speedometer reading faster than actual speed - PeachParts Mercedes-Benz Forum

rfremista · #1 09-05-2003, 03:13 PM

I have this problem with my 85' 190E where the speedometer is reading at least 10 mph faster than what I'm actually doing.

Does anybody else know if there is any relationship with this problem and my faulty cruise control. I am not able to set my cruise control ........... everytime I try to set it it actually tries to speed up ?????????

carman850 · #2 09-05-2003, 03:25 PM

At what speed you read 10mph faster?

I think it is by law that speedo meter has to read faster than actual speed in Canada, though I can not find reference to back it up. 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.

Also you read higher speed if your tires is smaller than stock tires.

sbourg · #3 09-05-2003, 03:32 PM

Almost anything that could cause a speed error would cause speed to be off by a percentage - not a fixed amount. If you speedo reads 10 mph at standstill, and 70 mph when you are going 60, then the indicator needle may have slipped or a counter-tension spring may have slipped. If it is reading 0 mph at 0 mph, 35 mph at 30 mph, and 70 mph at 60 mph - then likely you have on smaller-than-stock tires or the differential has been changed.

Steve

Kestas · #4 09-05-2003, 04:06 PM

I experienced this on another car and it turned out to be a defective speed sensor at the transmission. I don't know if this would apply to your situation.

SAE design standards require the speedometer to be accurate between +3% and -1%.

rfremista · #5 09-05-2003, 04:35 PM

thanks for the quick reply guys!

I have heard about this smaller-than-stock tire syndrome ........... does anybody know what the stock tire would be for that model (85' 190E)? I think I have 195/60/14 on their right now.

I would say the offset doesn't happen at 0 mph , more like when I get to the 60 mph range. It just so happens that I have an US model (mph) here in Canada where it is kmh. But I know for sure that there is a major speedometer fault.

haasman · #6 09-05-2003, 05:22 PM

I believe the original size is 185/65-15.

Try using this helpful tire size calculator for differences in actual vs. indicated speeds: Tire Calculator

The cruise control is a separate issue. I am not an expert on them but you may want to check your throttle linkages for play and proper adjustment. I believe the input comes from the sensor on the back of the speedo head.

Haasman

sbourg · #7 09-05-2003, 05:27 PM

I don't know the stock size in that year, but ours uses 195/65R15, and yours on our car would read 4.5 mph high at 60 mph.

Steve

haasman · #8 09-05-2003, 05:41 PM

per the calculator link above:

a car that came with 185/65x15 tires but is now running 195/65x15 tires, the speedometer would read 2.1% too slow, or when traveling at 61.3 mph your speedo would read 60mph.

Haasman

sbourg · #9 09-06-2003, 08:48 AM

Yes, and oops. Our car should have had 185/65R15's, but came to us with the other size.

Steve

rfremista · #10 09-08-2003, 11:54 AM

from the Tire Calculator link above ............ it seems to me that this could be my problem. I'm just verifying what was discussed below: