I tried to compared the OEM tire sizes:

17" uses 225/50/17
18" uses 235/45/18

I would have assumed that they would be very close together in terms of speed reading. But when I compared the 17" to 18", the 18" is 1.822" too slow. When you're using the 17" tire and going 60mph, the 18" tire will show 58.9mph! I think that's pretty bad considering they're OEM tires! So which one is configured for the car's ODO? One of the coupe V6 or coupe I4 will accumulate miles faster than the other. Or do you guys think that the factory ECU takes this into account?

What do you guys think about this issue?!

One of the coupe V6 or coupe I4 will accumulate miles faster than the other.

With the 17" wheels, my car's speedometer is perfect -- it matched exactly multiple speed displays that you sometimes see the police put up. I am not sure if the speedo and the odo are related, but I assume so.

the 18" is 1.822" too slow. When you're using the 17" tire and going 60mph, the 18" tire will show 58.9mph!
You did your math wrong man. The 18s have a slightly larger perimeter, making them 1.81% faster than the 17s you listed. So 60 mph on the 17s translate to 61.1 on the coupe 18s.

To answer your question, no manufacturer I know calibrates 2 different tire sizes for the same car; that's why they're so close in perimeter (distance one full revolution of the tire covers, which is 81.236" on 17s and 82.708" on 18s). Cars with 17s should read slightly higher. Manufacturers rarely calibrate their speedos accurately for liability reasons; they're usually 2 to 3% optimistic. But that's much better than motorcycles, which typically are 6% and over. Later.
JC

You did your math wrong man. The 18s have a slightly larger perimeter, making them 1.81% faster than the 17s you listed. So 60 mph on the 17s translate to 61.1 on the coupe 18s.

To answer your question, no manufacturer I know calibrates 2 different tire sizes for the same car; that's why they're so close in perimeter (distance one full revolution of the tire covers, which is 81.236" on 17s and 82.708" on 18s). Cars with 17s should read slightly higher. Manufacturers rarely calibrate their speedos accurately for liability reasons; they're usually 2 to 3% optimistic. But that's much better than motorcycles, which typically are 6% and over. Later.
JC

I didn't do the math. I just got it from a tire calculator online and I attached the picture to show you. I think you're comparing the 17s with regards to the 18s. I'm doing it the other way around because I have the 17s

So the coupe i4s have 225/50/17 stock and the coupe v6 have 235/45/18. Are you saying that one of them has the tire calibrated wrong? Which one? I'm thinking that they have different ECUs so maybe each one is calibrated properly.

Manufacturers rarely calibrate their speedos accurately for liability reasons; they're usually 2 to 3% optimistic. But that's much better than motorcycles, which typically are 6% and over. Later.
JC

My car was calibrated exactly -- I checked this with at least 4 speed displays already. I was surprised as well.

My car was calibrated exactly -- I checked this with at least 4 speed displays already. I was surprised as well.

How do you check? If you have OEM wheels/tires, your speedo should be calibrated exactly.

Anybody else have any input?!

How do you check? If you have OEM wheels/tires, your speedo should be calibrated exactly.

I was just responding to elp_jc's comment that most cars show a few more MPH than actual. Which is true, but it doesn't appear to be so for the Accord. It's possible that the Nav system requires a higher level of accuracy when estimating your position in case it cannot get a GPS signal.

I didn't do the math. I just got it from a tire calculator online and I attached the picture to show you.
If you see the circumferences in your table, they're the same as mine. What happened is your speeds were 'indicated' on the 18s vs 'actual' on the 17s; apples and oranges, but correct as well. Sorry.

Never seen a perfectly accurate speedo yet, and some of my cars have had nav; don't think they're linked, to be honest. My 6MT has it as well, but haven't seen one of those mobile radars on the highway (slow speeds don't show the error well) to check it. And the reason manufacturers don't calibrate different tire sizes on the same car is because they all are within 2% error, which is perfectly acceptable. Check the circumference of the HFP 19" wheels with the recommended tire size and it'll be very close as well. And makes sense, since the fender gap looks similar. Good day.
JC

If you see the circumferences in your table, they're the same as mine. What happened is your speeds were 'indicated' on the 18s vs 'actual' on the 17s; apples and oranges, but correct as well. Sorry.

Never seen a perfectly accurate speedo yet, and some of my cars have had nav; don't think they're linked, to be honest. My 6MT has it as well, but haven't seen one of those mobile radars on the highway (slow speeds don't show the error well) to check it. And the reason manufacturers don't calibrate different tire sizes on the same car is because they all are within 2% error, which is perfectly acceptable. Check the circumference of the HFP 19" wheels with the recommended tire size and it'll be very close as well. And makes sense, since the fender gap looks similar. Good day.
JC

I checked and the 19" HFP wheels with the recommended tire size is very close to the coupe V6. However, it is 3.2% off from the coupe i4, which is pretty bad.