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Jun 24, 2019 | 08:46 AM

Sin1UZFE

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Racer

Joined: Sep 2018

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From: California

Google maps

With Google maps adding speedometer to it's directions I noticed my 99 being 2 to 3 mph behind.

I wonder how accurate that would be.

My 99 says 70 but Google maps says 67

Jun 24, 2019 | 09:14 AM

peterls

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From: ca

Quote:

Originally Posted by Sin1UZFE

With Google maps adding speedometer to it's directions I noticed my 99 being 2 to 3 mph behind.

I wonder how accurate that would be.

My 99 says 70 but Google maps says 67

That's very common. 5% deviation (or was it 3% ?)is acceptable and manufacturers usually set up speedometers to show a little more rather than a little less to avoid getting blamed for speeding tickets.

Jun 24, 2019 | 11:53 AM

BNastee

Pit Crew

Joined: Mar 2015

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From: Ohio

My 99 is 3 mph fast at that speed, too.

Jun 24, 2019 | 01:49 PM

RA40

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From: California

What size tire? On ours with a 245/40-18 the rounded amount is about 3MPH high and increases fractionally more at higher revs. Google as well as the stand alone GPS should be pretty accurate.

Jun 24, 2019 | 03:45 PM

Sin1UZFE

Thread Starter

Racer

Joined: Sep 2018

Posts: 1,391

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From: California

Quote:

Originally Posted by RA40

What size tire? On ours with a 245/40-18 the rounded amount is about 3MPH high and increases fractionally more at higher revs. Google as well as the stand alone GPS should be pretty accurate.

Need to double check but pretty sure it's 225/60/16

It's funny at 60 it's 2mph high and 3mph at 75 lol

Last edited by Sin1UZFE; Jun 24, 2019 at 10:55 PM.

Jun 24, 2019 | 10:52 PM

Losiracer2

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Joined: Sep 2008

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From: Chandler, AZ

Quote:

Originally Posted by RA40

What size tire? On ours with a 245/40-18 the rounded amount is about 3MPH high and increases fractionally more at higher revs. Google as well as the stand alone GPS should be pretty accurate.

You should have a 245/45/18 for the most accurate overall diameter to stock. I've got IS350 wheels with that size tire and I believe I'm only 0.2% larger than stock. Your size is 3.4% smaller than stock which would explain more revolutions per mile.

https://www.miata.net/garage/tirecalc.html

Jun 25, 2019 | 12:52 AM

RA40

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From: California

Quote:

Originally Posted by Losiracer2

Yes. The 245/45 was the correct choice for size matching. I desired a slightly smaller diameter for a tire combo that wasn't available in a 45 series and kept with 40 series then on.