I have a '90 740 wagon and whenever I pass those signs that tell you your speed, they always say I'm going about 10 slower than the speedometer says. are those signs really inaccurate or is my speedometer wrong? thanks!

 

Your speedo is wrong.

 

and how do I fix it?

 

an old skool way to check is using mile markers on a highway. for example maintain travel at 60mph exactly on your speedo, have your passenger hit a stopwatch when you pass one then time one minute and see if you are passing the next marker. not super accurate but will give you a good idea what's going on.

 

new school is to use Waze on your phone :-) it uses GPS to track your speed. Just cruise at a steady speed and compare the app to your indicated speed (lol assuming you have 4G coverage)

 

google maps in directions mode will show your speed too. touch the 'speed limit' in the lower left corner to turn on your actual speed next to it. note that GPS speed is only accurate when you're going in a straight line at a steady speed, as it lags by a second or two. no 4G coverage is needed for GPS.

my experience is, those radar speed warning signs are calibrated a little lower than your actual speed. I was driving my wife's 1994 Mercedes E320 wagon last night for a few hours, and noted that when the speedometer said 70, I was actually going 66-67 per GPS, and the couple radar road signs said 65. The car has the correct sized tires and they are properly inflated, so thats not it. The Mercedes wagon has a mechanical rather than electronic speedometer (volvo 740/940 and later 240s use an electronic speedo).

 

Waze=Spyware=Very bad Juju - no thanks

 

So, how do you fix it?
There is probably a way but short of replacing it with another speedo, it is really impractical and cost prohibitive... Just keep in mind the discrepancy when you drive and adjust your speed accordingly...

 

If you have a cell phone you already are being spied on. Try wearing a tinfoil hat and it will repel the incoming rays.

 

maybe so lol but i don't have to HELP by installing apps like that...and turn location off..
i have a great map app in my car it was once called "Atlas" ...
now, where did i leave my flip-phone ??

 

is there really no way to adjust the speed sensor? it's like 10 mph too high, I based my speed on a sign that, in my other car, is only about 2 mph off.

 

There might be a way but I'm not aware of it. These days the way specialty labor is, I doubt that any such work would cost you less than a new speedometer...
In the old days, in the police cars the speedos were calibrated by the techs and all the cars carried a hand written label on the dash showing the actual speed vs the indicated speed in spite of the "calibrated" label from the factory all police cars still carry. The courts required a new calibration periodically, I forget how often, in speeding violation cases.

 

or use a credit card, or drive a licensed motor vehicle with plates, or .....

 

if the discrepancy is a hard offset (20 MPH actual reads 30MPH, 40 MPH actual reads 50MPH, etc), then its possible the needle was mis installed, perhaps after an instrument panel repair. remove the instrument panel, carefully remove the speedometer from the instrument panel, gently lift the needle over the 'stop' pin, note where it comes to a rest, gently pull the needle off the shaft, replace the needle on the shaft with the correct angular offset, and then lift it back over the 'stop' pin, reassemble and reinstall.

if its a proportional error, like 15% wrong at any speed, so 20MPH reads 23, 40 reads 46, 60 reads 69, 80 reads 92, then its a lot more complicated, the speedometer is converting pulses from the speed sensor in the differential to volts to drive the needle via an digital to analog circuit, there's at /least/ 4 completely different speedometers in various 740/940, and each one would require different mods to correct, and I doubt anyone has done this for a 1-year only 1990 dashboard.

 

^^^^ This. IIRC the needle has to come of when you replace either of the little gears for the odo. Easy to get it wrong when replacing the needle

 

Stock tire/rim?

 

you don’t have to remove it on an 850. Just did the gear replacement on my 850 wagon.

 

The original post is for a 740, very different from an 850, and from 2019... I am sure they've moved on...