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Speedo Question

T

thumper05

Full Member
Joined
May 7, 2002
Messages
49
Location
Ovilla, Texas
I tried searching for a thread on this and couldn't find anything. I have a 1500 Z66 with the bigger tires 17's. I have noticed lately that when I pass one of those speed tracking radar things, that the men in blue put on the side of the road to make you see the error of your ways, It always seems that it says i'm going 2-4 mph slower than I say I am. Is it just me or is there something to this? If there's something to it, does that mean I'm racking up miles faster than I should be, hitting the end of the warranty faster. Am I just crazy? don't answer that........ :eek:
 
Unless you have changed tires from stock your speedo should be dead on ... strange, maybe the cops have a problem. :eek:

If you can confirm it, take it to the dealer and have them verify that the PCM is set properly for the gearset and tire OD that you have.
 
Also check your tire inflation. With tires this big, a few pounds of pressure can make a significant change in the tire size.

As Gandolphxx said, if everything is stock, it's time to visit the dealer.
 
I have the same set-up, Z-66 with 17 inch tires. We just did a 4000 mile round trip from New Mexico to Florida. I noticed a 2-5 mph difference from my speedo and and my GPS. Also another car with us noted I was slower that what the cruise control setting was. :0:
 
i have a basic av that came with the 16" tires, 265/70/16 and the day i got it i did the swap to 17" with the same size tires as you/z66 package. i had the dealer reset my pcm and it was showing 2-3 miles slower than i was actually doing, invested in the HPP3 and since i reset it myself i am now showing 1 mile an hour faster , at 55 on speedo i am doing 54 according to gps and the same type radar system you are referring to. i made 3 passes on it and came up with the same thing each time. I seen half-breed posted the same readings as i, i do not know if he has the HPP3 or not, but yours is still factory,,,,,,,,,,warrantyyyyyyyyyyyyyyyyy
 
There was an article in Motor Trend or Car and Driver a few months ago. Jap cars are the closest, American cars typically indicate 1 to 2 high at 60, and European cars indicate much higher than they should. In Europe it is the law that the cars can't indicate low (if the cop says you were speeding your spedo will say you were really speeding). On a number of occasions I have passes one of those warning things that have radar and show you your speed. At 30 it shows me going 29. I would assume then when I am going 60 I am really going 58. The article also indicated that the odometers are required to be accurate and the magazine found that they all read the correct milage so warrenties were not affected. Can anyone find the article?

Half-breed
 
I have the same problem...As I increase the speed, the difference in what my GPS says and my speedometer says increases. At 80 on the speedo, my GPS says I am doing 77.5 or so. Not a real bother, just an annoyance. BTW, I have the original tires and rims on the truck.
 
Guest said:
There was an article in Motor Trend or Car and Driver a few months ago. Jap cars are the closest, American cars typically indicate 1 to 2 high at 60, and European cars indicate much higher than they should. In Europe it is the law that the cars can't indicate low (if the cop says you were speeding your spedo will say you were really speeding). On a number of occasions I have passes one of those warning things that have radar and show you your speed. At 30 it shows me going 29. I would assume then when I am going 60 I am really going 58. The article also indicated that the odometers are required to be accurate and the magazine found that they all read the correct milage so warrenties were not affected. Can anyone find the article?

Half-breed
Click to expand...

I was just ging to post the same thing. I read the article also. Seems that BMWs are the farthest off, followed by Porsche. The Porsche ans BMW people expect so many owners to swap thier wheels and tires to a larger size that they try to offset the difference. I do remember it saying that it was the law (somewhere) that the speedo not err the wrong way. The manufacturers make sure the speedo reads a little high.

Ron

 
I've noticed the same thing when using my GPS. My speedometer was consistently 3-4 mph higher than my GPS on a trip from LA to San Diego.

Doesn't really bother me I figure I need all the help I can get to stay out of trouble. >:D
 
Automotive speedometer are notoriously inaccurate. If you are old enough to remember, the old car tests in magazines always showed a bicycle wheel attached to the car to give an accurate speed reading. Now they do it with radar. If you are "only" 2 -3 miles off, consider yourself lucky.
 
MTBJoe said:
I've noticed ?the same thing when using my GPS. ?My speedometer was consistently 3-4 mph higher than my GPS on a trip from LA to San Diego.

Doesn't really bother me I figure I need all the help I can get to stay out of trouble. >:D
Click to expand...

Fascinating, learn something new every day - the fact that all of you report that the speedo is lower sounds like a conspiracy to let ust think that we are going faster than we are. ;D
 
Motorcycle speedometers typically read optimistic. Hondas used to read 60 when you were actually traveling 56. My Moto Guzzi (Italian) would read even faster. I always thought it was a pyschological ploy to reduce speeds (except for the Guzzi, it was just wishing it could do 140mph) such as the case in the old 85mph car speedos forced on us in the 1980's. I had never considered the more devious purpose of blasting through the warranty faster.
 
I also noticed my AV to be 1-3 mph more than what the auto-speed sign says....I always related it to me and my flutter foot..... :B:
 
My 04 Z71 is dead on according to my portable GPS.  However my 99 Jeep Wrangler is 3-4 higher than what it actually is travelling.  This is also affecting the odometer.  In a 65 mile trip, it is saying it went 67 miles.

The manufacturer has EVERYTHING to gain from this:

- It will falsely calculate better gas mileage
- It will reduce tickets/better insurance stats
- Warranty will elapse quicker
- Driver will be lead to believe better handling
- Driver will believe better 0-60
- Gets you in for scheduled maintenance more often
- Parts not lasting as long as you are lead to believe
- Car will impress by showing 100K before it's time
 
Putting the bigger tires will make your speedo show slower than you are actually going...unfortunately I found this by getting a ticket...

The bigger tires will also make less miles show up on your odometer (bonus)

The deal can reset it or you can live with it, I just live with it. 
 
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