It is fairly easy to see how your speedometer compares with your actual driving speed. Enter the ratio of the original ring gear and pinion (changed from) and the new ring gear and pinion (changed to). All vehicles come with speedometers that read a faster speed than actual. Speedo reading vs actual speed. Share this conversation. Joined Nov 1, 2009 Location Alabama TDI 2006 VW Jetta TDI Package 2 Nov 19, 2009 #1 Hello everyone. Show More. The speedometer read 5-15mph than my actual speed, running stock sized wherls. 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 25 tooth gear. I presently have an 18 tooth gear in the trans. Reply Prev 1. of 3. Category: Chevy. My 2003 Cadillac CTS 3.2 V-6 Speedometer is reading approximately 25 MPH over the actual speed. Now count the teeth on the plastic gear on the transmission end on the speedometer cable, is should have something like 18 teeth. Thus, with each revolution you are traveling further than before. … Someone once told me one tooth adjustment will deliver about a 5 mph correction. The speedometer is original but at some time or other the c4 transmission has been changed out. For safety reasons, manufacturers calibrate speedometers to read higher than the actual speed of the vehicle. This is because a taller tire will have a larger overall circumference, which will cause it to travel more distance per revolution than the original equipment tire. LordHaveMurci. You need to slow it down, so you need more teeth on the driven gear. Bigger tires=bigger around=faster actual speed than your speedometer In my case, the truck is going 12% further and faster than it realizes. Does anybody else know if there is any relationship with this problem and my faulty cruise control. Because speed is defined as the distance covered per unit of time, your actual speed will be greater than the speedo reading. It only happens above 45mph, and there's a strange whirring noise coming from the gauge itself. #21 radioprius1, Oct 1, 2009. 10,542 posts. 8 to 10 mph at what speed? Speedometer shows 2 MPH faster than actual speed. 10 MPH too fast at what speed? A higher value alerts the driver about the speed and prevents him or her from speeding. Sometimes, the speedometer on my '73 Chevy will read faster than I'm traveling. Joined Dec 1, 2018 Messages 22 Reaction score 9. A speedometer will read the same speed as it is with the original tyres. I recall reading an article on C&D, the test result of speedometer accuracy shows all cars read faster than their actual speed with Bimmer being the extreme, read almost 10% faster. Answered in 5 minutes by: 12/18/2008. Joined: Aug 13, 2009 1,355 154 0 Location: Iceland Vehicle: 2010 Prius Model: II. Back to the basics, if your speedometer is indicating too slow for a given roadspeed then you have to turn it faster. However, there is a limit as to how high or low a speedometer is allowed to read. Reactions: wangstarr03. My GPS tells me my speed, and I think it's very accurate. For this reason, you often find a specific colored speedometer gear for different tire sizes and rear differential ratios. Show Less . Should I go to a 19 tooth or 17 tooth gear to adjust the reading? This is because of the inner workings of the speedometer calibration. Tyre pressure can also have an effect on speedometer reading; under-inflated tyres can result in the speedometer over-estimating the true speed. A new rear axle speed sensor did not fix. 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 faster the tail shaft spins the higher the reading on the dashboard. Not all speedometers read 2 mph faster than the car is travelling. I swapped the rear axle in my '93 RMW due to a bad pinion bearing. Thanks for any help. Discussion. If they make one. speedometer reading faster than actual speed I have this problem with my 85' 190E where the speedometer is reading at least 10 mph faster than what I'm actually doing. If the speedometer reading is lower than the actual speed, the speedometer is slow. If the speedo isn't tracking accurately, especially reading faster and putting miles on the odometer faster, this will make my warranty expire faster. otherwise you get to find a drive gear (on the output shaft) with fewer teeth. A useful speedometer calibration certificate will show a speedometer reading that is slower than the actual tested speed. Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed. 15 inch rims. 3 Next Reply Author. 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