QuestionMy 300D is showing a speed of about 8% faster than what the actual speed of the car is going (GPS and tachymetre verified). I suspect that the tires are too small, or there is some other issue. My question: Can I just increase the tire size (14-inch wheel; current tires are 185/75R14) by .7 to 1 inch and get fix the 8% difference that way?
I spoke with a tire sales person and they said it would cause my tire to hit the well, or some other part of the car; will a 3/4 inch to 1 inch increase do that? It looks like there is more than enough room.
What do you recommend? and if it is possible, I would appreciate a brand and size of tire in your reply.
Thank you.
Regards,
Robert B
Austin, Texas
AnswerRobert,
We need to do this in steps:
On every vehicle sold in the US, there is a sticker – commonly called the vehicle placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.
Find that placard and verify that the tire size and the pressure in your tires match the placard.
According to Tire Guides, a 1983 Mercedes 300D originally came with 195/70HR14 inflated to 28 psi front / 32 psi rear. Please verify the placard information, because every I say next is dependent on that information being correct.
If my information is correct, then the tires you have on there now would result in a 3/4% SLOWER speedometer reading.
But there are a number of possibilities that occur to me.
1) The tire is actually smaller than the tire size indicates. For example: a 185/75R14 could actually be sized small, even though the sizes says something else. While this is perfectly legal, it can cause problems. But in order to get an 8% difference the tire has to be sized way outside what is normally considered a reasonable range - so this possibility is probably not it.
2) There is a problem in the speedometer. Check the odometer reading against the distance traveled. You have a GPS unit, and I would suggest also checking against the mile markers on an Interstate highway. Those markers are generally pretty good. Use 10 miles to reduce the potential for error in the markers.
3) The sending unit is bad. If #2 also yields an 8% error difference, then it might be that the unit in the transmission is sending a bad signal to the dashboard. You might try to get that diagnosed by a mechanic.
But I think just using a larger tire is problematic. To correct an 8% error requires a tire 2" larger in diameter. While I do not know how much room there is in the Fenderwells of your car - and therefore can not comment of what tire size would fit - there is usually only a bit more space available and almost certainly not enough for a full 2". (Remember, the worst condition is with the spring fully compressed and the steering wheel fully turned.)
Hopefully this will get you in the right direction.
