Speedo Accuracy?
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Speedo Accuracy?
I've got a '95 with Michelin Pilot A/S tires which are fairly new and factory size. I think my speedo is off by a pretty good margin.
Around here people on average drive around 120 km/h on the highways. If I drive 120 in my wife's car I'm pretty well going with the flow and get passed occasionally by people in a hurry. My 1/2 ton truck is the same.
In my Vette, when I set my cruise at 120 I'm the person in a hurry. I never get passed and I'm blowing by cars like they are going 110 or so.
I haven't done any "scientific" testing yet to determine exactly how much it's off but I was wondering if this is a common problem? Anyone else run into this? I would especially like to hear from people with Michelin Pilot or Pilot A/S tires.
Thanks
Around here people on average drive around 120 km/h on the highways. If I drive 120 in my wife's car I'm pretty well going with the flow and get passed occasionally by people in a hurry. My 1/2 ton truck is the same.
In my Vette, when I set my cruise at 120 I'm the person in a hurry. I never get passed and I'm blowing by cars like they are going 110 or so.
I haven't done any "scientific" testing yet to determine exactly how much it's off but I was wondering if this is a common problem? Anyone else run into this? I would especially like to hear from people with Michelin Pilot or Pilot A/S tires.
Thanks
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
You could be picking days to drive when nobody is in a hurry! But a 10km/h difference (6 1/4 MPH) is a big difference if you think the speedo is off by that much.
The rear tire size even at 285/40-17 should not cause a big difference in the displayed speed. Most tires at either 275/40 or 284/40 will typically have no more than a 3/8" to 1/2" difference in diameter and that would not cause that amount of speed difference you see.
Is the speed difference you see consistent? Are you going faster than other cars at say 40km/h or 70? If you are, I would suspect the speed sending unit on the tranny. Have the differential gears been changed to another ratio?
Have your wife drive at a specifc speed with her car and then follow her to see if your displayed speed is any different. If you have a pair of the small hand-held radios, you can tell her to change speeds so you can see if there is any specific pattern.
The rear tire size even at 285/40-17 should not cause a big difference in the displayed speed. Most tires at either 275/40 or 284/40 will typically have no more than a 3/8" to 1/2" difference in diameter and that would not cause that amount of speed difference you see.
Is the speed difference you see consistent? Are you going faster than other cars at say 40km/h or 70? If you are, I would suspect the speed sending unit on the tranny. Have the differential gears been changed to another ratio?
Have your wife drive at a specifc speed with her car and then follow her to see if your displayed speed is any different. If you have a pair of the small hand-held radios, you can tell her to change speeds so you can see if there is any specific pattern.
Pro
Joined: Nov 2004
Posts: 677
Likes: 0
From: Frisco TX
Originally Posted by c4cruiser
Have your wife drive at a specifc speed with her car and then follow her to see if your displayed speed is any different. If you have a pair of the small hand-held radios, you can tell her to change speeds so you can see if there is any specific pattern.
It would be easer to toss a GPS unit on the dash and and watch the speed setting. I found that my car was off my 1 to 2 MPH. The GPS was locked onto a few sats and was accurate within 12 feet.
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Originally Posted by RocketSapp
It would be easer to toss a GPS unit on the dash and and watch the speed setting. I found that my car was off my 1 to 2 MPH. The GPS was locked onto a few sats and was accurate within 12 feet.
So I guess other people with Michelin tires haven't noticed a problem?
Thanks
Race Director
Joined: Jul 2004
Posts: 16,795
Likes: 62
From: Lake Havasu City, AZ
I recently drove by a traffic speed warning device... a radar gun with a display unit mounted on a trailer...
My speedo and the radar display were exactly the same....
has this just happened since you changed tires? Is it possible you have a different set of gears in the rear end than it came with?
My speedo and the radar display were exactly the same....
has this just happened since you changed tires? Is it possible you have a different set of gears in the rear end than it came with?
CF Senior Member
Joined: Feb 2005
Posts: 654
Likes: 0
From: Crestwood Il.
St. Jude Donor '05
Originally Posted by RocketSapp
It would be easer to toss a GPS unit on the dash and and watch the speed setting. I found that my car was off my 1 to 2 MPH. The GPS was locked onto a few sats and was accurate within 12 feet.
Great idea!
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Originally Posted by rws.1
I recently drove by a traffic speed warning device... a radar gun with a display unit mounted on a trailer...
My speedo and the radar display were exactly the same....
has this just happened since you changed tires? Is it possible you have a different set of gears in the rear end than it came with?
My speedo and the radar display were exactly the same....
has this just happened since you changed tires? Is it possible you have a different set of gears in the rear end than it came with?
The gears, wheels, PCM, and tire size are all stock factory size. There's no reason why it should be off unless the Michelin tires are little bigger than the Goodyears GM used.
I haven't had time to go for a run with the GPS yet....soon.
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Pro
Joined: Nov 2004
Posts: 677
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From: Frisco TX
Originally Posted by Atok
I didn't drive the car long enough with the original Goodyears to know if there was a problem there too. I switched to Michelin shortly after I bought it.
The gears, wheels, PCM, and tire size are all stock factory size. There's no reason why it should be off unless the Michelin tires are little bigger than the Goodyears GM used.
I haven't had time to go for a run with the GPS yet....soon.
The gears, wheels, PCM, and tire size are all stock factory size. There's no reason why it should be off unless the Michelin tires are little bigger than the Goodyears GM used.
I haven't had time to go for a run with the GPS yet....soon.
Same size tires and wheels should not make the Speeo change. Even going wider will not cause a problem if you change the ratio of the tires to keep the sidewall the same height.
When I was getting the new wheels and tires on the Vette they said that going to 18s in the back with 295\30\18s (I think that was the size) would keep the speeo the same. I ended up keeping the 275/40/18s on the back and now I am off a MPH or two. (The cost of the wider tire just about doubled from the 275s)
Tim