Do the speedometers overestimate speed?
- Thread starter Flycaster
- Start date
It's traditional to have them overstate speed - flatters the driver, avoids any claims against the manufacturer, provides a margin of error for the driver, etc.
ian in lancs
Member
mine reads about 5% fast.
Medium Dave
Member
GPS is only able to measure speed across the earth. If you are going up or down hill, then the GPS will underread. GPS also has trouble with corners. The only time it will be accurate is when you are travelling straight and level. These errors account for some of the discrepancy but most speedos do overread anyway as has already been said.
Medium Dave said:
Can't speak for the Z4 but assume its the same as on the X5 where to overcome this problem and enable the sat nav to calulate it's position accurately enough to work small scale maps (say navigating city roads) or where GPS signal is poor the car takes reading from the anti lock brakes of their absolute and relative speed of rotation and calculates the position and speed until the next GPS reading updates.
Pretty cool me thinks
cj10jeeper said:Medium Dave said:
Can't speak for the Z4 but assume its the same as on the X5 where to overcome this problem and enable the sat nav to calulate it's position accurately enough to work small scale maps (say navigating city roads) or where GPS signal is poor the car takes reading from the anti lock brakes of their absolute and relative speed of rotation and calculates the position and speed until the next GPS reading updates.
Pretty cool me thinks
Uhm I'm just going to nod and say ok.
Anyways, Yeah both my Garmin and the old Magellan red approx 2mph slower than what the speedo said. It's kinda bad knowing that because now I have tendency to put the speedo 2 mph over.
Oh a cop pulled me over on Mt Charleston the other day...clocked me at 82mph. I think I had the speed at 85mph, but I wasn't paying that close attention. On a side note he asked some questions about the car, I showed him the engine bay, then asked how in the hell an E-5 affords this car. He let me off with a warning. I should offered him a beer or two. Ok I jest, I didn't have beer with me.
Having been through the speedo pain for the SVA of a kit car, the 'UK' law allows speedos to be +8% / -0% out in every gear..
When we did the kit car, using the gear / final drive ratios & rolling distance of the wheels, you can work out RPM v MPH, we did this and calibrated the speedo to be 3mph over. When the inspector checked it was that. The GPS reads about 1-3 mph out depending on the road conditions.
When we did the kit car, using the gear / final drive ratios & rolling distance of the wheels, you can work out RPM v MPH, we did this and calibrated the speedo to be 3mph over. When the inspector checked it was that. The GPS reads about 1-3 mph out depending on the road conditions.
There's a bulletin from BMW on the speed calibration telling techs the tolerence for error adjustment upon customer complaints or query. Generally speaking, there's a 2-5% below actual indicated speed error built in with intent by BMW. It's posted over at bimmerfest, however, the site is down right now.
In the US the federal mandated error tolerance is +4 / -0 mph. I'm sure that is at average highway speeds and turns into a percentage as speed increases. I've always noticed that on all my vehicles. My truck was actually -4 at 70mph (speedo read 70 and I was actually doing 74) when I bought it last year. Dealership was required by law to correct it at no charge. It is now +2 at 70mph.