Why is my speedometer not accurate?

About 2 weeks ago I finally bought a nice GPS. Anyways its showing me that my speedo is reading 5-6 mph over. I do not like that at all. Is it possible to adjust the speedo? Also how is it done and what will it cost to correct it? Or are they all this way? Also my tacoma is stock tires not worn.

 

Due to the inaccuracy of converting a digital signal to an analog needle. Use a ODBII tool (like a scan gauge) and see what the digital read out is for your speed...then make a decision from there.

There are calibrator tools out there...but some folks had had errors pop up from them time to time.

 

Mine is the same way...I rely on the GPS for an accurate read...tire size could also have something to do w/it [maybe].

 

I think tire size is the main factor. I have a TRD Sport and it's only reading 3mph over confirmed with GPS and driving by one of those police "your speed is" get ups. The larger tires on mine mean that the speedo will be closer to right. People with slightly larger tires than the ones I have seem to be right on the money with accuracy.

I actually like that mine is a bit over. Keeps me out of trouble.

 

Mine was the same way until I got 265s. Now it's dead on. Since your speedo is reading fast, wouldn't it be safe to asume that the odometer is also turning too fast? In other words, you've got more miles showing on your truck than you've actually driven...

 

Thanks for the replys. When the time comes I will up size the tires to 265 but I got a 4cyl will I lose much low end power? Should I ask the dealership to recalibrate the speedo? Im not happy about the inaccuracy! On my Suzuki GSXR its just a part you plug in "speedo healer" it was also about 5mph off.

 

Yup, as far as I know. And if you really want to give yourself an aneurysm do the math. It's actually a significant amount. Up close to 100k miles it's over 8k miles I believe. Your calculation will also depend heavily on what you use for your "average" speed that you think the truck will see.

 

On most vehicles I have noticed that the speed the computer actually sees--and the one that is used to compute the odometer reading--agrees with GPS speed very closely. It's the speedometer needle that is off.

 

Mine is the same way but that was after adding 33" tires. I honestly think yours needs adjustment.

 

My Tacoma speedometer is 2 mph difference to my gps but my integra speedometer matches with my gps speed fine. I was wondering that too because I would like it to match with my gps.

 

I complained about this back in 2006 while under warranty. They ran tests and said it was within spec...something like it can be 10% off at 60 mph...~5-6mph off. Sucks but I think that's the way it is...calibrations is expensive.

 

It's as much a legal as a technical issue ... Toyota (like many other manufacturers) spec's the speedometer's allowable variance so it reads on the 'fast' side. If they allowed it to read 'slow' they could be liable for tricking you into speeding.

 

Maybe we should start a class-action suit since our odometers might be off. Although I read another thread that said a scangauge read the speed correctly, so there's something fishy going on between the input and the actual gauge reading.

 

There's no reason to do that. It is defined by both UN and US federal laws the amount allowed to be off. Pretty senseless to sue a company for following the law.

 

Sorry, I should have put a smiley face...I was (somewhat) kidding. This reminded me of a recent class action with a similar issue...where speed-o's forced odometers to be off...everyone was granted an extended warranty to compensate, among other specific damages for improper warranty coverage, lost value of vehicle, etc.

 

On another car forum, people were discussing how to 'correct' the speedometer for a variety of reasons. People would disassemble the gauge cluster, remove the clear plastic gauge cover, and then get on the highway with some way to monitor speed (one guy had a cop friend with a calibrated speedometer pace him, others used GPS or OBD-2 scanners). They'd confirm their speed at 60 or 70 mph, and then install the needle (or have a passenger do so to be safe!).

If you're that concerned about it being off a little bit, you could probably figure out a way to do that with your Tacoma.

 

With regard to manufacturing, speedometers are certified by manufacturers to follow EITHER the U.S. SAE (Society of Automotive Engineers) J1226 OR the European version, ECR-39. The SAE rule, published in 1983, allows readings to be plus or minus 2 percent of true speed, or on a sliding scale from minus 1 percent to as much as plus 4 percent

 

Same here... Like 4-5 mph off.

 

im about 5mph slow on my speedo , hoping that it will be correct once i step to 285's

 

Changing my 5 lug's 215-70s to 235-70s brought my GPS and speedometer into agreement. It was off by about 4-5% before that.