You need to find a combo of sidewall height and wheel diameter that as closely matches your original as possible. I've always heard that you need to shoot for matching circumference to less than 0.5%

 

The diameter of the stock wheels/tires is 25.4"

 

The circumference difference is almost non-existent going from the stock 235/40-18 to 235/45-17. Do a Google search for "tire size calculator" and you can check all the sizes you are interested in.

 

All good answers guys.
While the factory tire setup is 25.4" i'am sure ford inputs that into the computer so the spedo reads proper,so there should be a way to change that setting instead of being stuck on using something that is 25.4" in diameter.
If we can't oh well.my tire setup shows 25.0" diameter and doubtfull it would change to much.
Back in the 90's i had a 1994 pontiac firehawk and flashed the computer with a hypertech tool and remember being able to change tire diameter so the spedo would be accurate.

 

235/45r17 is what I plan on getting for my winter tires. Difference is 0.3% and if you use one of the online calculators, it will show speed differences of 0.5-0.6 mph max for most normal conditions.

 

The accuracy of modern speedometers is +/- 5%. Part of that was including the differences in possible tires size configurations, manufactures, and tire wear.

 

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

 

Here's every single possible 18" tire that's within 2% of our OEM tire, with a width between 200 and 265. Some of these sizes are not even made, but I figured I'd include them anyway.

Wheel Description	Width	Ratio	Rim Diameter	TOTAL WHEEL DIAMETER, inches	Tire wall, inches	Tire wall, mm	Diameter Difference from OEM	True speed when speedo shows 50mph	Odometer reading for true 1000 miles
235/40-18	235	40	18	25.40	3.70	94.00	0%	50.00	1,000.00
265/35-18	265	35	18	25.30	3.65	92.75	-0.39	49.81	1,003.87
205/45-18	205	45	18	25.26	3.63	92.25	-0.54%	49.73	1,005.42
215/45-18	215	45	18	25.62	3.81	96.75	+0.85%	50.43	991.48
225/40-18	225	40	18	25.09	3.54	90.00	-1.24%	49.38	1,012.40
245/40-18	245	40	18	25.72	3.86	98.00	+1.24%	50.62	987.60
255/35-18	255	35	18	25.03	3.51	89.25	-1.47%	49.26	1,014.72

Like it's been said, the speedo will be off more than the difference in diameter for a properly sized tire.

 

Thats a good chart:RockOn:
I knew when i ordered my 255/35/18 yokohama advan ad08r tires that my spedo would be off some,but it's not to bad.
My spedo will show 60.9 when actually doing 60 mph and over a period of say 10,000 miles with those tires the odometer would show 140-200 miles farther vs the factory tires.
That really would not affect the value of the vehicle should i ever decide to sell it.
It would be nice if cobb could give us the ability to change tire diameter size thru there ap.

 

I believe you have to use revs per mile when programming.

This calc was spot on when I used it to program my ZX3 with the SCT handheld:

Tire Size Calculator - Tire & Wheel Plus Sizing