Re: tire size and speedometer accuracy

Okay, then my question now is if the circumference of tires are changed, will speedo recalibration be necessary?.

If the circumference of the tires are changed, then speedo recalibration would be necessary for the speedometer and odometer to be accurate. Modern speedometers use speed sensors to send a signal to to ECM and instrument clusters, and to be honest, I do not know what it would take to recalibrate the speedometer. My guess is that the sensor is magnetic and is sensing the teeth on a geared hub, much like the speed sensors for ABS. I doubt if different hubs are available and the ECM is not designed to be re-programmed so the owner has to calculate the speed difference and keep it in mind when looking at the speedometer.

Besides speedometer accuracy, vehicle shift points will be affected by the error caused by different size tires.

I've seen aftermarket trip computers that use driveshaft-mounted speed sensors and that can "learn" the characteristics of a vehicle, and that or a GPS system may be the best way to get accurate speed readings.

How much over- or undersized are you going?

I had a Toyota Tercel I put 185/60-13's on, when the standard was

Google Tire Sized Calculator Dodge Ram; there is an excellent one there, there is also a very good one if you change Dodge Ram to Miata, and there is another at 1010tires.com.

You enter the stock size for your vehicle, and the size you want to replace it with, and it will give you the difference as a percentage and in MPH.

how tire size and different differential gear ratios

gears for the tranny (this was on a GM TH400) to

accurate as possible as I didn't want to speed and

diameter tires on his car (Honda Element), the tire dealer

Are today's electronics intelligent enough to

something needs to be manually re-calibrated? This

issues. Thank you.

circumference of the original tires, then the speedometer

and have the same circumference if the aspect ratio is

the "hole" to the bead, not the tread.

of the hole to the bead...

I thought say a 225 / 50 series tire would mean the tread width was 225 and the sidewall was

Am I mistaken? Dan

gearing calculator program I use shows that if you were actually going 57 mph with 24" tall tires and the only change is the diameter of the tire.

a 25" tall tire is 59 mph. a 30" tall tire is 71 mph.

Dan

No, because there is no way for the ECM to know that the tire size is different.

No.

how tire size and different differential gear

drive gears for the tranny (this was on a GM TH400)

close accurate as possible as I didn't want to speed

diameter tires on his car (Honda Element), the tire

believe. Are today's electronics intelligent enough

least something needs to be manually re-calibrated?

specific issues. Thank you.

circumference of the original tires, then the

different diameters and have the same circumference if the

from the center of the "hole" to the bead, not the

center of the hole to the bead...

the sidewall was

Thanks

You're welcome!

Not completely.

The 225/50 would have the same tread width and sidewall, but the rim would cause the tire to be 1 inch different.