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hi
There is TPMS (tyre pressure monitoring system)
they are inside every tyre. Which tell the car about your tyre pressure. If they have failed or you have recently changed tyres where sensors are not placed properly then light will come on and most likely car will stop, due to safety issue.
the main reason for incorrect speedo readings is the wrong size tyres or rims on the vehicle
for example if you need 185 -75 r14 on the car and you went to 205- 25 r14 that drop of 50 on the side wall height alters the circumference of the tyre so throwing out the speedo
alternately if you had 185-75 r14 and you fitted 185 -75 r16 then again the circumference has altered and so the speedo is out
next is if you changed the final drive ratio in the diff will also cause an error
They don't use speedo cables anymore, all electrical. On or in the transmission there is a VSS- vehicle speed sensor, and it sends a signal by wire to the dash cluster, and also to the engine computer.
If you recall, the speedo cable would attach at the center rear of the speedometer gauge. I don't remember just when, but all car makers stopped using cables to indicate speed, I think it was in the '80's when they went to electrical devices rather than cables. You can use a voltmeter to run tests on the sensor and/or the wires, but I don't know the procedure. You would have to consult a repair manual for that. It is not uncommon for the sensor to fail, but could also be a wiring problem, or the speedo gauge itself, but that is less common.
Putting different tyres on will alter the accuracy of the speedo and the recording of distance, so I wouldn't pay much attention the fuel economy. For example, a single revolution of the axle will remain constant regardless of the wheel size, but the revolution of the wheel versus distance traveled will vary depending on the diameter of the tyre fitted. If the new tyre was, say 10 cms circumference larger, then the car will travel 10cms further each full revolution (that is 100 meters every 1000 turns. If your speedo sensor is determined by the number of revolutions of the axle, then every 10,000 turns you will have travelled a kilometer more than with the old tyre size (less if the circumference is smaller). The figures you give convert to (UK) 25 mpg dropping to 20 mpg with the new tyres. Does seem poor consumption figures for a small car, but if a big engined truck that might be acceptable. All depends on how you drive it as well.
Yes you can but expect an error in speedo reading as you are decreasing the circumference of the tyre meaning it turns faster to cover the same distance.. The 195 is the rim width The 75 is the profile height of the side wall and the 14 is the rim diameter in inches So smaller profile size ==lower car= wrong speedo reading. You can use this information to change the rims IE go to 15 inch rims so the tyre reading would look like this 195 35 15 would be about the same as 195 95 14. By lowering the profile size you can go up in rim dia size and still retain the same speedo readings and car height.. Talk to a tyre shop to get the idea.
to get the speedo right you will need to fit the original size tyres with the correct speed rating which conforms to the certificate of homoglonation ,deviate from this and this is the problem you get also the vehicle will not pass the annual vehicle mechanical inspection because the tyres are not whats on the original certificate
remove it and put a ohm meter on it and spin it it should change when you spin it. Not sure if it is normally open or closed but I would remove it to test it
I think you may be suspecting the ABS speed sensors but , this is not connected to the road speed of the vehicle The speedo will be on the gearbox final drive or output shaft. the wheels will rotate at different speeds relevent to each other depending on road conditions , cornering or tyre grip etc . So check if the speedo sensor it could have been damaged if a trolly type of jack was used ! Good Luck
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