5" wide wheel
IMHO, spending the roughly extra $120 for the proper size tires is worth the money.
Your talking about a $250 savings over the life of the tires, maybe 3 years? That comes out to less than two tankfulls of gas a year. Go with the correct tires and your car will handle better and not look funky.
Speedo would be off by how much?? If it is negligible.....I might take that savings
Tire Rack or another site should have a table or calculator for you to know exactly. 255s have a shrinking market, so they've been disappearing/becoming more expensive. I suspect that the error is within the factory tolerances, anyway.
I'd rather go larger than smaller if saving were my goal. that said, please martyr your billfold and get the 255s so the market remains.
Several years ago, I used 245's more than once. The speedo may be off a little, but no problems at all. Look around, sometimes you can get 245's for as low as $50 each.