Speedo vs GPS

[John A]

In the last few weeks I've gone from 275/40/20 to 245/45/20 to accommodate going a bit lower on coilovers.

I noticed that, unusually, the speed reads lower than the GPS speed recorded on Waze. On the dash the needle (and cruise set speed) showed 70mph, but Waze was showing 72/73.
My maths (if it's correct) says there is 0.5mm difference between 275/40/20 and 245/45/20 being =(245*0.45)*2 + (20*25.4) = 728.5mm
0.5mm bigger isn't going to make that much difference is it? There's a bit of difference in tread depth between a new and part worn set of tyres I guess.

So out of curiosity, how much difference between your speedo needle and GPS do you see and what's your wheel and tyre spec?

 

[Sasquatch]

Waze on a phone is probably fine, but use a proper dedicated GPS that doesn't take secondary location data from the mobile network, and test it on a flat and straight road to be sure.

Mine under reads by 2mph on 235/55 18, but now I know I just set my speed accordingly.

 

[dubber36]

Don't forget that there are variations in sizes between brands of tyres, despite what's written on the side wall. At least with it under reading, you're not clocking up as many miles.

 

[Frigo110]

The 275 tyre had less stretch as well. I believe there is more difference in the side wall height than you think.

255/45/19 - Michelin CC2 auf 8.5J Rim (that would l look better with your springs ) - Speedo bang on at 70mph.

 

[The Flying Scotsman]

I’m running 275/40/20’s and my speedo reads 70mph and gps says 73mph

 

[Lubrown]

Agree with others - manufacturers will have different tolerances for a marked size, especially when fitted and in use where temperatures and material choices come into play - affecting pressure and elasticity.

If you went from a well worn tyre (e.g. 1.6mm tread left) to a brand new one (with maybe 7mm tread) the diameter change would be different again, on top of the size change you noted from your calcs.

Also worth noting GPS speed does have a delay while the signal bounces up and down and goes through the various computing and display functions it needs to in order to be displayed on your phone. There may be some averaging involved to reduce that lag. It’s a good indicator but not always spot on. Dedicated devices may be more accurate but aren’t really needed. A rolling road would be a more accurate way of calibrating a speedo.

 

[Greybeard]

Hi @John A

Bearing in mind the pertinent replies above;

Wheel-size.com calculates the 245s at 10mm bigger diameter and 30mm bigger circumference than the 275s.
HTH, Andy

Rim & Tire Size Calculator. Custom Offsets

Tire Tech Information - The Wheel Fitment, Tyre Stretch, Rolling Radius and Speedometer Error Calculator. Play with specs, try different settings, compare results

www.wheel-size.com

 

[roadtripper]

See also my reply here around why the speedometer will generally over read by a couple of MPH when compared with GPS for legal/regulatory reasons:

Court case for (not?) speeding in converted camper SCOTLAND

Modern speedos are now considered so accurate that some forces - although not all - will prosecute for even 1mph over in high risk roads, red routes etc. The old 10%+2 was only ever guidance, and was never universally observed anyway. I think the chances are unlikely, particularly if they do...

www.t6forum.com

TLDR; a speedometer legally must never indicate less then the real physical speed, but it's fine for it to read more so that's the side the calibration is shifted to at the factory.

 

[archersam]

My speedo underreads compared to the GPS since moving to 235/65/17 Falkens however I never used it anyway as I have a dash mounted sat nav that I use for the GPS speed everywhere I drive and converts to kmh so thats nice and handy when in Europe and all that.

 

[John A]

Thanks Andy but you've compared a 275/40 with a 255/40 - not the 245

Hi @John A

Bearing in mind the pertinent replies above;

Wheel-size.com calculates the 245s at 10mm bigger diameter and 30mm bigger circumference than the 275s.
HTH, Andy

Rim & Tire Size Calculator. Custom Offsets

Tire Tech Information - The Wheel Fitment, Tyre Stretch, Rolling Radius and Speedometer Error Calculator. Play with specs, try different settings, compare results

www.wheel-size.com

View attachment 261967

245/45/20 shows 1mm of difference (when I calculated it myself it's 0.5mm)

 

[Greybeard]

Sorry John. Just tested +ve for Covid for 2nd time.
I’ll try harder!
Andy

 

[John A]

Get well soon!