2005 - 2014 MustangsDiscussions on the latest S197 model Mustangs from Ford.
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Sounds like your cars ecmwas set up for 18" wheels. If so they should be able to fix at the dealer.
Stock 17 and 18" rims have the same Revs Per Mile as the tires are different sizes.
You can swap OEM 16s for OEM 17s or OEM 18s and not need to change the Revs
Per Mile setting.
This is true as long as the diameter of the tire remains the same. As the diameter of the tire decreases the revolutions per mile increase, and as the diameter of the tire increases, the revolutions per mile decreases. Please, correct me if I'm wrong.
2006 Tungsten GT 5spd
Mac Boom Tubes
Tuneable Induction CAI
B@machip$ 93 Race Tune
Raptor Shift Light
CHE K-Member w/ Torque Limiters
Granatelli strut tower brace
Mac O/R Prochamber
Steeda CMCV Deletes
I noticed it about a year ago...35-40 mph, my car's speedo was dead on accurate. At 70-80 mph on the speedo, my car's speedo is off by about 5-7mph (I'm going anywhere between 65-75 mph per GPS, but the speedo reads higher).
I figure I'll just fix it when I do the tune. Fire & forget, I guess.
2007 Tungsten Grey Mustang GT
1974 Chevrolet Corvette (in the process of rebuilding)
1967 Jaguar E-Type 2+2 4.2L, British Racing Green, aka The Green *****
Wow - I thought it was just me. I installed 4.10s a while back, used the XCal2 to reset the rear diff and also put in the rev/mile for good measure. Speedo has been off just like you guys say, in the higher MPH ranges. Someday when I feel like it I'll probably mess around with the settings in the XCal, see if I can get it closer to reality.
07 GT 5spd: stock
99 C5 FRC: track toy
Good to know other people are having this problem.
Anyways, here's an update:
Took my car to the dealer today to have em look at it. They said sometimes it doesn't come from the factory with the right calibration and it's probably just a simple software fix.
I didn't have time to leave it with them but if your cars are still under warranty I'd reccomend you do that. Some people might not think it's a big deal but keep in mind that your odometer might be reading off too. This means that there's anywhere between 5-15% extra mileage on your car than there actually is; not something to scoff at for resale value etc. That's my major concern anyways.
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