PhantomZX

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!

rynberg

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?

PhantomZX

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.

rynberg

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?

poss

iTrader: (5)

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.

PhantomZX

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.

dgeesaman

iTrader: (7)

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