speedometer off?

cmatb08

Original poster
Member
Jun 3, 2012
101
So according to my parents car and the mobile speed sensor that the police put up on the side of the road our speedometer is off the only thing we have done is put 20s on it. Lookin at puttin 22s or 24s should I wait until we decide for sure if we change the wheel size or fix the.issue now?
 

C-ya

Member
Aug 24, 2012
1,098
Umm, yeah. When you change the diameter of the tire/wheel combo, you'll have to recalibrate. This link will show you how much you are off. If you are going to change again, just see how much you are off, learn it and live with it.

Tire size calculator
 

jrSS

Member
Dec 4, 2011
3,950
C-ya said:
Umm, yeah. When you change the diameter of the tire/wheel combo, you'll have to recalibrate. This link will show you how much you are off. If you are going to change again, just see how much you are off, learn it and live with it.

Tire size calculator

Or u can get a gps that has a mph on it. It'll tell u how fast ur going. They are accurate.
 

C-ya

Member
Aug 24, 2012
1,098
jrSS said:
Or u can get a gps that has a mph on it. It'll tell u how fast ur going. They are accurate.


Yep, I have 2 or 3 of them myself. But since the OP was comparing his vehicle to other cars and a police radar, I figured this would be easier (and cheaper) for him as apparently he DOESN'T have one.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
cmatb08 said:
..put 20s on it. Lookin at puttin 22s or 24s ...

C-ya said:
...When you change the diameter of the tire/wheel combo, you'll have to recalibrate. ..
Just to be clear - the WHEEL size is irrelevant by itself. It doesn't determine the outside diameter of the tire, which is nominally 29" from the factory.

And you didn't tell us what the speed numbers were or even if you here high or low. :frown:

We have to know the tire size ON the rims to compare diameters properly. Can you get the size off the sidewall, then locate an on-line "tire size calculator" and that will give you the nominal outer dimension of the tire. You should be able to compare whatever diameter you have with 29" and see the percentage difference.

I figured anybody investing in 20's would have a GPS or a smartphone somewhere in the family, but maybe not. :confused:
 

cmatb08

Original poster
Member
Jun 3, 2012
101
The tire size is a 275/45/R20. We had a GPS but someone decided they need the aftermarket part inside our truck mire than we did and we have never replaced the GPS.

I checked out the link it says we are -2.4%
 

jrSS

Member
Dec 4, 2011
3,950
Your sppedo shouldn't read too far off with 275 45 20's. That's what I had on the envoy I used to own and its only off by 1mph or so.
 

Short Bus

Member
Dec 2, 2011
1,906
Stock Tire - 245/65R17
Section Width: 9.64 in 245 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 7 - 8.5 in
Overall Diameter: 29.53 in 750.06 mm
Sidewall Height: 6.26 in 159.00 mm
Radius: 14.76 in 374.90 mm
Circumference: 92.77 in 2356.3 mm
Revs per Mile: 704.3

Tire 1 - 275/45R20
Section Width: 10.82 in 275 mm
Rim Diameter: 20 in 508 mm
Rim Width Range: 8.5 - 10 in
Overall Diameter: 29.74 in 755.39 mm
Sidewall Height: 4.87 in 123.69 mm
Radius: 14.87 in 377.69 mm
Circumference: 93.43 in 2373.1 mm
Revs per Mile: 699.3

Actual Speed: 60 mph 100 km/h Speedometer1: 59.5 mph 99.2 km/h
Speedometer Difference: - Speedometer Difference: 0.715% too slow
Diameter Difference: - Diameter Difference: 0.71%
 

C-ya

Member
Aug 24, 2012
1,098
the roadie said:
Just to be clear - the WHEEL size is irrelevant by itself. It doesn't determine the outside diameter of the tire, which is nominally 29" from the factory.

Just to be clear - I said when you change the diameter of the tire/wheel combo.
 

cmatb08

Original poster
Member
Jun 3, 2012
101
jrSS said:
Your sppedo shouldn't read too far off with 275 45 20's. That's what I had on the envoy I used to own and its only off by 1mph or so.

I must have read that wrong! Thanks for all the input guys. I'm lookin at a set of 24s on air. So I'm hoping it rides smooth.
 

Decembersend

Member
Dec 3, 2011
316
As another alternative to a gps you can download a free speedometer app on a smartphone. They are just as accurate as a gps, the only reason why I know this is my speedometer is off and I use my gps to see how fast I'm going and for fun I downloaded the app to see if was the same as my gps and it was.
 

Sparky

Member
Dec 4, 2011
12,927
cmatb08 said:
I must have read that wrong! Thanks for all the input guys. I'm lookin at a set of 24s on air. So I'm hoping it rides smooth.

Just my opinion but 24s are too big for any vehicle :no:
Plus a super thin sidewall of under 2.5 inches won't help it ride smooth at all. You're going to feel everything plus greatly increase the risk of damaging the wheel on a pothole, not to mention the very limited tire choices at a huge price premium.
 

McGMT

Member
Jun 17, 2012
621
Sparky said:
Just my opinion but 24s are too big for any vehicle :no:
Plus a super thin sidewall of under 2.5 inches won't help it ride smooth at all. You're going to feel everything plus greatly increase the risk of damaging the wheel on a pothole, not to mention the very limited tire choices at a huge price premium.

I don't even know if a cloverleaf would be a wide enough turn for 24's haha
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
C-ya said:
Just to be clear - I said when you change the diameter of the tire/wheel combo.
I know you're fine. :thumbsup: It's just that I see questions 2-3 times a year that make me think the OP is confused about what the 20" really refers to and how important knowing the tire size is. :undecided:
 

WarGawd

Member
Sep 2, 2012
468
Decembersend said:
As another alternative to a gps you can download a free speedometer app on a smartphone. They are just as accurate as a gps, the only reason why I know this is my speedometer is off and I use my gps to see how fast I'm going and for fun I downloaded the app to see if was the same as my gps and it was.

Of course the app should be "just as accurate" and report the same as your gps, because the app is relying on the gps sensor IN the phone. One of the reasons I love my Nokia N8 is that I can download the maps (free lifetime updates too!!) for any part of the world, and use my phone for walking/driving navigation WITHOUT a data connection. Standalone GPS units for me have been made completely obsolete in a device I have with me nearly 100% of the time. Anyway sorry to sidetrack, but I had the impression from your post that you thought the app works independently from a GPS sensor, which isn't the case, so I just wanted to clarify.


Short Bus said:
Stock Tire - 245/65R17
Section Width: 9.64 in 245 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 7 - 8.5 in
Overall Diameter: 29.53 in 750.06 mm
Sidewall Height: 6.26 in 159.00 mm
Radius: 14.76 in 374.90 mm
Circumference: 92.77 in 2356.3 mm
Revs per Mile: 704.3

Tire 1 - 275/45R20
Section Width: 10.82 in 275 mm
Rim Diameter: 20 in 508 mm
Rim Width Range: 8.5 - 10 in
Overall Diameter: 29.74 in 755.39 mm
Sidewall Height: 4.87 in 123.69 mm
Radius: 14.87 in 377.69 mm
Circumference: 93.43 in 2373.1 mm
Revs per Mile: 699.3

Actual Speed: 60 mph 100 km/h Speedometer1: 59.5 mph 99.2 km/h
Speedometer Difference: - Speedometer Difference: 0.715% too slow
Diameter Difference: - Diameter Difference: 0.71%

Minor comment - the revs per mile, sidewall heights etc are going to vary a little by manufacturer and tire model (IE the stock sized tires range from roughly 702 - 708 rev per mile) and so the error introduced may also vary a bit depending on the manufacturer and tire model...In your example the speedo difference was .7% but it might be a little higher or lower than that for the exact same tire from a different manufacturer...still so close as to be irrelevant in practice
 

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