Speedometer Off?
01-21-2010, 12:31 PM
#1
Speedometer Off? i was crusing down the highway at 75 one day and look over and im passing a civic whos speedo said 80 
so i had my girlfriend follow me in her 08 Mazda 3 stock. while i was crusing at 75 i called her up and had her pace me. she said her speedo said around 80. but whenever i pass those speed checkers that the cops set up it said that my speed was correct. but that was going 35. i have put 18" rims so could that be the problem and it cannot be detected going at low speeds?
so i had my girlfriend follow me in her 08 Mazda 3 stock. while i was crusing at 75 i called her up and had her pace me. she said her speedo said around 80. but whenever i pass those speed checkers that the cops set up it said that my speed was correct. but that was going 35. i have put 18" rims so could that be the problem and it cannot be detected going at low speeds? 
01-21-2010, 12:40 PM
#2
LS1Tech Administrator
iTrader: (3)
Join Date: Nov 2001
Location: Schiller Park, IL Member: #317
Posts: 32,010
Likes: 0
Received 1,462 Likes
on
1,054 Posts
It's not about the wheel diameter, but rather the overall tire diameter.
The stock tires on these cars are 25.66" tall. If your current tires are significanty taller, then this could be the reason.
What size tires are you running?
The stock tires on these cars are 25.66" tall. If your current tires are significanty taller, then this could be the reason.
What size tires are you running?
01-21-2010, 01:42 PM
#4
LS1Tech Administrator
iTrader: (3)
Join Date: Nov 2001
Location: Schiller Park, IL Member: #317
Posts: 32,010
Likes: 0
Received 1,462 Likes
on
1,054 Posts
Ah, that explains it then.
Stock 245/50/16 or 275/40/17 is 25.65" and 25.66" tall respectively.
Your tire (285/40/18) is 26.98" tall. That's a big difference.
So with those specs, the calulator at Discount Tire says that if your speedo is showing 75mph, you're actually going 78.89mph.
Stock 245/50/16 or 275/40/17 is 25.65" and 25.66" tall respectively.
Your tire (285/40/18) is 26.98" tall. That's a big difference.
So with those specs, the calulator at Discount Tire says that if your speedo is showing 75mph, you're actually going 78.89mph.
