Speed calibration

So on my trip I have noticed that 60 indicated on a flat road is actually 55 according to the GPS. I understand that there is a need for some latitude in order to keep on the right side of the law but equally I'd like to get on with my life. I could I suppose have a dedicated app open just giving speed on my phone but if I'm also using nav then running 2 on a split screen starts to make them a bit smaller to see than I might like . I do have an obd display that I can calibrate, but if I put it in line of sight it obscures other useful information like fuel level and temperature  and when i have used it in the past having 2 speed displays tends to be like wearing 2 watches, you are never really sure what the time is. Soooo is it possible via a different sensor or something to calibrate the actual dashboard dial to like 1 under instead of 5 under? 

Like many things on the Jimny JB74 (fuel gauge and sun visors come to mind). the speedo is really a bit crap.

You are correct, it is not very accurate, although I found mine was much more on the ball at 70-80mph. So it isn't a straight percentage drop across the board. It also seems more accurate as sub 20mph. Which to me hints at it deliberately being programmed to do this.

Also if you ever use the cruise control or speed limiter, you'll get a digital readout on the dash, this also doesn't tally with the mechanical speedo.... so who knows what speed you are really doing, i.t. On the motorway I'd often set the speed limiter to 72mph, which you can reach as it'll stop accelerating, but the speedo will show a different speed.

Part of me wonders if the large speedo error is to allow for larger tyres to be fitted. Can't you get slightly bigger tyres as standard in New Zealand or something? Sure I read that once.

On 235/75's the speedo is much better, but still not 100% believe it or not. 30mph on the speedo is still only 28-29mph GPS or on the radar road signs.

For my needs I use speedo app on my tablet for the Jimny 
On my other cars I use same app and take notes on rev counter in relation to accurate speed readings in certain gears and memorise the rpm

I played around with a speedo corrector in the past. The one I got was a kit and I could never get it to work correctly but I now see they make it in a pre-built module. It corrects the pulses from the speed sensor to correct the speedo reading and can correct both up and downwards.

www.jaycar.com.au/speedo-corrector-modul...2rzjP_DEUAUB2h1EM5Un

 

Speedos can’t under-read and can over-read by 10% + 6 mph, or roughly 20% at 60mph. As they can’t under-read they all over-read.

My Jimny was about 8% before a tyre change and is now about 5%. My other car is over 10% and my last Jimny was about 6%.

If using GPS to check it remember you have to be driving on a flat straight road and at a constant speed for a several seconds, otherwise GPS will under-read.

TBH, you only need 30, 40, 50 & 60mph and even then you are usually allowed 10% + 2mph before they will issue a ticket, but not guaranteed.

So think 35, 46, 57 & 68mph.

If your speedo over-reads by 5 - 10% and you keep to an indicated 35, 46, 57 or 68mph, you shouldn’t get a ticket.

As you may have noticed my brain tends to work best in straight lines, asking it to drive, be aware of other road users, watch for speed cameras, watch for police hiding in bushes with speed guns and listen to the computer lady tell me my next junction is one thing but to then also require me to do mathematics on the fly simultaneously is a bit much. Hence if the speedometer says 60 and I'm actually doing 59 then any slight deviation say for transition over a crest can fall into the alleged latitude for prosecution is less to think about in the moment.

Also I'm not liking that on the gen4 the disparity is potentially none linear but that is secondary as this is aimed at temeraire as she gets all the none local road trips and is therefore more likely to be somewhere I don't know with likely more cameras than home.