Should I trust Google Maps Speedometer?

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69 Barracuda vert

69 Barracuda vert

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Bout to get my 1st radials for my 69 Barracuda (Lady Blue) and would like to make the speedometer be as accurate as possible. I have this tire converter I found on line which is really pretty cool 205/75-R14 vs 265/50-R15 Tire Comparison - Tire Size Calculator | Tacoma World. I want to make sure I'm not gonna screw things up and get the wrong size. So, Goggle Maps says that when the speedo is reading 61 mph in the cabin, the car is really going 65 (according to google) and I'm currently on 205/75 r14. I bought 15" x 8" wheels and wonder if I should get a slightly smaller diameter tire to slow the car down in order to be more accurate with my speedometer?

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Nice car. You could always get the tires you want then check the speedometer for accuracy. After a gear swap I just ordered a new speedo gear for the transmission.
 
If you have to ask......
 
It is close, but it seems to lag a bit, as far as the Google Maps app goes. I downloaded a free speedometer app, it only has a 1 second lag. I'm close to a major highway - usually I just go for 10 mile markers and compare the odometer.
 
Use whatever size tire you want and compare your speedometer to google and adjust your speedometer gear as require. FYI, you will probably not get them to exactly match but you should come close.
 
Check out tiresize.com. LOTS of info on tire sizes, widths, and rpm and rolling distances for any tire you can find.
And I bought a GPS speedo for my 62, and tested it in my pickup. I think the gps speedo is accurate, and shows my truck speedo is slow (larger tires than stock).
 
I search around the thrift stores and pickup old GPS units, typically the small Garmin Nuvi units, don't worry about the map, it does not need to be up to date. I typically pay 5-15 bucks for these, and now have several. They are absolutely accurate, far more accurate than a speedo

I've also got a small Garmin "Etrex" batterypowered handheld unit which I use in the boat
 
Will the 265/50x15s fit? 225/60x15 is also slightly shorter (25.6) and much more likely to fit (assuming the backspace of the 15x8s are close to being correct.
Better fit for an eight inch rim than the 265s, too.
 
69 Barracuda vert said:
Exactly! I'm not very smart with these kinds of things. I didn't know you could swap out speedo gears.
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There's a chart floatin around here somewhere regarding tire size, gear ratio and correct speedometer gear.
 
33IMP said:
Will the 265/50x15s fit? 225/60x15 is also slightly shorter (25.6) and much more likely to fit (assuming the backspace of the 15x8s are close to being correct.
Better fit for an eight inch rim than the 265s, too.
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I'm thinking they will by looking at what everyone on here is putin on theirs. I have made no modifications to the body but, I am putting an 8.75 rear in the car that is the same width as the 7.25 that's in there now.
 
Looks like your speedo is off by a little under 6.5%. Tires would have to be quite short to make that up, 25"ers are only about 2%.
My advice, same as above . Make the rearend swap, buy the tires you want, and change the speedo gear in the trans to fit.
 
I've been using it for years as I travel the country transporting musclecars and it is always 1 mph faster than the speedo on my truck, ALWAYS! It used GPS so it should be right on accurate...
 
67Dart273 said:
I search around the thrift stores and pickup old GPS units, typically the small Garmin Nuvi units, don't worry about the map, it does not need to be up to date. I typically pay 5-15 bucks for these, and now have several. They are absolutely accurate, far more accurate than a speedo

I've also got a small Garmin "Etrex" batterypowered handheld unit which I use in the boat
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Just download a MPH app on to your phone. That’s what I use.
 
mcodecuda said:
Just download a MPH app on to your phone. That’s what I use.
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That only works on smart phones LOL

Also, "it might be" that those aps do NOT work on GPS, but rather the tower information / location in your phone. If you are out somewhere rural, and you only have access to 1 tower, they are not accurate, and not much better if you only have access to 2 towers
 
Those digital speed limit signs that municipalities have, especially around schools seem to be pretty accurate. There are a couple around me and using them I found that my Roadrunners speedo reads 10 miles slower than what I'm actually going and my Duster is just right.
 
^^That's a fair idea except around here you only see them in 25 zones typically^^
 
FWIW After changing my rearend gears I found a calculator posted on this site. It gave me a tooth count for the gear change . I found a new gear kit on Ebay. After installation I used an Etrex GPS to check my speedo on an open county road after acquiring 3 satellites. It all worked out great.
 
69 Barracuda vert said:
I bought 15" x 8" wheels and wonder if I should get a slightly smaller diameter tire to slow the car down in order to be more accurate with my speedometer?
Click to expand...

just get the proper speedo gear and be done with it.

when i set the aftermarket speedo in my old dart i had a Garmin on the windshield and a speedo app on my phone. a the end all three (car speedo, garmin speed reading and phone app) were all within a single mph at any given time.
 
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