portaman03

Ok guys I really need your help. My roomate and I have an argument going.

I have two sets of stock wheels. One has normal size tires, while the other has undersized tires.

When I drive at 65 mph on the stock tires, let's say for arguments sake that the RPM is at 3,000.

However, when I drive at "65" mph (according to the speedometer) with the undersized tires, the RPM is at, say 4,000 rpm, revving higher.

I understand that traveling on undersized tires throws off the speedometer and makes it read faster than you are actually traveling.

My roomate is disagreeing with me on the fact that the RPM is affected (raised) by the smaller tires.

I told him that when I switch them over for the winter, he should ride in my car so I can show him. We will go "65" mph with the smaller tires (reading 4k rpm), and then switch the tires and go "65" mph with stock tires (which will read 3k rpm).

He doesnt think there will be any affect on the tach.

I know some of you guys are running smaller tires, so lets settle this.

portaman03

To add to this, he agrees with me that the speedometer (and odometer) IS INDEED AFFECTED. He bases this that the speed sensor is on the axle, and that is why they are affected. He claims there is NO affect on the tach and they read the same.

Am i retarded?

joeshmo

Check with GPS. It seems odd to me that the tach would be off, as well as the speedo. In cars I have worked on, the speedo measures the actual revolutions of the tire (well the tranny output shaft) and converts it to a signal that is sent to the speedo. Are you in the same gear with both sets of tires? What are the exact sizes of both sets?

TunerN00b

Um, speedometer and tachometer readings will always stay consistent with each other.

If the tach shows 3,000 RPM while the speedometer shows 65 MPH, and then the tire size is changed, the tach will still show 3,000 RPM when the speedometer shows 65 MPH. The actual speed the car is traveling will have changed, but the readings will stay the same relative to each other.

The car's speedometer counts rotations of the differential. Smaller tires travel a shorter distance for the same number of rotations, but the rotations counted will stay the same for the same RPM in the same gear.

joeshmo

That's what I was trying to get at. The only way I can think of the VSS to report the same speed with a different tach reading is being in the wrong gear.
If I knew the tranny/tiresize I could figure out if it was in the wrong gear.

It Wasn't Me

If your stock tires are 185/65/15 and you install 185/60/15, your RPM (revs per mile) will go from 824 to 850, and the speedometer will read 60 MPH when you're actually only doing 58.2 MPH.

If you were to accelerate until you were actually traveling 60mph, your tachometer would read higher than normal as well.

joeshmo

Except the VSS measures axle revolutions. Smaller tires=a higher number of revolutions in order to go the same distance. Think about it, the distance traveled per revolution of a tire is equal to the circumference of the tire. A smaller circumference=less distance traveled=slower speed.

What I am trying to say is that your tach should read the same at any speed indicated on your speedo but your true speed would be less. There is only one indicated speed your car could be going per rpm/gear, regardless of tire size.

toyomatt84

iTrader: (1)

To be honest... your tachometer/speedometer will not be altered when you change tire sizes, as the VSS does not take in to effect tire sizing. But, the speedometer will no longer be displaying factual information. The tachometer will always display accurate information (unless its broken or you mess with the crank position sensor).

joeshmo

Well put, this puts what I was trying to say in a much more eloquent language.

It Wasn't Me

Car A: 2002 Civic EX coupe with stock 185/65/15 tires.

Car B: 2002 Civic EX coupe with undersized 185/60/15 tires.

Both cars are identical except for the tires.


Both cars traveling at 60 MPH, tracked by radar, will not have the same speedometer and tachometer readings as the other. Car B's will be higher, even though they're physically traveling (according to radar) at the same speed.

No?

toyomatt84

iTrader: (1)

Yes. To travel at the same speed as Car A, Car B must be beyond the tachometer/speedometer readings of Car A.