How accurate is your speedometer?

[Huskymaniac]

I checked mine for the first time today with a GPS and was surprised to see it reading about 5% too low. I need to have the speedo read about 73-74 to be going around 70.

So what have other people experienced? And does this inaccuracy exist in the odometer too or is that more accurate?

[Hocrest]

Mine varies with tires. With some "snow's I've been about 5% off. With the current tires I'm right on.

[LetItSnow]

My Pearly read very close to the GPS. I haven't checked the Silver yet.

[Manarius]

I'd say that as I go faster, the speedo becomes more and more inaccurate. At 77 on the speedo, I read about 73-74mph on the gps. That's running on 45/R17's. Same as my mom's 2008 Impala; stock tires/rims and as you go faster, it starts to get more and more inaccurate.

[Huskymaniac]

Originally Posted by Manarius

I'd say that as I go faster, the speedo becomes more and more inaccurate. At 77 on the speedo, I read about 73-74mph on the gps. That's running on 45/R17's. Same as my mom's 2008 Impala; stock tires/rims and as you go faster, it starts to get more and more inaccurate.

If the speedo is off, will the odometer be off too?

[ZMan]

Originally Posted by Huskymaniac

If the speedo is off, will the odometer be off too?

Of course.

[SoCal LS-L]

Mine is almost a consistent 3mph too high, via the stationary radar signs that show your speed underneath the posted limit sign.

[Huskymaniac]

Originally Posted by ZMan

Of course.

Are you sure? I looked this up over lunch today and found this:

http://auto.howstuffworks.com/car-dr...peedometer.htm

Our speedo is analog. It looks like analog speedos work through eddy currents that vary the torque applied to the needle as your speed changes. This torque is balanced against an internal spring. If either the magnetic fields or the spring torsion are off, the indicated speed will be off. The odometer, on the other hand, is a directly geared off the cable and should be very accurate if the tire diameters are correct. But they can be different from one another because they are not both directly gear driven.

[ZMan]

Originally Posted by Huskymaniac

Are you sure? I looked this up over lunch today and found this:

http://auto.howstuffworks.com/car-dr...peedometer.htm

Our speedo is analog. It looks like analog speedos work through eddy currents that vary the torque applied to the needle as your speed changes. This torque is balanced against an internal spring. If either the magnetic fields or the spring torsion are off, the indicated speed will be off. The odometer, on the other hand, is a directly geared off the cable and should be very accurate if the tire diameters are correct. But they can be different from one another because they are not both directly gear driven.

Yes, that is right. Because i did not elaborate properly in the first place, i was simply talking about an error in speed due to tire size affecting the odometer similarly. - Sorry!

[NeedForSpeed]

Originally Posted by ZMan

Yes, that is right. Because i did not elaborate properly in the first place, i was simply talking about an error in speed due to tire size affecting the odometer similarly. - Sorry!

I've experienced the same speedo error reported by others running stock size tires. Increasing tire diameter has always brought my observed speed closer to the road side radar speed.

Tiire size vs. odometer test :

Stock diameter 225/50/16 is 24.9"
LS-L Pearl 235/40/18 TOYO VERSADO LX is 25.4"

Recently, I made a trip on the TOYOs. I wanted to know the same thing you are asking about, how much more accurate would my odometer be running the larger diameter tires.

Mapquest travel distance was 352.55 miles. Trip odometer was 350.5 miles. So, about 0.6% error, much less than 1%.

Also, the gas mileage appears 'less', as the odometer reads less miles traveled on the same amount of fuel.

[Huskymaniac]

Originally Posted by NeedForSpeed

I've experienced the same speedo error reported by others running stock size tires. Increasing tire diameter has always brought my observed speed closer to the road side radar speed.

Tiire size vs. odometer test :

Stock diameter 225/50/16 is 24.9"
LS-L Pearl 235/40/18 TOYO VERSADO LX is 25.4"

Recently, I made a trip on the TOYOs. I wanted to know the same thing you are asking about, how much more accurate would my odometer be running the larger diameter tires.

Mapquest travel distance was 352.55 miles. Trip odometer was 350.5 miles. So, about 0.6% error, much less than 1%.

Also, the gas mileage appears 'less', as the odometer reads less miles traveled on the same amount of fuel.

Was that trip odometer test with the bigger tires? If so, your 352.55 mile trip should have read 345 miles so it appears as though the odometer is reading 1.6% too high as a starting point. But when you increased your tire diameter by 2.2% you caused your trip meter to now read .6% too low. Of course, that assumes Mapquest was accurate.

[kwren]

All 3 of our SVX's are between 1 - 2 mph off... consistently slow @ a speedometer reading of 70mph. I just assumed all 92's were.

Keith

[1986nate]

When you change to a 4.44 geared transmission, it reads about 4-6 mph fast at 70 mph

[NeedForSpeed]

Originally Posted by Huskymaniac

Was that trip odometer test with the bigger tires? If so, your 352.55 mile trip should have read 345 miles so it appears as though the odometer is reading 1.6% too high as a starting point. But when you increased your tire diameter by 2.2% you caused your trip meter to now read .6% too low. Of course, that assumes Mapquest was accurate.

Yes, trip was on larger TOYOs.

I would agree that with stock diameter tires, the speedo and the odometer read too high. With the larger circumference, it seems that the speedo and odometer are more accurate, closer to reality. If Mapquest is accurate, 1/2 of 1% is close enough for me. I believe Mapquest is more accurate than an SVX running OEM 225/50/16 rubber

Also, when the speedo reads 77mph, I don't want to be doing 73. If I'm going 78 and it reads 77, that suits me better.

I've always thought the speedo/odometer calibrations were designed for 215/55/16 rubber, OEM on 97s, but 55 series wasn't sexy enough, so 225/50s were selected at intro. I believe the speedo/odometer would be spot-on with those 25.3" tires.

[Huskymaniac]

Originally Posted by NeedForSpeed

Yes, trip was on larger TOYOs.

I would agree that with stock diameter tires, the speedo and the odometer read too high. With the larger circumference, it seems that the speedo and odometer are more accurate, closer to reality. If Mapquest is accurate, 1/2 of 1% is close enough for me. I believe Mapquest is more accurate than an SVX running OEM 225/50/16 rubber

Also, when the speedo reads 77mph, I don't want to be doing 73. If I'm going 78 and it reads 77, that suits me better.

I've always thought the speedo/odometer calibrations were designed for 215/55/16 rubber, OEM on 97s, but 55 series wasn't sexy enough, so 225/50s were selected at intro. I believe the speedo/odometer would be spot-on with those 25.3" tires.

Well, not for me. That would correct for about 1.8% of the error but my speedo is off closer to 5%!

So you don't get tire rub with tires that are more than 13mm larger in diameter and 10mm larger in width? That would be good to know as it would allow me another option for snows. Up until now I have only looked at 205/55/16 and 225/50/16.