The speedometer recently started acting wierd, sometimes showing much less than actual speed and sometimes showing much faster. 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. The indicated speed must never be less than the actual speed, i.e. 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. 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. My speedometer is off by 10 MPH (less than actual speed). The speedometer driven gear will make the speedometer read incorrectly by a percentage - not a specific miles per hour. Speedometer reading slower than actual speed? 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. my speedometer reading is lower than the actual speed of the car, is this due to a bad sensor or..... - honda civic question The tires are 215/55R16, and … These standards say that a motorcycle speedo may not show a speed lower than the actual over-the-road speed… Speedo reading vs actual speed. 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. asked Feb 8, 2017 in Trades & Technology by 1laboy. Odometer accuracy is no longer defined. 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. I'm about 15 mph off when the speedometer reads 50 (gps says I'm going 65). I'm … Never saw that before. Speedometer readings are typically optimistic since no manufacturer wants the liability of it reading lower than actual speed. The 1997 Acura CL has 2 problems reported for speedometer shows slower than actual speed. The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. The odometer is reading 8000km but it's reading speed incorrectly and logging the speedo speed to the odometer. Save Share. 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. You may recalibrate a speedometer in a few simple steps. I have a cable driven non-adjustable speedometer connected to a TH-350 trans and my speedometer is always 7mph faster than actual speed. Reply Prev of 3. 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. 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 . A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. Tyre pressure can also have an effect on speedometer reading; under-inflated tyres can result in the speedometer over-estimating the true speed. 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. 8 to 10 mph at what speed? 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). 2 3 Next Reply Author. so, changing tire size affects the correct reading of the speedometer. 2,778 posts. 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. Bigger tires will cause your speedometer to register a speed that is slower than your actual speed. Don't get me wrong, I love this. One of the most mentioned complaints about vintage Detroit iron involves a speedometer that is way off from the vehicle’s actual speed. Interestingly, use a scan tool to see the actual speed the car sees. I bet it will be a bit slower than what the speedo displays. Average failure mileage is 100,000 miles. I presently have an 18 tooth gear in the trans. I have a 1993 Cummins 4x4 with the 5 speed manual tranny. 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 speedo reads about 3 km/h too fast but the digital readout is dead-on (it's a Jag). the larger the tire, the faster you are going compared to what the speedometer shows. Original Poster. I've seen that before too. Discussion. Not all speedometers read 2 mph faster than the car is travelling. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. If it has, say 22 teeth now, you'd need 22 x .15 = 3.3 more teeth on the gear, so you'd get a … How does changing the size of the tires affect the speedometer reading? Should I go to a 19 tooth or 17 tooth gear to adjust the reading? Someone once told me one tooth adjustment will deliver about a 5 mph correction. Post by Yuckydog » Wed Jul 01, 2020 6:59 pm I just picked up another Caprice and driving home I noticed that my GPS showed me going 74 when my cruise control was set at 70. The EU says "The indicated speed must never be less than the actual speed, i.e. Therefore, the speedometer in the vehicle will read less than the actual speed that is being traveled. 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. 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. 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 … DaveH23. 131 months. In fact, my non-Ford car has both a digital speed display and speedo. If it's at 60 mph, then that's about 9/60= .15 so it's 15% off. it should not be possible to inadvertently speed because of an incorrect speedometer reading. Before you ask why, ask if, or something. speedometer reporting a much lower speed than my actual speed. 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. 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