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
I have a vivid memory of staying home sick from school and watching daytime educational programming on PBS. There was a (dry, low-budget, old) math show for kids on. They had a “skit” where a couple of teenagers went and got replacement tires for their car. They came in with a set of numbers that I assume had to do with the tire measurements. (Maybe hub diameter, hub thickness, and tire outer diameter.) They found tires that matched on two of those numbers, but the guy was impatient and said it had to be basically the same because it matched on two parameters. Then in the next scene, the same teens were driving the car with brand new tires and they got pulled over for speeding. The driver was sure the speedometer said he wasn’t speeding, but the new outer tire diameter changed the calculation, meaning the speedometer read lower than they were actually going.
This is the first time in my life the memory of that show has ever come in handy.
I remember that show! Not that episode specifically.
If it’s the one I’m thinking of, it was called Square One.
I’m sure there are some cars that can adjust the speedometer automatically or something but in my experience if you change the outer diameter of the tire it will throw off your speedometer.
I actually don’t know if it messes with odometer. I imagine it would if the odometer is running off the same senser that does the speedometer.
I had to when I changed the tyres on my ebike. The slick tyres were smaller diameter than the chunky off road fat tyres I had on there, and I had to tell it the new diameter so that it could correctly determine my speed.
You can also tell it the wrong diameter to increase top speed.
I have a ECU tuner with my Camaro. It has a tire size setter for this exact purpose. People use different tires for many reasons.
It depends if your car uses GPS or ABS(Wheel speed) to determine MPH. Generally speaking, if you increase the diameter(circumference) of your tire(not wheel[rim] necessarily), then you need to recalibrate your car to have an accurate speedometer reading.
The larger the diameter, the slower your speedometer will read because your tire makes less revolutions per distance traveled; because speed is distance over time. If your speed is determined by the number of revolutions based on the stock diameter of the tires(+/-5% due to tire wear) then going up considerably(>5%+/-), then your speed will be off.
Realistically, if everybody is speeding than nobody is, so if you throw 37s on your 32 stock vehicle and keep with traffic, you will be fine until you aren’t. Don’t be stupid, have your car calibrated for your tire size so you don’t get pulled over for speeding with a kilo of fent in your trunk.
Only break one law at a time, breaking two is begging to get pulled over.
Than is not then
While you are technically correct, the best kind of correct, fuck off and die in a well.
whenever i’ve driven an unfamiliar vehicle, i’ve always checked with the mile markers on the freeway. 1 minute for 1 mile at 60 mph. if it’s off too much you can adjust your driving accordingly so you’re at least ‘close enough’
Using GPS to drive the spedo/odo on a car seems like it wouldn’t be super reliable?
It becomes reliable if the car uses all the other sensors together with the GPS. Current German cars do that for their internal computers, for positioning, controlling their driving dynamics etc. Such systems are able to drive through several kilometers of tunnels and still know their own position with a deviation less than a meter, and afterwards adjust it again from GPS and mobile antennas etc.
But they usually don’t display this data on the spoedometer.Not so good in a tunnel or surrounded by high rise, trees etc.
A lot of tunnels have GPS beacons to mitigate the issue of constellation blindness.
That’s why it’s not a thing and OP is wrong.
Can you tell me some cars that use GPS for the speedometer?
Yeah, pretty sure that’s not a thing.
Sure. The vehicle will under report speed and driven mileage with larger wheels and tires, and over report both with smaller wheels than tires. I’m not if it’s possible to adjust the speedometer and odometer, or if people even bother.
No, they are calibrated and have technicians that set them professionally.
Do you need to? No. Should you? Maybe? Can you? Also maybe. I’m guessing not all manufacturers have the means to calibrate a speedometer that accurately but I could be wrong.
Yes, actually you are supposed to do that.
People who put lift kits and huge tires on a truck they’re using to drive around town don’t really use the kind of critical thinking to worry about that. They’re mostly thinking about their credit card payment and how it’s all brown people’s fault.
I hope you atleast see the irony in suggesting people are racist based on the kind of vehicle they drive.
Yeah. That’s almost as bad as claiming someone is racist because they performed a Nazi salute. Ohh wait …
You can know that, statistically, you are more likely to be mugged by a minority. And then there is also knowing that treating people badly based on those statistics just makes the problem worse.
Interesting logic.
But on the other hand, I can’t help feeling like he also used some personal observation about some racists.
Not OP, but I come from a place where this is the case. Can confirm, it is a valid observation
Judging people for their actions/choices is nowhere near the same as judging someone for how they were born.
Concluding that someone is a lesser human because of their skin color seems just as illogical to me as calling someone a racist because they drive a lifted truck. These things are completely unrelated.
Your speedometer likely wasn’t accurate before the tire size change
I use my phone as a nav device pretty much everywhere I go with different rental cars and the speedometer is never accurate to the GPS measured speed or the radar based speed limit warning signs I see sometimes
The GPS always matches the radar signs, so I tend to trust it and use it more than the factory speedometer
Check your tire pressure.
Being low or too high is enough to put it off.
AfaIk, the tachometer is allowed to be overestimating your velocity by 10 %. On my car, it’s pretty accurate when using 15" the factory default summer tires, but it’s off by 10 % when using the factory default 14" winter tires.
I don’t think so because the 14" and 15" wheels for the same car should have a different second number to their tire measurement to compensate, the manufacturer wouldn’t recommend installing tires that will make your speedo different depending on your wheel size.
First number is width
Second number is height of the sidewall as a % of the width
Third number is wheel size
So for example my summer tires are 255/45R20, my winter tires are 255/55R18, increasing the sidewalk height from 45% to 55% of the 255mm width compensates for the wheels being 2" smaller
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)
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
Yes. Also your math checks out.
Yes,you can use the tire size information to calculate the circumference of the stock and the new tires. The speedometer will be off by the same ratio as the difference in circumference. If the new tires are already installed and you don’t like geometry, use your gps to check the accuracy of the speedo at a few speeds
Nit: a difference is not a ratio
Tangential speed of the tires is proportional to the angular speed, and the factor is the radius (assuming a perfect circle with negligible differences from tire deformation). Same thing with distance. Tires that have half the radius of stock would report speeds and distance twice as fast/far, and twice as large tires would be half as fast.
