Saw a truck around town today with a ridiculous lift kit and chunky off-road tires that were clearly much larger than factory standard, and it got me thinking; if you install this kind of modification in a car, do you need to adjust the speedometer to compensate? What about the odometer?
My logic is the only absolute measurement the car has is how fast the wheels and drive shaft are turning, so presumably there is some sort of multiplier - 1 revolution = X meters - that is then used to show speed and track distance travelled, but that factor would need to change if the circumference of the tires did
Hi! I was a controls engineering in the automotive industry in the US for a while.
Yup. You sure should! Some cars even have tire dimensions and quick selections of winter/summer tires for exactly that. Some cars make it much harder/impossible to do.
Same with motorcycles if you swap sprockets of course (a common modification)
Wait, is this actually a thing?
I thought it would use some other metric to measure speed.
Would this be true for say a Honda civic?
Not sure why you got downvoted for asking a question, but yes. Most cars use a system of counting the rotations of your tires. So a larger tire will have a larger circumference and therefore change the distance traveled per rotation. Note this means if your tires need air, it will alter the speedometer as well, as the circumference is actually altered by such. (Smaller radius than when fully inflated).
This also means if someone spins the tires the speedometer will show they are going fast, when they are actually moving much slower.
Well there’s lots of ways to measure speed. Some use a worm gear in the transmission, some use a sensor on the wheel hub. But all of them take tire diameter into account, unless you count like GPS, which afaik (though probably some really shitty privacy invading car may prove me wrong) isn’t a thing for speedometers and odometers
So yes, all production cars, I believe.
Eh, for the motorcycle bit… No…
On my TW200 (pretty much unchanged since 1987) the speed is based on front tire rotations (there’s a speedometer cable with a part that literally spins inside going from the front wheel to the speedo), on my modern motorcycles it’s based on the ABS ring/brake disc.
When looking at what you see in the cluster, changing the sprocket just influences how much the engine is revving when comparing two different speeds, otherwise it’s about acceleration and top speed.
Well, I’d argue it’s not a blanket “no”. I’ve owned 9 crotch rockets and all? Of them had speed sensors on front sprockets. A lot of similar or the same designs within, so surely it’s off ABS rings if they’re newer, but a fair few of them have had speed deviations because of that
So it’s not a blanket “same with motorcycles” either then ;)
You’re absolutely right!
I don’t know of a single car that it wouldn’t affect, but there could be some using a gps speed instead? Sounds like a bad idea to me