How much speedo error w/ 18' wheels
08-31-2006, 12:38 PM
#3
Thread Starter
Join Date: Aug 2006
Posts: 44
RE: How much speedo error w/ 18' wheels
I want to change my tires and wheels to the factory 18" wheels and tires combo(BF goodrich tires, I think). My car came with 17' painted wheels(yuck!) Dealer says that I'd have to change the pinion gear to avoid speedo error.
08-31-2006, 12:41 PM
#4
3rd Gear Member
Join Date: Mar 2005
Location:
Posts: 864
RE: How much speedo error w/ 18' wheels
If you want to know exactly go here http://www.richmondgear.com/01calculators.html
Click the "Street Performance Calcuator"
You will need to know your tire diameter, overdrive ratio and final drive ratio
O.D. Ratios for 05, 06 Mustangs:
Manuals V6 ansd GT have an O.D. ratio of 0.68
Automatics have an O.D. ratio of 0.71
Rear axle ratios are:
GT Auto = 3.31
GT Manual = 3.55
V6 Auto and manual are 3.31
Click the "Street Performance Calcuator"
You will need to know your tire diameter, overdrive ratio and final drive ratio
O.D. Ratios for 05, 06 Mustangs:
Manuals V6 ansd GT have an O.D. ratio of 0.68
Automatics have an O.D. ratio of 0.71
Rear axle ratios are:
GT Auto = 3.31
GT Manual = 3.55
V6 Auto and manual are 3.31
08-31-2006, 12:41 PM
#5
I ♥ Acer
Join Date: Mar 2006
Location: Concord, NC
Posts: 1,278
RE: How much speedo error w/ 18' wheels
Your dealer is an idiot, the pinion is matched to the ring gear. You can't change one without the other. As long as you keep the overall tire height the same your speedo will remain correct.
08-31-2006, 12:54 PM
#6
Thread Starter
Join Date: Aug 2006
Posts: 44
RE: How much speedo error w/ 18' wheels
Is the tire height the same for the 17" Pirelli Nero Zeros and BF goodrich 18"....it couldn't be right?
All I wanted to do was get the 18" combo that they had promised in the first place.....but they couldn't locate a car with the wheels I wanted. I just don't want to go with the 18s and have too much speedo error.
All I wanted to do was get the 18" combo that they had promised in the first place.....but they couldn't locate a car with the wheels I wanted. I just don't want to go with the 18s and have too much speedo error.
08-31-2006, 12:56 PM
#7
Join Date: Aug 2006
Location: Valrico, Fl
Posts: 37
RE: How much speedo error w/ 18' wheels
You can validate your true speed with a hand help gps device. Almost every car manufacture reduces the true speed from the speedometer display. I guess this may help some of us from getting tickets. 
08-31-2006, 01:05 PM
#8
5th Gear Member
Join Date: Mar 2006
Location:
Posts: 2,544
RE: How much speedo error w/ 18' wheels
Yeah, your dealer is definatley a moron.
On OLD cars the way you recalibrated your speedo was to change little plastic gears on the speedo drive cable. That is probably what the dealer meant by "change the pinion".
On newer cars like this one, the speedo setting is done with a computer. A tuner (which is often used for HP upgrades) such as the SCT-II or the Diablosport Predator can be used to change this. It just takes a few button pushes. I would imagine that the dealership has electronics that can do it as well.
As others have said, the WHEEL diameter doesn't matter. It's the overall outside diameter of the tire that matters. When you look at a tire, there are three different "numbers" describing that tire. There's the wheel (rim) diameter (for example 17" or 18"), the width, and the section height. The section height determines the diameter. But, to cut a long story short, you most certainly can get 18" tires that have the same OD (height) as your stock 17s.
That means that if you get the proper diameter tires--such as those that Ford uses factory on the GT with 18" wheels--you don't have to adjust the speedometer at all.
Personally, I would look for a used set of 18" wheels and tires off an existing Mustang. You can probably find this very inexpensively. It is VERY common for people to get rid of their factory wheels/tires to get custom ones. I'd offer you mine, but I already sold them months ago.
On OLD cars the way you recalibrated your speedo was to change little plastic gears on the speedo drive cable. That is probably what the dealer meant by "change the pinion".
On newer cars like this one, the speedo setting is done with a computer. A tuner (which is often used for HP upgrades) such as the SCT-II or the Diablosport Predator can be used to change this. It just takes a few button pushes. I would imagine that the dealership has electronics that can do it as well.
As others have said, the WHEEL diameter doesn't matter. It's the overall outside diameter of the tire that matters. When you look at a tire, there are three different "numbers" describing that tire. There's the wheel (rim) diameter (for example 17" or 18"), the width, and the section height. The section height determines the diameter. But, to cut a long story short, you most certainly can get 18" tires that have the same OD (height) as your stock 17s.
That means that if you get the proper diameter tires--such as those that Ford uses factory on the GT with 18" wheels--you don't have to adjust the speedometer at all.
Personally, I would look for a used set of 18" wheels and tires off an existing Mustang. You can probably find this very inexpensively. It is VERY common for people to get rid of their factory wheels/tires to get custom ones. I'd offer you mine, but I already sold them months ago.
08-31-2006, 01:10 PM
#10
2nd Gear Member
Join Date: Feb 2005
Location: West Virginia
Posts: 295
RE: How much speedo error w/ 18' wheels
I'm sure I'm repeating what's been said above, but speedometer cares about the overall wheel package diameter, i.e. rim + tire. So, to keep from having to recalibrate the speedometer, most people go to a lower profile tire as they increase the wheel size such that the new package has the same overall diameter as OEM. For example, when I swapped out my 17" OEM wheels/tires for 18" wheels, my overall diameter was almost identical to OEM (maybe off by a 1/16 of an inch).