Speedometer Issue

[sloth]

I was following my father-in-law the other day and found out when he is going 65 I was going 62mph.So I reset my odometer at a mile marker to see if it was accurate at the next mile marker. I did this about 5 times and I was consistently almost a tenth of a mile past the next mile marker when my trip odometer finally rolled over to 1 mile.I just talked to the mechanic I have work on it and he showed me a service bulletin that said that the speedometer/odometer were only accurate to +/- 4%.He didn't think there was anything they could do besides replace the cluster. With it out of warranty I'm not going to pay for that. The good news is I probably got a lot more out of my warranty miles because of this.Anybody know of a way to adjust the speedometer? The only thing I can think of is to spend a lot of time calculating tire sizes to figure out which tire size to put on to make up the difference.

[silver_vibe]

Assuming you have 205/55-16, your current tire diameter = 24.88"You'd need a tire diameter of 23.75" in order to get 62mph to equal 65mph.205/50-16 would be close at 24.07" - that would be less than 1 mph off showing 62 mph (you'd be going 62.84).

[VforVIBE]

Quote, originally posted by silver_vibe »Assuming you have 205/55-16, your current tire diameter = 24.88"You'd need a tire diameter of 23.75" in order to get 62mph to equal 65mph.205/50-16 would be close at 24.07" - that would be less than 1 mph off showing 62 mph (you'd be going 62.84).This is it. That moment they told us in high school where one day, algebra would save our lives.

[silver_vibe]

Quote, originally posted by VforVIBE »This is it. That moment they told us in high school where one day, algebra would save our lives.You quoted me before I corrected my verbiage! I've always enjoyed math problems - English is my weekness

[zionzr2]

GPS has confirmed a small discrepency in my Speedometer too.

[Mavrik]

yup, tire size can effect the speedo. Now you could always see if there is a way to recalibrate the speedo as you very well could have an issue.

[joatmon]

The speed signal is a pulse train, electronic, and the speedometer runs off the frequency of the pulse train, the odometer counts pulses. There is probably a calibration factor stored inthe programming of the instrument cluster, and you may be able to find someone who can do that, but it's not going to be anything you can do yourself unless you want to drop a couple grand for the device to do it. A company like specmo.com might be able to do a one time adjustment for you.You can also get a device that plugs in between the sender and the cluster, taking in one frequency of pulses and ouputting a slightly different frequency. No idea of a soure for such a device, but I rememeber reading about one here somewhere. You can determine the % error and replace the tires with a different size to offset the error. There are some tire size calculators in the stickies in the wheels forum that are pretty useful for such things.

[ke4yyd]

My speedo is of about 1 mph at 50 mph as confirmed by my GPS. I have a ScanGauge installed and the mph it shows is always the same as the GPS.

[sloth]

I'll probably just end up living with it for the time being as my tires are pretty new. After doing some calculations my guage seems to be off by more than the 4% allowed. Actually around 4.5%

[Digger]

why not just drive 4.5% faster than the car says you are going.

[binary]

It's true - my Vibe base with 5 speed and 16" wheels. Is consistently 1MPH under... I've tested with two GPS units and at 50% of the speedometer it reads 1mph higher than GPS speed.At 25% range, same 1MPH over GPS Speed.

[binary]

Quote, originally posted by ke4yyd »My speedo is of about 1 mph at 50 mph as confirmed by my GPS. I have a ScanGauge installed and the mph it shows is always the same as the GPS.That's good to know that it's just the speedo and not the speed sensor. With data recorders, that could be bad to be recording the wrong speed.

[sloth]

Quote, originally posted by Digger »why not just drive 4.5% faster than the car says you are going. Then I'd be going 9% over the speed limit

[ColonelPanic]

I noticed some speedometer inaccuracy with my Vibe when I had it... Below 50 MPH, it would be dead on according to the GPS or ScanGauge. The faster I went above 50 though, the greater the deviation. It would be of ~3+ MPH by the time I was at 70-80 MPH. The speedometer was reading slower than the actual speed. Not sure if anyone else has found that to be the case with theirs or not. It didn't have the x MPH difference consistent across the scale.No telling how far off speedometers have been on stuff we've driven in the past, it's just that now we have tools at our disposal to prove them wrong. If it weren't for those stupid radar sign thingies that the cops put out, scangauges, and GPS units, I'd find the speedometers to be accurate.

[Digger]

Quote, originally posted by sloth »Then I'd be going 9% over the speed limit Opps. I ment slower. Mine is off a bit also I went by a speed traylor and it saig 51MPH when the speedo was dead set on 55.