Tire size and speedometer reading

[CowboyTJ]

How tire size affects speedo.
The sticker on the door frame says my TJ came with 205/15s.
But probably had 215/15s. It now has 235/15s and that affects the speedo by 5 mph. Speedo says 45 mph, actually doing 40 mph.
But the odometer reading is the same as the GPS reading for miles. Odometer says 100 miles GPS says 100 miles give or take a few tenths. How can this be?

 

[jwilkinson01]

You are only listing the section width of your different tires. Without the second number, which is the height aspect ratio, there is no way to know the difference in height, which is what is going to affect your speedo.

I always buy my tires in standard sizes: 30”, 33”, so I am not as familiar with the different metric sizes available. When I switched from 30 to 33 my Speedo was off quite a bit.

 

[OldBuzzard]

Assuming you are now on larger diameter tires (but we won't know until you provide the full tire size) — your speedometer should read lower than the actual speed. So that's weird. Did a previous owner change the speedo gear on the transfer case? The odometer thing is weird, too.

 

[Apparition]

Width is in millimeters
Aspect ratio is a percentage
Rim is in inches

Width conversion to inches 225/25.4 = 8.858 inches

Width x aspect ratio x 2 + Rim = height of tire

8.858 x 0.70 x 2 + 16 = 28.4"

 

[CowboyTJ]

The middle number was 75 in all of them. My question is why would the miles per hour be different and the miles traveled be the same.?

 

[jwilkinson01]

205/75/15 to 235/75/15 is a substantial difference. Did you replace the factory size tires with the 235s or was it already like that when you got it? If a previous owner put on larger tires they may have also corrected the Speedo: changed the gear or Speedo healer (I think that’s the name).

Edit: apparently my reading comprehension is not very good this morning. I misread that the Speedo is off but odometer is correct. Sorry.

 

[OldBuzzard]

Since the odometer agrees with the GPS so well, that implies that the correct signal is coming up from the sensor. So perhaps the speedometer itself has a problem?

 

[TheBoogieman]

Why does this matter and who actually cares about that small of a discrepancy. Maybe drive 53 in a 55 zone so you obey the law.

 

[NashvilleTJ]

Why does this matter and who actually cares about that small of a discrepancy. Maybe drive 53 in a 55 zone so you obey the law.

Boog, you are clearly not OCD enough…

 

[JEEPCJTJ]

Why does this matter and who actually cares about that small of a discrepancy. Maybe drive 53 in a 55 zone so you obey the law.

That's not a bad question but his speedo is reading 45 while traveling only 40. That means he's obeying the law even if he thinks he's speeding.

Now I have to find out how to modify my wife's JK and her Forester to do the same.