Forum: Advice / Cars & Driving

Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 11:20 AM

Is it possible that your speedometer can run slow if your tires are to big for your car?

I think mine is about five mph slow. Because I was following my mom home and the limit was 75 and she doesn't speed and I don't either and I was way to far behind to be going the same speed.

14 Replies to Speedometer Question

re: Speedometer Question
By LlamaLlamaDuckmember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 11:25 AM
We take it for granted that a speedometer measures vehicle speed, but technically, the speedometer measures the number of driveshaft revolutions per mile, which is displayed on the gauge face as speed in miles per hour. When you change the tire height or the rearend ratio, you change the ratio between the number of driveshaft rotations during one complete tire rotation, which changes the number of driveshaft revolutions per mile. The speedometer reading has to be corrected to reflect the change. Taller tires will cause the speedometer to indicate too slow a speed, and shorter tires will cause it to indicate too fast. Similarly, taller gears (lower numerically) will have the same effect as taller tires, and the speedometer reading will be slower than actual. In either case, you need to figure out the amount of error in your speedometer and how to correct it.


In a nut shell... yes it can.
re: Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 11:31 AM
LlamaLlamaDuck wrote:


In a nut shell... yes it can.

Thanks. No wonder I make people mad on the turnpike :\
re: Speedometer Question
By DancingDiva736member has saluted, click to view salute photos
On Mon May 04, 2009 11:37 AM
Side note: It also affects the odometer therefore it is going to show less mileage on the car as well. By having larger tires, both your speedometer and odometer are affected.
re: Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 11:41 AM
DancingDiva736 wrote:

Side note: It also affects the odometer therefore it is going to show less mileage on the car as well. By having larger tires, both your speedometer and odometer are affected.


Good to know. Thank you very much. I shall blame my dad :) he bought the tires.

I drive about 200 miles a week mostly on the turnpike. It's probably more than that so now I know.
re: Speedometer Question
By LlamaLlamaDuckmember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 11:49 AM
^ So if one leases a car and knows they are going to go over the limit than they should get a set of bigger tires...

JK... that would be dishonest.
re: Speedometer Question
By DancingDiva736member has saluted, click to view salute photos
On Mon May 04, 2009 11:55 AM
^But really your right! They would just need to put the actual tires on before they turn it back in, because they would notice at inspection otherwise.
re: Speedometer Question
By rincedragonflymember has saluted, click to view salute photosPremium member
On Mon May 04, 2009 01:51 PM
^Well, wouldn't someone figure it out though? "Hmmm...these tires have NO wear on them and they've driven HOW many miles? Weird!"

Unless...you swapped them, and then swapped them back...haha. Okay, enough from me about being deceitful. :)

This is a very interesting thread. I feel like I've learned something new today!
re: Speedometer Question
By dangslowmember has saluted, click to view salute photos
On Tue May 05, 2009 05:41 PM
Bigger tires means that you are actually going faster than your spedometer says. Generally you will deviate by 2% for every inch bigger in tire diameter you upgrade. If you got rid of those stupid 28 inch stock tires on a jeep and slapped on some more respectable 33's, then you could easily expect to have about 10% of error in your gauge.
Just figure out the math, it will do you some good. You'll get better at it as time goes by, particularly when a COP is around.
re: Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Wed May 06, 2009 07:37 AM
dangslow wrote:

Bigger tires means that you are actually going faster than your spedometer says. Generally you will deviate by 2% for every inch bigger in tire diameter you upgrade. If you got rid of those stupid 28 inch stock tires on a jeep and slapped on some more respectable 33's, then you could easily expect to have about 10% of error in your gauge.
Just figure out the math, it will do you some good. You'll get better at it as time goes by, particularly when a COP is around.


Hmm...Thanks.

Conflicting opinions on this thread.
re: Speedometer Question
By dangslowmember has saluted, click to view salute photos
On Wed May 06, 2009 08:58 AM
Edited by dangslow (68577) on 2009-05-06 09:10:57
No conflict at all. Your axles spin at a given speed to move your car down the highway. Your speedometer is calibrated to that speed. If you put bigger tires, each revolution will propel you down the road a bit farther than smaller diameter tires. If you had 28 inch tires, multiply that by pi. You will get about 90 inches of foreward travel with each revolution. If you put on 33 inch tires, then you will travel about a FOOT farther EACH revolution because 33's measure about 104 inches around.
Your car will feel a bit more sluggish with bigger tires because the same torque is operating on a larger radius from the wheel center to the ground. That's why four wheelers usually try to get lower gears in the axles when they get bigger tires, so they dont lose any pulling power when they upgrade tires.

Your speedometer doesnt know what size tires you have, so it measures the same speed even though each revolution amounts to an additional foot traveled down the road.
re: Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Wed May 06, 2009 10:52 AM
dangslow wrote:

No conflict at all. Your axles spin at a given speed to move your car down the highway. Your speedometer is calibrated to that speed. If you put bigger tires, each revolution will propel you down the road a bit farther than smaller diameter tires. If you had 28 inch tires, multiply that by pi. You will get about 90 inches of foreward travel with each revolution. If you put on 33 inch tires, then you will travel about a FOOT farther EACH revolution because 33's measure about 104 inches around.
Your car will feel a bit more sluggish with bigger tires because the same torque is operating on a larger radius from the wheel center to the ground. That's why four wheelers usually try to get lower gears in the axles when they get bigger tires, so they dont lose any pulling power when they upgrade tires.

Your speedometer doesnt know what size tires you have, so it measures the same speed even though each revolution amounts to an additional foot traveled down the road.


You are a smart man. Thanks.

they 14' tires but supposed to be 13' according to my dad

Now I'm wondering about my dads truck because he has four different size tires.
re: Speedometer Question
By dangslowmember has saluted, click to view salute photos
On Wed May 06, 2009 06:07 PM
Edited by dangslow (68577) on 2009-05-06 18:08:58
One inch on such a small tire shouldn't make a difference. Maybe... just maybe... maybe your mom WAS speeding after all *GASP*

Your dad can run four different tires on his truck because his axle differentials allow each wheel to turn at it's own speed. One will turn faster, but it's no big deal because the gears inside just equalize it anyway. That's why they are called differentials. If both tires turned at the same speed all of the time, his tires would "chirp" and scuff whenever he turned around sharp corners because the outside wheel needs to turn faster than the inside wheel. The differential allows the tires to turn at a different rate, which is great, because your Dad is a cheapskate like me. I run absolute garbage tires on my old truck. If it's round, black, and holds air, I would put it on my truck and run the bajeesus out of it too. He sounds like my kind of guy.

This is a cool animation of a diff. Notice how the axles are turning at different speeds...

www.youtube.com . . .
re: Speedometer Question
By majeremember has saluted, click to view salute photosPremium member
On Wed May 06, 2009 08:26 PM
LOL

I feel like I have learned today. Thank you.
re: Speedometer Question
By Sinead_X
On Sat May 09, 2009 05:21 PM
Oh thats mad I never thought of that..
how do people figure these things out?...ha

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