My speedo is reading way more than it should..and probably adding more mileage to my odo at the same time.

My friend's 2000 E-class was reading 78 when mine was reading 83. Just by flipping through the shop manual, I couldn't find anything, but does anyone know how I can correct the speedo reading?

Thanks!

 

How do you know your speedo is wrong and not the Mercedes? German cars are notorious for playing speedo games -- BMWs frequently show 5-7 mph above the correct speed, even below 100 mph.

Do you have any reason your speedo would be wrong such as different sized rear wheels or different rear end?

 

Yes, I read about German cars purposely reading higher because of some laws that say the speedo must not read too slow even if owners change wheel/tire sizes, but that emphasizes my problem even more!!! His speedo was reading 78 and mine was even higher when we were travelling the same speed!

I do have different tires on the stock rims, but I can't remember the size....either 245/45 or 245/50. I just bought the car recently and all I remember is the guy went wider but I can't remember the sidewall profile. I think the differential and other driveline components are stock, but it does have a lightened flywheel which shouldn't cause any speedo issues.

 

If the tire+wheel diameter has increased, your speedometer would actually err to the slow side. If your tire+wheel diameter has decreased, the speedo would err too fast. It does sound like your speedo is having a problem...have you paced a mile at 60mph using highway mile markers?

 

my speedo is fast too. my apexi AVC-R shows about a 7-8% lower speed, which if you work out the rpm, gear ratios, tire/wheel size, it appears that the AVC-R is fairly close. this tells me that the error is in converting the pulses to movement on the analog guage.

 

I guess this implies the odometer is still correct then? I guess I should try driving at an indicated 60 and see if my odo takes exactly 60 seconds to change 1 mile.. or maybe I should go 120 and see if it takes 30 seconds? I don't know what mile markers look like on our freeways..do we even have them in the Bay Area, CA?

I guess that should narrow it down to either a speed sensor or speedometer problem.

 

I'd just measure it directly. I've found highway road mile markers to be accurate enough - just drive along at an exact speed for one mile and use a stopwatch to time it. In my old honda, I did '70' for about 4 miles, timed one mile at a time. My stopwatch (on my wristwatch) was consistent within .2s each time. Do the math, and it's very accurate.

FWIW, I believe the mile markers are located based on surveying information, which is very, very accurate.

Dave