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)
Edit: seems bikes are a pretty mixed bag where the speed sensor is. Your mileage (and speed) may vary there
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.
Pretty sure you can’t use GPS by itself since it can be inaccurate or not work at all inside tunnels.
I knew a guy with a busted speedometer that used a GPS and yes you’re correct. Anytime he was in a major city it was problematic.
The speedo on my motorcycle is on the front…
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
The speedometer on my motorcycles were all driven from the front wheel so the final drive ratio did not effect it.
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.
Yes. Most vehicles I think estimate speed using rotations in the transmission, which assumes the original tire size. Lots of car mods, but especially tire size changes, can impact speedometer readings as well as other things like performance.
Most people modding their cars are idiots about it.
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.
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.
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.
Sorry to burst your bubble, but I’m pretty sure that’s not the one. There was no (attempt at) comedy in the one I was talking about. I kindof doubt I’d be able to find the name of the one I’m thinking about, but I might try later if I get a minute.
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.
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.
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
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.
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 circumferences. 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
Thank you for your service. It’s super important for every internet comment to contain every pedantic detail every time
No, i stand by my original statement. I said it would be “off by” as in the “error will be” so the difference is the important bit to calculate that. There are a lot of ways to correctly do the math though
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.
Can you tell me some cars that use GPS for the speedometer?
Yeah, pretty sure that’s not a thing.
Using GPS to drive the spedo/odo on a car seems like it wouldn’t be super reliable?
That’s why it’s not a thing and OP is wrong.
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.
Can you tell me some tunnels that have GPS?
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’
Yes. Also your math checks out.
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
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
I know, but still the circumference differs. It’s 195/60R15 vs. 185/65R14
Obviously, the 10 % difference are just a feeling.
Check your tire pressure.
Being low or too high is enough to put it off.
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.
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.
Good to know, thanks.
