If the tire size on a vehicle calls for 215/75R and you instead install larger tires(225/75R) how far off will the speedometer be off and in which direction? Faster or slower?
Well I could work out the mathematics of this and waste lots of time, but I think I’ll give you the short answer. The aspect ratio is a percentage of the width of the tire, so 75% of an increase of ten miilimeters across would mean that the tire is 7.5 millimeters taller overall. This is very minor. The wheel will now be turning slightly slower than it did before so your speedomter will register slightly slower. How much? I’ll say 1 mph if that.
Well you need the wheel size as well, but assuming 15 inch wheels and both original tread the new tyres will make the speedometer and odometer 2.1% slower/lower than they are now.
Note: a 2 - 3% error rate in the speedometer is more or less intentionally standard in most cars, so you are currently likely showing about an equal amount fast on your speedometer so with the new tyres your speedometer should be more accurate, but the odometer will likely under report the distance traveled by about 2%.
Google “Tire Calculator”. You will find a lot of web sites where you can calaculate the difference in OD.
While these calculators are based on slightly false assumptions - and tires aren’t always built to the dimensions the tire size indicates - the calculator will be good enough for the purpose of estimating the difference.
My 1995 Dakota came with 215/75R15 tires that were terrible. The local tire dealer recommended 225/75R15 tires (235/75R15 tires were too big). The 2% difference in the circumference was not noticeable in day-to-day driving. I later replaced the 225/75 tires with 235/70R15 tires. The difference between 215/75R15 and 235/70R15 tires was ~1%. Here are the specs for different sizes for a particular tire (Firestone Destination LE) from Tirerack.
http://www.tirerack.com/tires/Spec.jsp?make=Firestone&model=Destination+LE&tirePageLocQty=
Ed B.
in other words , the van will be traveling slightly faster than the speedometer shows. On a long stretch of highway drive at 60 mph and time the mile markers as they pass. Having a helper with you makes this easy. For a true 60mph you should pass each mile marker in exactly 60 seconds. Adjusting your speed until you see 60 seconds a mile will tell you how far off the speedo is. Now when the cop stops you and says “do you know how fast you’re going” you can just say “oops”.