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#1 2021-08-17 14:36:20

Neo1
Member
Registered: 2019-11-22
User Number: 46701
Posts: 34

Speedometer accuracy - should I expect it to be closer to actual?

On a 7-hour round-trip yesterday to Long Island, I noticed on a plug-in GPS that my mph was a good 3 miles an hour slower than my gauge reads (at just about every highway speed).  eek 
Today I tried it with another plug-in GPS, and same situation (so it wasn't the GPS).  Other than possibly saving me from getting speeding tickets over the years, is that common?  I have 17 inch wheels (not sure if they came with 16 originally).  Is there a way to adjust my gauge (no biggie - just wondering)? confused


2̶0̶0̶5̶ ̶B̶l̶a̶c̶k̶ ̶A̶e̶r̶o̶,̶ ̶5̶M̶T̶
2005 Silver Aero, 4EAT

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#2 2021-08-17 15:02:04

krazykarguy
Worst ninja editor, ever.
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From: South of North Carolina
Registered: 2007-04-06
User Number: 1944
Posts: 8724

Re: Speedometer accuracy - should I expect it to be closer to actual?

It's pretty typical for speedometers to run a few mph slow.

The base tire your car originally came with is a 205/55/16, which has an overall diameter of 631.9mm. The general rule of thumb is that you should try to stay within +/- 2%.

The stock Aero size tire is a 215/45/17 (625.3 mm OD), but a 225/45/17 (634.3mm) is also used on the Impreza line. If you're running a different 17" tire size than these two, your speedo will be off even more than stock.

if you go UP in overall diameter, the tire will rotate fewer times per mile, and the indicated speed (and accumulated mileage) will also be lower.
If you go DOWN in overall diameter, the tire will rotate more times per mile, and the indicated speed (and accumulated mileage) will be higher.


2005 Aero 5MT
2014 Ford Mustang GT Premium 6MT - Sterling Gray Metallic - 475hp of crowd munching fun

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#3 2021-08-17 15:48:17

Neo1
Member
Registered: 2019-11-22
User Number: 46701
Posts: 34

Re: Speedometer accuracy - should I expect it to be closer to actual?

The stock Aero size tire is a 215/45/17 (625.3 mm OD), but a 225/45/17 (634.3mm) is also used on the Impreza line. If you're running a different 17" tire size than these two, your speedo will be off even more than stock.


I have the stock Aero size you show here (215/45/17) on my automatic, and I believe 16" ones on my manual.  Are the rims the same size (as in, when I next buy tires, could I get 16's for the automatic - not as cool looking, but seem to have a smoother ride)?


2̶0̶0̶5̶ ̶B̶l̶a̶c̶k̶ ̶A̶e̶r̶o̶,̶ ̶5̶M̶T̶
2005 Silver Aero, 4EAT

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#4 2021-08-18 07:22:35

krazykarguy
Worst ninja editor, ever.
Supporter
From: South of North Carolina
Registered: 2007-04-06
User Number: 1944
Posts: 8724

Re: Speedometer accuracy - should I expect it to be closer to actual?

You can’t put 16" tires on 17" wheels.

Last edited by krazykarguy (2021-08-18 10:49:14)


2005 Aero 5MT
2014 Ford Mustang GT Premium 6MT - Sterling Gray Metallic - 475hp of crowd munching fun

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#5 2021-08-18 08:03:35

Neo1
Member
Registered: 2019-11-22
User Number: 46701
Posts: 34

Re: Speedometer accuracy - should I expect it to be closer to actual?

Gotcha.


2̶0̶0̶5̶ ̶B̶l̶a̶c̶k̶ ̶A̶e̶r̶o̶,̶ ̶5̶M̶T̶
2005 Silver Aero, 4EAT

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