Change of tire size and speedometer error

Hello, I have an ’ 87 Dodge Caravan cargo van that came originally with 195-70-14 size tires. I recently got for free, a like new set of 205-75-14 tires that I put on to replace the worn old set. My question is how will this throw off my speedometer, say at 35 mph or at 55-65 mph on the highway? Thanks in advance.

You can find your answer here:

That tire’s taller and wider, looks like you’ll be about 3.6 mph off at 65 mph indicated (you’ll actually be going 68.6 mph) according to this:

Make SURE your new tires aren’t rubbing when you turn or hit bumps.

Personally, I’m all for it’s long as they fit and the rims can handle the slightly wider tires.
Another good site is found googling “powerdog”.

I’m thinkin’ those 14 inch 70 series tires are not factory tires to begin with. Maybe they are (or should be) 78 series instead?

Either way, put on the new wheels, head to the highway with a friend, find a stretch you can drive 60 mph. Use a stopwatch or an app on your phone app to time the mileposts. Run through 3 or 4 timing the event. If the speedo is correct, you will pass a milepost marker every 60 seconds. If it is shorter, you are going faster. Reduce your speed by a however many seconds shorter you are. Test again at your adjusted speed on the way back. Remember that number and drive accordingly.

At 60 mph time the number of seconds in a mile and divide 3600 by the number of seconds. That is your speed.

Oldtimer’s calculation works for any constant speed. Be sure to use the mile markers along the road and not the odometer.