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.
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.
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,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
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.
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?
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.
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?
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.
Than is not then
While you are technically correct, the best kind of correct, fuck off and die in a well.
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.
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.
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.
