Whats more accurate

[Forg]

Speedo would be more accurate; if it’d been designed to be accurate. Instead it’s designed to read a bit high. So answer is GPS, regardless of hills.

 

[VFSV6FORME]

Most speedos are optimistic...by a couple of kays.

I’ve changed my wheels so I can dial in 64kph and be hovering on 60-61.

I use DashCommand also, to keep all in check.

(Please read Carefully as I have a habit of making things confusing)

Say you changed you Redline wheels to 20 inch from 19 inch and when you had your 19 inch wheels on you car you speedo it read 62 when you were actually doing 60 (car made to read higher) on a accurate GPS and considering you were running the older 19 inch and then you purchased the NEW 20 inch although is 1 inch more but only is really 1/4 or say 5 mm higher in height compared to the 19 inch road wise. Now what will you car Speedo show when you are doing 60 on the GPS? This bit confuses me since I had the Stroke.

 

[426Cuda]

The difference will be less.

 

[kleanphil]

when you change 19" to 20" the tyre changes from 40 to 35 compensating for the bigger rim , when I changed mine , putting 275/35/r20 , the tyre calculator figured it would add 4kph at 110kph, if my speedo was calibrated 4kph higher , it was in fact now correct .
Checking with the GPS on my dash cam and on my phone , all read pretty much identical .

HMM I think I just confused myself ?????

 

[woooo]

GPS is spot on usually. You'll be right...

 

[VFSV6FORME]

The difference will be less.

Please give me more information. Remember I had a stroke 3 years ago and think and talk like yoga on Stars Wars

when you change 19" to 20" the tyre changes from 40 to 35 compensating for the bigger rim , when I changed mine , putting 275/35/r20 , the tyre calculator figured it would add 4kph at 110kph, if my speedo was calibrated 4kph higher , it was in fact now correct .
Checking with the GPS on my dash cam and on my phone , all read pretty much identical .

HMM I think I just confused myself ?????

Its extremely Confusing as if you go from 18 to 19 or you go from 19 to 20 you use a lower profile tire but at the same time the circumference is larger (About 5MM higher for 25MM higher Rim)which will slow your speedo so if you are doing 60 on the Speedo your are actually doing 58 and when you go to 60 you are actually speeding at 62-63 KPh but consider the Car Speedo is faster than the GPS you are actually doing around 60-61. CONFUSING Yes ITS VERY CONFUSING.
A BIGGER TYRE even its has a lower profile in Circumference make more foot print so it cover more area thus for the exact speed you are doing is faster than what you think and considering the Car Speed is normally faster than a GPS you will find yourself back where you have started.

Any Scientist Here!!!

 

[kleanphil]

I think basically - GPS = spot on . Speedo = +4 (to compensate tyres and wheel changes) . Speed camera = +5 (set to trigger above 5kph speed limit to compensate wheel/tyre changes DONT QUOTE ME ON THAT )

If a tyre/wheel is bigger than standard and is making you go faster by 4k and your speedo is calibrated 4k faster than you would with standard wheels/tyres , that would mean the speedo is now reading the correct speed because it is all measured by how many rotations the wheel is making and not the distanced travelled by the diameter of the wheel .

 

[Forg]

Any Scientist Here!!!

The short answer is - there’s no way to tell from just “changed from 19 inch to 20 inch” or “changed from 20 inch to 19 inch”.
You’ll still have choices in tyre profile / width, and even then, there can be enough differences between different brands (for tyres with the same specs) to cause 1km/h difference when you’re doing around 100km/h.

You basically need to break it down into calculation steps to compare:
(a) use an online calculator to get the tyre’s outside diameter for the original wheel, based on the tyre’s specs.
(b) use the same online calculator to get the new tyre’s outside diameter based on specs.
(c) calculate a/b ... it will be a number a bit lower or a bit higher than 1 ... the speedo will read that percentage lower or higher (if a > b, speedo will read a bit higher after the change because the wheels have to scurry a little bit faster to maintain the same real speed as before the change, if b > a the speedo will read lower after the change).

But remember it can still be a percentage point or two out due to tyre brand differences (yes - Plod can be booking you for doing a speed higher than you thought you were doing - but PR is more important than actual safety!).

Just go by the reading the gps is reading. Its more accurate than the speedo.
Also won't matter what size the tyres or wheels are.

The GPS in our Garmin Nuvi (operated by a not-that-interested-in-being-there wife from the passenger seat) is how I tuned the speedo in my project-car.

 

[Lex]

Just go by the reading the gps is reading. Its more accurate than the speedo.
Also won't matter what size the tyres or wheels are.

 

[woooo]

I think basically - GPS = spot on . Speedo = +4 (to compensate tyres and wheel changes) . Speed camera = +5 (set to trigger above 5kph speed limit to compensate wheel/tyre changes DONT QUOTE ME ON THAT )

If a tyre/wheel is bigger than standard and is making you go faster by 4k and your speedo is calibrated 4k faster than you would with standard wheels/tyres , that would mean the speedo is now reading the correct speed because it is all measured by how many rotations the wheel is making and not the distanced travelled by the diameter of the wheel .

These figures are all wrong.