it should not be possible to inadvertently speed because of an incorrect speedometer reading. Changing the stock parts, including the vehicle's tires, can throw off the speedometer calibration; larger tires cause the speedometer to read a speed slower than the vehicle's actual speed while smaller tires cause the speedometer to read a speed faster than the vehicle's actual speed. Someone once told me one tooth adjustment will deliver about a 5 mph correction. Should I go to a 19 tooth or 17 tooth gear to adjust the reading? Like the title says, my speedometer reads slower than the actual speed being driven. Don't get me wrong, I love this. My speedometer is off by 10 MPH (less than actual speed). DaveH23. for me, i went about 3% larger on the tires and this brought my actual speed in line with what the speedometer shows (verified with GPS). Above: Nobody enjoys a speedometer that has the indicator needle bouncing back and forth on the gauge face.It is impossible to determine the vehicle’s actual speed while traversing down the road because of the excessive needle movement. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. 196 Posts . 2 3 Next Reply Author. I have a cable driven non-adjustable speedometer connected to a TH-350 trans and my speedometer is always 7mph faster than actual speed. Speedometer readings are typically optimistic since no manufacturer wants the liability of it reading lower than actual speed. speedometer reporting a much lower speed than my actual speed. European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h. I bet it will be a bit slower than what the speedo displays. 131 months. Therefore, the speedometer in the vehicle will read less than the actual speed that is being traveled. 8 to 10 mph at what speed? I'm about 15 mph off when the speedometer reads 50 (gps says I'm going 65). Before you ask why, ask if, or something. 2,778 posts. Now, everything I've read says that bigger tires should translate to your speedometer reading higher than what you're actual speed (in fact, an online calculator says to expect a ~6% delta) but, in real world testing using a GPS speedometer app, it turns out my actual travel speed is faster than the speedometer is posting. For example, if your speedometer says 35 miles per hour on 20-inch diameter tires, changing to 23-inch diameter tires will make your true speed 40.25 mph, even though your speedometer will still show 35 mph. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. I never have to worry about speeding, and in fact, know that I'm not, since I always match my speedometer reading to the speed limit (basically what I just said was that I'm always going 3 MPH under the speed limit). Reply Prev of 3. I have a 1993 Cummins 4x4 with the 5 speed manual tranny. What this means: For a vehicle travelling at a true speed of 100km/h, the speedo must read between 100km/h and 114km/h. Tyre pressure can also have an effect on speedometer reading; under-inflated tyres can result in the speedometer over-estimating the true speed. It is common for Accords to read 3-5 mph faster than actual at highway speeds, ... mine is off like 2mph slower as compared to the GPS at any speed, just dont know if the speedo or the GPS is accurate:dunno:. Battery Change Speedometer reading 2x actual speed Yaris 2006 23-12-19, 05:58 ... After driving it and being over taken multiple times that day, I found my speedometer to be 2x slower than the navigation application was suggesting. You may recalibrate a speedometer in a few simple steps. Speedo Reading Slower Than Actual Speed - posted in Problems, Questions and Technical: Hi all, my speedo is reading around 10mph lower than the actual speed that im going, so im having to try to match my speed with other cars or guess my speed when going past speed cameras. The 1997 Acura CL has 2 problems reported for speedometer shows slower than actual speed. The speedometer driven gear will make the speedometer read incorrectly by a percentage - not a specific miles per hour. If the speedometer shows a speed higher than the tested speed, then the certificate is actually harmful to a speeding case, since the driver had reason to believe the vehicle was traveling faster than the actual speed. The indicated speed must never be less than the actual speed, i.e. It's always seem like I was going a lot faster than what the speedo is reading so I verified it with my GPS. The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. I've seen that before too. so, changing tire size affects the correct reading of the speedometer. The EU says "The indicated speed must never be less than the actual speed, i.e. After I selected economy mode on my edge computer and it reprogrammed the truck my speedometer is now reading 10 - 20 mph faster than actual. GPS, on the other hand, uses a purely mathematical technique to measure the speed of the vehicle and is therefore more accurate than a speedometer. Speedometer is reading faster than actual speed I have a 2009 GMC Sierra SLT 1500 with an edge performance chip and 20" tires and my speedometer. The speedo reads about 3 km/h too fast but the digital readout is dead-on (it's a Jag). I'm … #20 RRxing , Oct 12, 2017 kithmo and RCO like this. The standards for car manufacturers, and the effect of wheels and tyres or tyre pressures all mean that each car’s speedometer reading will be different, so while you may think another vehicle is travelling slower than the speed limit that … it should not be possible to inadvertently speed because of an incorrect speedometer reading. Speedo reading vs actual speed. Speedometers are not allowed to read lower than the actual speed of the car, but they are permitted to indicate a value that is 10% higher than the actual speed, plus 4. asked Feb 8, 2017 in Trades & Technology by 1laboy. Odometer accuracy is no longer defined. The odometer is reading 8000km but it's reading speed incorrectly and logging the speedo speed to the odometer. That is, at a true 100kmh the speedo may be reading up to 110kmh, ... We're not aware of any instance of a recent model having a speedo that showed a slower speed than the actual speed. Discussion. the larger the tire, the faster you are going compared to what the speedometer shows. A useful speedometer calibration certificate will show a speedometer reading that is slower than the actual tested speed. Reply. Bigger tires will cause your speedometer to register a speed that is slower than your actual speed. From a stop to about 25 mph, it reads fairly accurately, but beyond that, the speedometer reads from 5-15 mph slower. Why do they set them 3 MPH below the actual speed? These standards say that a motorcycle speedo may not show a speed lower than the actual over-the-road speed… Save Share. The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. How does changing the size of the tires affect the speedometer reading? Interestingly, use a scan tool to see the actual speed the car sees. Likewise, a smaller tire on a low-rider, for instance, will have to spin faster than a stock one to maintain the same speed, and therefore will cause the speedometer to read faster than the actual rate traveled. The tires are 215/55R16, and … I have changed my wheels, but it was still reading incorrectly before I changed from standard steels. I presently have an 18 tooth gear in the trans. For instance, if it's 10 MPH off at 100 MPH, it's 10% off - and that will make it read 66 MPH when you're actually traveling 60 MPH. ... 1000km with the speedometer reading 10kmph over actual speed. Not all speedometers read 2 mph faster than the car is travelling. Average failure mileage is 100,000 miles. Monday 23rd … You need to slow it down, so you need more teeth on the driven gear. I thought that larger tires may affect the speedometer reading by 2 to 3 MPH but not 10. The difference between a new and worn set of tyres usually equates to about 3km/h at 100km/h, as the car will actually be travelling slightly slower for the same reading on worn tyres. Speedometer reading slower than actual speed? Never saw that before. The speedometer recently started acting wierd, sometimes showing much less than actual speed and sometimes showing much faster. If it's at 60 mph, then that's about 9/60= .15 so it's 15% off. Original Poster. One of the most mentioned complaints about vintage Detroit iron involves a speedometer that is way off from the vehicle’s actual speed. Discussion Starter • #1 • Jan 13, 2003. adjusting speedometer My speedometer is 10 miles slower thatn I am actually going, what do you do (rookie) I admit . 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