Having speedo corrected on Monday - any other adjustments?

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

Monday, I'll be taking the KJ in to the local dealer to have the speedo adjusted for the new tyres. While they've got it on the DRB-III, is there anything else I can or should have tweaked?

I'd like to get the fuel and temperature gauges reading linearly, as well as dropping the cruise control low speed set limit from 30mph down to about half of that. Don't know if these are realistic things to ask for, but if they are, is there anything else that may be worth asking for?

PZKW108 · **Joined:** Sun Apr 06, 2014 5:23 pm **Posts:** 461

casm wrote:

Monday, I'll be taking the KJ in to the local dealer to have the speedo adjusted for the new tyres. While they've got it on the DRB-III, is there anything else I can or should have tweaked?

I'd like to get the fuel and temperature gauges reading linearly, as well as dropping the cruise control low speed set limit from 30mph down to about half of that. Don't know if these are realistic things to ask for, but if they are, is there anything else that may be worth asking for?

You could ask to get the cruise control not to downshift when accelerating downhill.....

Or you could avoid going to the dealership !

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

PZKW108 wrote:

Or you could avoid going to the dealership !

And your suggestion for doing this otherwise is what, exactly?

PZKW108 · **Joined:** Sun Apr 06, 2014 5:23 pm **Posts:** 461

casm wrote:

PZKW108 wrote:

Or you could avoid going to the dealership !

And your suggestion for doing this otherwise is what, exactly?

How much bigger are the new tires anyway ? With tires slightly bigger your speedo reading is likely to be closer to the real speed of the vehicle than it was from the start as the speedo reading from factory is always something like 5% faster than your actual speed !

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

PZKW108 wrote:

How much bigger are the new tires anyway ? With tires slightly bigger your speedo reading is likely to be closer to the real speed of the vehicle than it was from the start as the speedo reading from factory is always something like 5% faster than your actual speed !

245/75R16 vs. 225/75R16. The difference is around 4% in diameter.

Below 40mph, the difference isn't really noticeable. Above 40mph, the error is much greater. It's also not linear: 50mph indicated is closer to an actual 54mph; 62mph indicated is more like 70mph. Actual speeds are being derived via GPS.

So, yeah, I'd like to have the speedo reading closer to the truth. Ditto the odometer. If it helps with EVIC accuracy at all (doubtful), that's a bonus.

PZKW108 · **Joined:** Sun Apr 06, 2014 5:23 pm **Posts:** 461

casm wrote:

PZKW108 wrote:

How much bigger are the new tires anyway ? With tires slightly bigger your speedo reading is likely to be closer to the real speed of the vehicle than it was from the start as the speedo reading from factory is always something like 5% faster than your actual speed !

245/75R16 vs. 225/75R16. The difference is around 4% in diameter.

Below 40mph, the difference isn't really noticeable. Above 40mph, the error is much greater. It's also not linear: 50mph indicated is closer to an actual 54mph; 62mph indicated is more like 70mph. Actual speeds are being derived via GPS.

So, yeah, I'd like to have the speedo reading closer to the truth. Ditto the odometer. If it helps with EVIC accuracy at all (doubtful), that's a bonus.

With stock tire I use to be at 96kmh when the speedo read 100kmh. Now I'm 102kmh with 245/75r16 so I am closer to real speed.

The speedo is linear, you can calculate the error as a %.

And again the speedo is not accurate from factory in purpose !!

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

PZKW108 wrote:

With stock tire I use to be at 96kmh when the speedo read 100kmh. Now I'm 102kmh with 245/75r16 so I am closer to real speed.

The speedo is linear, you can calculate the error as a %.

And again the speedo is not accurate from factory in purpose !!

Yes, I am aware that the speedo is not balls-on accurate from the factory.

Mine is not showing linear error as speed increases. If you doubt this, please feel free to come for a ridealong and I'll show you exactly what's happening, which is as I've described it.

TKB4 · **Posted:** Tue Mar 24, 2020 8:46 pm

I doubt there is anything the dealer can do to produce linearity on any of the goals you wish to accomplish. They can recalibrate the speedometer but it its not linear now it still won't be and you should be very close to accurate with the new tires . The difference in real and indicated will never be the same number of MPH it will always be a percentage of indicated MPH. The guage is operated according to rpm or maybe the speed sensor data . Used to be a cable that actually connected needle on cluster to a gear . If it is nonlinear which I believe you I just dont see how it would be the best dealer could do is make it correct at a particular speed maybe mid range one like 40 or wherever you drive on highway a lot like 65 etc. I am also not sure how they would know what to calibrate it to unless they look up revs per mile of that brand tire size and if they go by that it seems to me it would still be off as much as originals were. Only thing I can think of is change it a certain amount up or down percentage wise and direction to change it. Without driving it and checking with GPS etc or possibly a dyne they dont know how indicated MPH compares to actual MPH either.

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

We're under 'Stay at Home' orders right now, so I decided to put this off for the time being. That said:

TKB4 wrote:

I doubt there is anything the dealer can do to produce linearity on any of the goals you wish to accomplish.

Which may very well be the case. I know that GDE was able to get the fuel and temperature gauges to read linearly, but have no idea if that was something they changed in the ECU, BCM, or elsewhere.

Quote:

They can recalibrate the speedometer but it its not linear now it still won't be and you should be very close to accurate with the new tires .

That's the thing - speedo accuracy is worse on the new tyres compared to the old ones. I suspect that the current programming is for 225/75R16s at ±714 revs per mile, though I have no way of really proving that right now. Having said that:

Quote:

The difference in real and indicated will never be the same number of MPH it will always be a percentage of indicated MPH.

Agreed, but in this case that's not what happening. At an indicated 50mph, I'm actually going 52mph per GPS. Proportionately speaking, that's correct error (4%) for the increase in tyre size. At 63mph, I'm going 67mph per GPS, which is nearer to 6.5% error. By the time I'm seeing an indicated 70mph, actual speed is 78mph, or approximately 11% error.

I didn't believe it myself when I first saw it, so ran the same tests against three different GPS units (two phones and a Garmin GPSMap 60csx). Results were the same across the board.

Quote:

The guage is operated according to rpm or maybe the speed sensor data .

I'd have to go re-read what I dug up on it, but I believe it's a combination of speed sensor, programmed tyre size, programmed tyre revs per mile, engine revs, and whatever calculations are done based on those inputs to operate both the speedo needle and odometer.

Quote:

Used to be a cable that actually connected needle on cluster to a gear . If it is nonlinear which I believe you I just dont see how it would be the best dealer could do is make it correct at a particular speed maybe mid range one like 40 or wherever you drive on highway a lot like 65 etc.

What I'm wondering is if there isn't some crappy programming (most likely revs per mile) left in there from a previous tyre size change. My thinking is that the stock-sized tyres may have been close enough for the calculations' purposes that the speedo error was small enough to not be notcieable, but having bumped up it's now throwing everything off more and more as speed increases.

Quote:

I am also not sure how they would know what to calibrate it to unless they look up revs per mile of that brand tire size and if they go by that it seems to me it would still be off as much as originals were.

That part's easy - I've got BFG's numbers on revs per mile, so will use those as a starting point. Agreed that there's no way of knowing what the changes may result in, however.

Quote:

Only thing I can think of is change it a certain amount up or down percentage wise and direction to change it. Without driving it and checking with GPS etc or possibly a dyne they dont know how indicated MPH compares to actual MPH either.

And I'm fine with that. Frankly, I was working on the assumption that it would need a couple of tweaks to make it accurate across the board; that's not something I have any issue with.

lfhoward · **Posted:** Fri Mar 27, 2020 7:02 pm

I agree the speedometer error is nonlinear.

I am running 31’s but have a scangauge. I’ve tried calibrating the scangauge using its % error function. If I calibrate the % at highway speeds (using either the gps or the “your speed” radar signs the DOT uses), it subsequently overestimates my in-town mpg. If I calibrate at in-town speeds, the highway mpg are underestimated. I just decided to put it somewhere in the middle, so it’s roughly ok in either situation.

Having speedo corrected on Monday - any other adjustments?

Who is online