Speedometer not Matching Trip Odometer
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
Speedometer not Matching Trip Odometer Ever since I've had my car the speedometer has been off. I figured it was because of the tires being the wrong size. So I switched out the tires for the right sized ones. Speedometer is still off. I even tried resetting the ECU by disconnecting the negative charge to the battery. Let it sit for about 45 mins. Plugged the battery back up and the speedometer is still off. Weird thing is that my radio stations didn't didn't get wiped, they were all still programmed in. I don't know if resetting the ECU actually effects the speedometer... I just tried it to see if it would work. Anyways, with the old tires the trip was always off. A tank of gas would last me about 100 miles (according to the trip). With the new tires the trip reads around 150 right now sitting on 1/4th a tank and this sounds way more accurate. So apparently the trip is rolling on the miles right...but the speedometer is still off. Anyone have any idea what the problem could be? Maybe I just don't know how to reset the ECU correctly? Anyone that can help, please enlighten me. I'm tired of going by cops and not knowing how fast I'm going. 
TECH Senior Member
Joined: Dec 2001
Posts: 5,399
Likes: 3
From: Jacksonville, FL
Resetting the PCM won't adjust/reprogram the speedometer. Aftermarket gears in the car? Visit www.f-body.org/gears/ and plug some different ratios in and see if you can find a match.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
I hate to ask, but is there any way to check if there are aftermarket gears on the car? And if so check what gear ratio they are?
And as for the site. When it lists the chart on the right, what do all the numbers exactly mean? Use those numbers to program it? Or is that how fast your going at those RPMs in whatever gear?
Sorry for so many questions.
Thanks again.
And as for the site. When it lists the chart on the right, what do all the numbers exactly mean? Use those numbers to program it? Or is that how fast your going at those RPMs in whatever gear?
Sorry for so many questions.
Thanks again.
TECH Senior Member
Joined: Dec 2001
Posts: 5,399
Likes: 3
From: Jacksonville, FL
It's how fast your going for any given gear/tire size/RPMs. Get someone beside you on the highway and do 70 for the gear you're in, note your RPM's and then bounce it against the chart.
TECH Enthusiast
Joined: Nov 2001
Posts: 662
Likes: 3
From: Queens, NYC and sometimes Orlando
Originally Posted by noname024
I hate to ask, but is there any way to check if there are aftermarket gears on the car? And if so check what gear ratio they are?
It's easy, just get under the car and mark the driveshaft and position of the wheels....
Turn the driveshaft and count the number of rotations it makes before the tires make one complete rotation...
If say you had 3.73's, the driveshaft would turn about 3 and 3/4 turns before the wheels made one full turn....
If you had 4.10's, the driveshaft would make a little over 4 turns before the wheels make one full turn....
Note, when doing this test, make sure both rear wheels turn evenly....
Rick
Aztek T/A WS7 & 70 RAM AIR IV T/A
Moderator @ LS2, FAST LS1, FUELSLUT,
The F-Body Hideout, LS1Turd & ChopperForums
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
Originally Posted by 98 SNAKE EATER
It's easy, just get under the car and mark the driveshaft and position of the wheels....
Turn the driveshaft and count the number of rotations it makes before the tires make one complete rotation...
If say you had 3.73's, the driveshaft would turn about 3 and 3/4 turns before the wheels made one full turn....
If you had 4.10's, the driveshaft would make a little over 4 turns before the wheels make one full turn....
Note, when doing this test, make sure both rear wheels turn evenly....
Rick
Aztek T/A WS7 & 70 RAM AIR IV T/A
Moderator @ LS2, FAST LS1, FUELSLUT,
The F-Body Hideout, LS1Turd & ChopperForums
Turn the driveshaft and count the number of rotations it makes before the tires make one complete rotation...
If say you had 3.73's, the driveshaft would turn about 3 and 3/4 turns before the wheels made one full turn....
If you had 4.10's, the driveshaft would make a little over 4 turns before the wheels make one full turn....
Note, when doing this test, make sure both rear wheels turn evenly....
Rick
Aztek T/A WS7 & 70 RAM AIR IV T/A
Moderator @ LS2, FAST LS1, FUELSLUT,
The F-Body Hideout, LS1Turd & ChopperForums
And thanks 2001NBMZ28. Will do.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
Ok, just did the speed test. And at 70 in 5th gear it was hitting about 2000 rpms. I checked the chart for my 98 M6 and it said that 5th at 2000 rpms was about 60 mph for a 3.42 gear ratio. I tried the other ratios, 3.73 and 4.10, and the speed just decreased. Anyone know what this means? I thought 3.42 was stock.
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TECH Enthusiast
Joined: Nov 2001
Posts: 662
Likes: 3
From: Queens, NYC and sometimes Orlando
Originally Posted by noname024
Ok, just did the speed test. And at 70 in 5th gear it was hitting about 2000 rpms. I checked the chart for my 98 M6 and it said that 5th at 2000 rpms was about 60 mph for a 3.42 gear ratio. I tried the other ratios, 3.73 and 4.10, and the speed just decreased. Anyone know what this means? I thought 3.42 was stock.
Sounds more like you have 3.23's in there then
Perhaps someone slapped an A4 rear in there before you bought it?
Do the driveshaft test and you'll know for sure....
Rick
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Verdad Gallardo Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
Originally Posted by 98 SNAKE EATER
Sounds more like you have 3.23's in there then
Perhaps someone slapped an A4 rear in there before you bought it?
Do the driveshaft test and you'll know for sure....
Rick
Perhaps someone slapped an A4 rear in there before you bought it?
Do the driveshaft test and you'll know for sure....
Rick
Launching!
Joined: Aug 2005
Posts: 227
Likes: 0
From: Burlington, NC
noname024, here is a formula to help ypu deside your rear axle ratio. First, these figures are based on straight 1 to 1 drive. In other words; if you have an automatic, lock it into third gear to get the figures. Should you have a stick shift (5 or 6 speed) use forth gear to get your figures.
Now, you will need the circumfrance of your rear tires in feet. For an example, if your rear tires are 84.5 inches around, that is 7.04 feet.
Here is the way to figure:
1. Tire circumf. (in feet) times RPM (at a given speed)
2. That answer divided by 5280.
3. That answer times 60.
4. That answer divided by the speed at the given RPM in number 1
5. The answer will be your axle ratio.
Now, you will need the circumfrance of your rear tires in feet. For an example, if your rear tires are 84.5 inches around, that is 7.04 feet.
Here is the way to figure:
1. Tire circumf. (in feet) times RPM (at a given speed)
2. That answer divided by 5280.
3. That answer times 60.
4. That answer divided by the speed at the given RPM in number 1
5. The answer will be your axle ratio.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 93
Likes: 0
From: GA
Originally Posted by JEB Garner
noname024, here is a formula to help ypu deside your rear axle ratio. First, these figures are based on straight 1 to 1 drive. In other words; if you have an automatic, lock it into third gear to get the figures. Should you have a stick shift (5 or 6 speed) use forth gear to get your figures.
Now, you will need the circumfrance of your rear tires in feet. For an example, if your rear tires are 84.5 inches around, that is 7.04 feet.
Here is the way to figure:
1. Tire circumf. (in feet) times RPM (at a given speed)
2. That answer divided by 5280.
3. That answer times 60.
4. That answer divided by the speed at the given RPM in number 1
5. The answer will be your axle ratio.
Now, you will need the circumfrance of your rear tires in feet. For an example, if your rear tires are 84.5 inches around, that is 7.04 feet.
Here is the way to figure:
1. Tire circumf. (in feet) times RPM (at a given speed)
2. That answer divided by 5280.
3. That answer times 60.
4. That answer divided by the speed at the given RPM in number 1
5. The answer will be your axle ratio.
it sounds to me like he has the 3.23's as long as he has the same size tires as i do 245 50zr16
my car with 3.42's in 5th gear doing 60 my rpm is right around 2100-2200 rpm and 6th gear at 60 it settles down to about 1500 rpm
my car with 3.42's in 5th gear doing 60 my rpm is right around 2100-2200 rpm and 6th gear at 60 it settles down to about 1500 rpm
