How many people have noticed their speedometer is off? Last week I was going to see the family and had my cruise set at 71mph in a 65mph zone. I knew I was speeding but 6mph is usually overlooked. I was upset then when I got pulled over and was clocked at 78 in a 65!!! I protested slightly to the officer but he had the radar reading. I asked politely if he could follow me and verify my speedometer but he said he couldn't do that.

So I used my brother's GPS and found that my speedometer was indeed off based on it, but thought that the GPS couldn't be that accurate of a measurement. Then tonight on my way home from work I passed one of those things on the side of the road to remind you of your speed. I looked and it said I was doing 38 and my speedo showed 34. So it seems as though my speedo is 10% slow (38/34 and 78/71).

Makes me wonder how fast I really was going when I hit 138 (indicated) on new years eve night (should have been 152 if percentages are correct).

Just curious if anyone else has this problem...

 

I just did the math with this information:

Tire Size 255/35/18
Gear Ratios
1st 4.459
2nd 2.614
3rd 1.723
4th 1.245
5th 1.000
6th 0.838
Final Drive 3.27
Overall Top Gear 2.74

Max speed in each gear (6,500 rpm):

1st 32mph
2nd 54.6mph
3rd 82.9
4th 114.7
5th 142.8
6th 170.4

Something isn't adding up...because the math and my speedo match (top speed in 1st, 2nd, 3rd, and 4th...and 138 indicated almost topped out 5th). But the radar and GPS isn't jiving. And my tires are stock for now, but worn so their diameter would be slightly smaller and therefore yield actually slightly lower speeds...not higher.

 

You went out on new years eve and did 138 ???

Are you just trying to get arrested??

BTW...every speedometer will have a certain amount of error, and typically GPS is not the best indicator of true speed because there's a couple second delay time, sometimes they only update every 30 - 45 seconds.

 

Not trying to...but it has happened before... I know speedo's are usually inaccurate, but they are usually indicating faster speeds than actual. I've never seen a car that the speedo was slow on.

 

yeah, this guy isn't winning any genius grants any time soon. I have never heard of a speedo reading that much LOWER than actually speed, I would ask for calibration records of that radar gun.

 

I have checked mine with my GPS many times and it is + or - 1 mph. The GPS also stores your top speeds. So far it is only at 120. Next spring I will fix that problem.

 

GPS is accurate to whatever the satellite fix radius is present. On average, this is 30 meters, but can be as fine as 3 meters (without differential GPS.)

In any case, a properly calibrated police radar speed gun is significantly more accurate than standard, good old-fashioned non-DGPS. It is possible that the speedometer is inaccurate, but without a radar gun, the most accurate way to determine the car's real speed is to time how long it takes to travel down a straight stretch of road a given distance at a given speed marked by your speedometer.

A simple stopwatch and some good old fashioned math could be more accurate than the radar gun

 

Yes all speedos are off very slightly but with new age electronics better than ever a DRB test at your Dealer will put this to rest. Have dealer perform scan test. Changing tire sizes requires re-calibration.

I buy all the speedguns for City I work for and all new units are factory calibrated, no recalibration required and only go astray if they are dropped.

Some previous Generation speedguns a few years ago would indicate a tree was going 120KPH when pointed at it, and other test was it would lock on to larger unit, howver with new units use laser tagging ends this.

The onus lies on honesty of Police Officer clearing previous offender,as if not cleared can continue tagging speeders with same reading as long as not reset. Even with the badge, some are not very honest and quota fillers.

 

Would your math be off because the rear (drive) wheels are 19"?

 

DYEZAK, will they rev to 6500? Redline is 6K.....& I assume a rev limiter would shut it down at 6K....

 

Well I will offer my three cents. lol
1. my speedo is right on the money, never failed me yet. Even had a local officer clock me to make sure
2. you can always try using a g-tech meter, not that it is always the most reliable but an option
3. do a simple math test going down the highway at 60. Assuming that the mile markers are accurate. Thats what I used to do in my 73 chevelle when the dash lights wouldn't work. if you are doing 60, than one mile is one minute. 75 is 48 seconds, etc.
4. go to your local track and run a quick 1/4 it will tell you what your trap speeds and such are.
5. dont speed?