Question For Engineer Types...

[picklesuit]

So I have been having a debate with a friend at work about how tire size would affect mileage/speedometer readings. It is my assertion that if you went from smaller to larger tires (with a larger outside circumference) you would get a speedometer reading that is lower than actual groundspeed, as the speedometer is based on the rotation of the axle, not the groundspeed...and that your mileage would actually decrease because it takes more power to push the vehicle on those tires as they have a greater surface area and would create more resistance/friction.

Am I right, or am I talking more out of my ass than usual?

Thanks,
Pickle

(please explain it in English to me, I was an Animal Science major in school)

 

[Eliot Watts]

If you'd like to calculate the error in the speedometer due to tire size this calculator will do the job. I can't speak to your broader question though.

 

[TTUalumn07]

I may be wrong but the odometer has nothing to do with power or speed, therefore the only aspect that would change the rate is the size of the tire.

Think of it this way: What vehicle would have a higher odometer reading (if both started at 0)... a honda civic or Bigfoot and both went from point A to point B.

So you can conclude that the bigger the tire diameter the chances are the odometer will be reading a lower than accurate reading of mileage.

 

[Herc_Dude]

With larger tires on my Jeep my speedometer said I was going slower than my GPS (ground speed) said I was traveling.

I think your mileage will decrease, along with what you said, because vehicles are designed to have a certain size tire. When you change that it "messes up the equation" (for lack of a better term) so your vehicle will work harder to get the same output (read speed). That is my PolSci take Animal man

 

[picklesuit]

I may be wrong but the odometer has nothing to do with power or speed, therefore the only aspect that would change the rate is the size of the tire.

Think of it this way: What vehicle would have a higher odometer reading (if both started at 0)... a honda civic or Bigfoot and both went from point A to point B.

So you can conclude that the bigger the tire diameter the chances are the odometer will be reading a lower than accurate reading of mileage.

I meant different tires, same vehicle.

 

[TTUalumn07]

Well what do i know...i have a mercedes with a moonroof, all leather interior...I got a good deal from my cousin prince murray, I paid cash....

 

[Herc_Dude]

Well what do i know...i have a mercedes with a moonroof, all leather interior...I got a good deal from my cousin prince murray, I paid cash....

Did this come with a subscription to GQ or did you have to order that separately?

 

[Beans]

Changing the tire circumference w/o changing anything else will change your speedometer/odometer reading. At least on older cars, there is a usually a gear attached to the driveshaft. Then there is some multiplier between the driveshaft speed (rotational speed) and the wheel speed, which then goes with ("goes with" = engineer-speak for "correlates to") the ground speed. If you change the gearing (by changing the tire size), then you will change the speedometer reading. You'd get the same effect if you changed the differential (assuming it's an old RWD with a differential). I know with older cars you could get different speedo gears for different wheel sizes and differential ratios.

As for gas mileage - all else held the same, you will improve your rolling resistance with a larger tire (narrower helps too). However, as devil_dog said, your car is probably designed for a certain tire size, and you may lose some mileage and/or mechanically damage or overstress your car with a non-standard wheel size.

 

[HokiePilot]

You are right on the mileage/speedometer. You actually should get them recalibrated after changing tire sizes to prevent speeding while your speedometer is displaying a legal speed. If you want to see, get a GPS and use its speed. It always be more sccurate than a speedomoter.

Theoretically, your actual mileage (not your calculated mileage based on your false odometer reading) shouldn't change. All you are doing in effect is changing your gear ratio. Your engine still has to do the same amount of work, it is just doing it at a slower RPM while delivering more torque. In actually, it depends on if your engine is moving towards a more or less efficient RPM at the speed you usually drive.

EDIT: Damn, beat me to it. BTW, from what I can tell, rolling resistance shouldn't change with tire size. It may be less by itself, but it acts on a greater radius to cancel that out. Also, larger tires = more weight = slightly less mileage. ... Oh, yeah, bigger tires = taller vehicle = more wind resistance. In the end, everything put together shouldn't change your mileage that much.

 

[TTUalumn07]

Did this come with a subscription to GQ or did you have to order that separately?

That'a quote from a movie.

 

[picklesuit]

That'a quote from a movie.

Dude, stop while yer behind...seriously...:banghead_

 

[TTUalumn07]

i meant "that's a quote from a movie"

but o well...i'm clueless to the GQ remark

 

[mmx1]

I was curious about this, going from bald to new tires. But judging by my GPS, my speedometer hasn't changed noticeably in relation to groundspeed.

 

[Veovis]

There's also a question on Form 5 of the ASTB as to what changing tire size does to the torque output of the engine... I didn't have a freakin' clue (Econ Major)

 

[MIDNJAC]

Well what do i know...i have a mercedes with a moonroof, all leather interior...I got a good deal from my cousin prince murray, I paid cash....

Making fun of me are we?

On a not very related note, since in many aircraft you have two different tire pressures (one for the mains, one for the nosewheels), which one is more important when trying to determine your hydroplaning speed? It would seem to me that as long as you have rudder authority, the mains would be the most important thing, but I could be wrong....

/useless knowledge questions