2003 Ford Focus

I can't answer your question directly, but, I can tell you that with the larger wheels, 60 mph is no longer 60mph. Rpm will still be accurate, but mpg will not. Personally, I think it would less of a hassle to go back to 15 inch wheels, than to try to reprogram the rest of the car to compensate for 16 inch.

There is no problem provided that lower profile tires are used as the wheel diameter increases. This is a plus 1 setup.

Ford uses both 15 and 16 inch wheels on the Focus. In the aftermarket, you could fit 17-inch wheels.

What you cannot do is change wheels without also changing the tire profile.

The 2.0 liter Focus gets better mileage than the 2.3 liter. Your previous Focus may have had the small bore cylinders.

RPM's are lower when the transmission shifts into overdrive. You did not indicate which transmissions the comparison vehicles had.

Vehicle has 4 speed auto with od. I will check with dealer today to see if any adjustments are necessary or helpful. Also I forgot to mention this one is a wagon and previous was 4 dr sed. Final drive appears to be 3.81 or there abouts. At this time due to finances changing wheels is not an option.

IF the speedometer reading can be calibrated by a simple computer adjustment, that should be the easiest and cheapest solution.

If a computer tweak doesn't work and if there is no suspension, brake, or body interference, I'd leave it as-is until you have the funds for a change. Make the mental adjustment as to the proper speedometer reading so you don't get a speeding ticket. Mileage "calculations" can be adjusted for the error, but do you really care? You are getting the same highway miles-per-gallon as with OEM wheels, but you are just not able to measuring it directly.

If/when you really want to fix the problem, there are 3 hardware options:

1) Calculate the proper tire(rubber) size to return the overall wheel diameter to the stock value. This is relatively easy to do and will work fine regardless of what a tire "sales person" says. Cost will be 4 new tires.
2) Go back to 15" rims and stock tires. Cost will be 4 rims AND 4 tires.
3) Ask a Ford Dealer IF they can change the speedometer gear going into the transmission/trans-axle. While the gear is inexpensive and the effort is not that difficult, their labor cost will be about the same as 2 new tires!

I live in a snow region and run the aluminum OEM rims with all-season-radials for 7 months. I use a set of inexpensive steel rims (that fit properly) with 4 good snow tires mounted for the remaining 5 months. The snow tires are actually one or two sizes smaller than OEM (much lower price and much higher availability here) so I have the opposite speedometer and mileage error. For me, this error is trivial when compared to the added performance of the snow tires in winter.

Thanks for the comments I just called the dealer and when Ford uses a 16 inch wheel tire combo they just go to a 50 series profile to drop the diameter to same as 15 inch. As long as I use the 50 profile all is ok. even if I do not still no problem. Thanks all for your comments.

Use this tire size converter and enter in the size of your stock 15" tire.

Then find the equivalent 16" tire that will match the diameter.

http://www.miata.net/garage/tirecalc.html

I personally agree that the tyres used will have the same circumference, so the speed shown should be as good as it was before....that is the normal way....

But as many speedos are up to 20% wrong, actually showing a higher speed than the true speed, I use my Tom Tom GPS to show me my "REAL" speed.

(Car tachos may show a higher speed but never a lower speed than the car is actually travelling at....it varies between manufacturers, but Volvos have the most accurate I have ever seen, usually within 3% in my experience)

As long as you are driving steadily on a fairly straight road, the speed shown on the Tom Tom GPS should be within 1% or 1 MPH or 1 KMH of the correct speed.

(Builtin GPS systems rarely if ever show the speed, I have no idea why not!!)

Then you can compare the car speedo and the Tom Tom and see where the errors are.....you can usually drive a few MPH faster quite legally, with most cars.

Do remember, that as tires wear, the displayed speed shown will increase a tiny fraction, for the same GPS speed.....I have not actually worked it out exactly myself, but it will depend upon the circumference change from new to old.....but its probably not worth worrying about!!!

I use my Garmin gps to validate the speedos on the various vehicles I drive. I have found that most 4 wheel vehicles show 5-10 mph low when the gps is at 60 mph. My Honda motorcycle is at 55 mph when the gps is at 60 and is at 1 for 1 at 35 mph and below.

Oddly enough, my new Kia Soul is spot on with the gps from 0-80 mph. 80 is about as high as I care to go and usually only for passing.

In the old days we used to presume that speedos were calibrated on the money at 60 mph. Apparently that isn't true anymore (if it really ever was).

Hooker

In Europe, they are not allowed to show a lower speed than the correct speed.....but up to 20% more is OK(!!)....

I guess its European law and not the law for Canada....dangerous for Canada as you can drive 10 mph faster and still blame the car's speedo quite legally it would appear......

Here if you drove at a speedo speed of 60 MPH, you might only be actually doing 55 MPH on the Tom Tom. A tick safer I feel.