[doug121041]

Just made a quick 2,000 mile trip with my new SEL hatch and found that the speed readout on my Scan Guage II and my GPS don't agree. The Scan Guage plugs into the OBD port and on my previous vehicle (Mercury Mariner) the Scan Guage readout and my GPS were in sync with each other.

On this Focus, when the Scan Guage says I'm going 70 MPH, my Garman GPS says I'm going 72 MPH. I also noticed that traveling a defined route which I've traveled many times before, the odometer also reads about 2.5% low (1,000 miles traveled only reads 975 on the odo).

I'm wondering if the 17" wheels are responsible for the discrepancy (if the car was designed for 16" wheels and the speedometer and odo calibrated accordingly, then installing 17" wheels would seem to be the reason that the speedometer and odo are now reading low).

Has anyone else noticed this on their SELs with 17" wheels?

 

[immaengineer]

Just made a quick 2,000 mile trip with my new SEL hatch and found that the speed readout on my Scan Guage II and my GPS don't agree. The Scan Guage plugs into the OBD port and on my previous vehicle (Mercury Mariner) the Scan Guage readout and my GPS were in sync with each other.

On this Focus, when the Scan Guage says I'm going 70 MPH, my Garman GPS says I'm going 72 MPH. I also noticed that traveling a defined route which I've traveled many times before, the odometer also reads about 2.5% low (1,000 miles traveled only reads 975 on the odo).

I'm wondering if the 17" wheels are responsible for the discrepancy (if the car was designed for 16" wheels and the speedometer and odo calibrated accordingly, then installing 17" wheels would seem to be the reason that the speedometer and odo are now reading low).

Has anyone else noticed this on their SELs with 17" wheels?

The positioning hardware in the Garmin isn't accurate enough in my opionion to make a ruccus over it.

 

[completely_focused]

That does sound like someone forgot to set the option for 17" wheels in the main computer. Check with InsectQueen here on the forums if there is an easy way to change that setting in the computer or at least check that it is correct. In any case the dealership should be able to fix this for you.

Changing wheel size definitely makes a difference in speedometer readout. Check this site to calculate the difference (over 4 mph).

http://www.therangerstation.com/tech_library/Speedometer_Change_From_Tires.html

 

[Benchmark]

The diameter of a 215/50R17 is only 0.6% larger than a 215/55R16. Even if there is no difference in the computer setting, the difference in speed or odometer reading should only be 0.6%. Approx. 70.4mph vs 70mph & 1000 miles vs 994 miles.

 

[completely_focused]

The diameter of a 215/50R17 is only 0.6% larger than a 215/55R16. Even if there is no difference in the computer setting, the difference in speed or odometer reading should only be 0.6%. Approx. 70.4mph vs 70mph & 1000 miles vs 994 miles.

I just used 16" > 17" @ 70mph and pluged them into this tire size calculater and the difference was 4.4mph!!!

http://www.therangerstation.com/tech_library/Speedometer_Change_From_Tires.html

 

[Joeywhat]

I just used 16" > 17" @ 70mph and pluged them into this tire size calculater and the difference was 4.4mph!!!

http://www.therangerstation.com/tech_library/Speedometer_Change_From_Tires.html

You use the outer diameter of the tire, not the wheel size.

 

[Benchmark]

I just used 16" > 17" @ 70mph and pluged them into this tire size calculater and the difference was 4.4mph!!!

http://www.therangerstation.com/tech_library/Speedometer_Change_From_Tires.html

You cannot simply put 16" & 17" into the calculator. 16" & 17" are wheel diameters. The tire diameters are approximately 25.31" & 25.46" respectively. 215/55R16 vs 215/50R17

 

[completely_focused]

You use the outer diameter of the tire, not the wheel size.

There is no such thing as outer diameter. Diameter is double the radius which is in this case 17". You are talking about circumference which is not what the calculator was asking for. Back to math class for you.

 

[Joeywhat]

There is no such thing as outer diameter. Diameter is double the radius which is in this case 17". You are talking about circumference which is not what the calculator was asking for. Back to math class for you.

You can't be serious...the calculator even says 'tire diameter'.

 

[benkap]

When discussing a series of concentric circles, such as a tire on a rim, there are multiple places to measure the diameter. Outer diameter is a perfectly acceptable term in this instance.

Back to math class with *YOU*.

 

[completely_focused]

Dude, you don't have a clue what diameter is do you. I don't want to argue about it but do your self a favor and just google 'diameter' and find out what it really is. It's very simple really. You have 16" diameter tires, 17", and even 18" diameter tires.

 

[wag4513]

Here is what you do:

Make a chalk mark on the tire and the ground, drive the car forward or backward so the tire makes one complete revolution and mark the ground at that point. Then measure the distance between the two marks. That should be the circumference of the tire, taking into account the fact the tire is flattened where it contacts the road.

 

[Benchmark]

There is no such thing as outer diameter. Diameter is double the radius which is in this case 17". You are talking about circumference which is not what the calculator was asking for. Back to math class for you.

Oh boy. So you're saying all XXX/XXR17" tires, regardless of what the X's are, would give you the same speed and distance readings?

 

[Joeywhat]

[facepalm]

Ignorance is bliss, I guess.

 

[benkap]

No, you have a 15, 16, or 17 inch diameter RIM.

The tire is that rubber thing that goes around the outside of it.

The diameter of the tire is the important variable in this equation.

You could have several different sizes of tire on a 16 inch rim that would effect the final, or "outer" diameter of the entire wheel assembly. This is then used to calculate the circumference of the wheel assembly. Which is then used to calculate the distance traveled per rotation. Which is what is being discussed here.

Not an argument. A statement of clear fact.

 

[wag4513]

For the diameter measure from one side of the tire to the other side passing through the center of the wheel. Bottom to top is probably best.

 

[Hypnotoad]

Dude, you don't have a clue what diameter is do you. I don't want to argue about it but do your self a favor and just google 'diameter' and find out what it really is. It's very simple really. You have 16" diameter tires, 17", and even 18" diameter tires.

Wrong. 16" and 17" are RIM diameters, not the tire diameter. A 16" diameter tire would be suitable for a wheelbarrow.

 

[completely_focused]

All three tires are going to compress essentially the same amount so it doesn't factor in to the calculation. The point of this thread is that if you change the size of the tires on the focus or any car for that matter, it will significantly change the speedometers accuracy. 4.4mph is significant to me at least.

 

[completely_focused]

Wrong. 16" and 17" are RIM diameters, not the tire diameter. A 16" diameter tire would be suitable for a wheelbarrow.

OK, that's my mistake that tire sizes are not actually tire size, rather rim size. So that does change the calculation. I'm obviously not a tire expert.[dunno]

So what are the numbers for tire diameters to plug into the calculator?

 

[benkap]

All three tires are going to compress essentially the same amount so it doesn't factor in to the calculation. The point of this thread is that if you change the size of the tires on the focus or any car for that matter, it will significantly change the speedometers accuracy. 4.4mph is significant to me at least.

No, all tires are not going to compress the same.

If you have a 16" rim with a tire that has a 4" sidewall, you have 24" of diameter.

If you have a 16" rim with a tire that has a 2" sidewall, you have 20" of diameter.

If you have a 16" rim with a tire that has a 1" sidewall, you have 18" of diameter.

Not only are these all different sizes of diameter, which will give you different circumferences, which will give you different amounts of travel per rotation, they will also give you different amounts of "compression". This is why the ride quality of a low profile tire is less comfortable than that of a normal tire.

Feel free to read about the different tire sizes and ratings on our sponsor's website to better educate yourself before giving advice on the subject.