Adjusting speedo after tire change

Rob99 · 10-11-2023, 11:39 PM

Is there a way to adjust speedo to correct after tire change without using a full blown tuner that I would not use those other features it comes with.
What’s weird after checking the online calculators I should only be offf 1 mph at most, but my GPS radar detector shows when going 75 dash shows 71.
I have staggered sizes 245/45/20F 275/40/20R
Does the computer read the rear or front?
Thanks.

2SS Capt · 10-12-2023, 06:29 AM

Quote:

Originally Posted by Rob99

Is there a way to adjust speedo to correct after tire change without using a full blown tuner that I would not use those other features it comes with.
What’s weird after checking the online calculators I should only be offf 1 mph at most, but my GPS radar detector shows when going 75 dash shows 71.
I have staggered sizes 245/45/20F 275/40/20R
Does the computer read the rear or front?
Thanks.

It reads from the transmission so rear tires, proper tire size should be 245/40R20 & 275/35R20, looks like you went up a full size on aspect ratio on both, likely accounting for the speedo error...

arpad_m · 10-12-2023, 06:50 AM

Quote:

Originally Posted by Rob99

Is there a way to adjust speedo to correct after tire change without using a full blown tuner that I would not use those other features it comes with.

There are midlevel scanners that can do it, for example my Xtool D8 claims to and has the UI for it, too (never tried myself), but those aren't super cheap either.

Quote:

Originally Posted by Rob99

What’s weird after checking the online calculators I should only be offf 1 mph at most, but my GPS radar detector shows when going 75 dash shows 71.
I have staggered sizes 245/45/20F 275/40/20R
Does the computer read the rear or front?

The speedo is based on driven wheels/tires. Not sure which online calculators you checked, but tiresize.com says your sidewall enhancement project increased tire height by +4%, which means the speedo should show 72 mph at an actual speed of 75 mph. If you saw 71, it's within the margin of error given how many factors can be involved (actual tire size vs specs, GPS accuracy, GPS delay, speedo refresh delay).

Did you switch to a different tire brand and/or model, too?

WhiteMale · 10-12-2023, 10:10 AM

That's a rather small change in aspect ratio It shouldn't be off my 4mph.

ctrlz · 10-12-2023, 11:02 AM

Any possibility new tires are overinflated?
Tire pressure will have an impact.

BatMobileLT1 · 10-12-2023, 01:25 PM

From: https://tiresize.com/calculator/
going from 245/40/20 to 245/45/20 goes up an inch in overall diameter from 27.7 to 28.7 which is 3.6% (and 275/40/20 also is 28.7) so doesn't seem surprising to me that you might see ~4% speedometer delta.
As you went up a full inch in diameter - If you wanted more sidewall did you consider going to 19" wheels and you could have gone with 245/45/19 and 275/40/19 at the same 27.7 diameter to avoid speedo issues?

Rob99 · 10-12-2023, 11:51 PM

Hey all thanks for the reply’s and links, I tested the gps in another vehicle and a 1mph gps reading error does exist, that would leave me with a 3 mph error at my test speeds of 75 on gps reading 71 on dash bumping up to 76 gps reading showed 72 on dash then minus the 1 mph gps error leaves me with a 3 mph error in both speeds.
So being the reading comes from the rear tires yes that calculator gets me to the 3 mph and looking at that calculator reading of 50 actually being 52.8 and it’s rounding up to 53 on the dash now makes sense to me.
Initially while driving and saw that large disparity I was concerned and TBH I never thought it be off by as much as the 3
I did change brands from the stock Eagle F1’s,
As for the tire pressures it’s 35 all around,
i didn’t see 19’s in the latest stock look 6th gen style so I stayed with the 20’s based on wheel style I was looking for.
So there appears to be no way to correct unless going to a full blown tuner, I guess 3 will be a number I’ll get used to while driving.

Here’s the new look, from the black wheels to these gunmetal gray.

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Rob99 · 10-13-2023, 04:23 PM

I checked the speedo with speed signs in my neighborhood with cruise set to 25 the sign showed 27, 30 showed 32 same with gps
Comparing to the calculator the gps, speed sign and dash are rounding up.

chevyssto2ss · 10-27-2023, 02:10 PM

I'm thinking of doing the same setup. Driving crappy roads through Philadelphia to get to work, I can definitely use more sidewall. The stock Eagle F1 ROFs are punishing. I can deal with the speedo being off but any issue with rubbing in the front?

Rob99 · 10-27-2023, 08:55 PM

Quote:

Originally Posted by chevyssto2ss

I'm thinking of doing the same setup. Driving crappy roads through Philadelphia to get to work, I can definitely use more sidewall. The stock Eagle F1 ROFs are punishing. I can deal with the speedo being off but any issue with rubbing in the front?

The sidewall on the tires i put on are approx 1'' more than the Eagle F1's IIRC,
the ride is improved and they are much quieter than the RF's, plus i got a wider look in the rear which looks great, no rubbing at all. I changed the rims to accommodate the wider tires in the rear.
I carry a Worx portable air pump and a tire plug kit, additionally i have AAA.
Good luck with your swap.

10-11-2023, 11:39 PM	#1
Rob99 I’m Magik a physco dog Drives: '15 1LT/RS & '23 3LT/RS Join Date: May 2016 Location: N.J. Posts: 251	Adjusting speedo after tire change Is there a way to adjust speedo to correct after tire change without using a full blown tuner that I would not use those other features it comes with. What’s weird after checking the online calculators I should only be offf 1 mph at most, but my GPS radar detector shows when going 75 dash shows 71. I have staggered sizes 245/45/20F 275/40/20R Does the computer read the rear or front? Thanks. __________________ 2023 3LT RS Shadow Gray 2015 1LT R/S Blue Velvet Coupe. Previous: 2015 1LT Ashen Grey 2016 1LT Blue Velvet My entire life has been 'A Midlife Crisis'

10-12-2023, 01:25 PM	#6
BatMobileLT1 Drives: 2023 Camaro LT1 M6 Join Date: Dec 2022 Location: Massachusetts Posts: 141	From: https://tiresize.com/calculator/ going from 245/40/20 to 245/45/20 goes up an inch in overall diameter from 27.7 to 28.7 which is 3.6% (and 275/40/20 also is 28.7) so doesn't seem surprising to me that you might see ~4% speedometer delta. As you went up a full inch in diameter - If you wanted more sidewall did you consider going to 19" wheels and you could have gone with 245/45/19 and 275/40/19 at the same 27.7 diameter to avoid speedo issues? __________________ 2023 Black LT1 – M6, Dual Mode Exhaust, Sunroof, Tech Pkg w/Bose Audio, Red Front Brake Calipers, Carbon Flash Wheels, Ground Effects & RS Pkg Added after delivery: Continental ExtCont DWS06+ Tires: 245/40/20 Front & 275/35/20 w/9.5” Rear Wheels, Red Rear MGP Caliper Covers

10-12-2023, 11:51 PM	#7
Rob99 I’m Magik a physco dog Drives: '15 1LT/RS & '23 3LT/RS Join Date: May 2016 Location: N.J. Posts: 251	Hey all thanks for the reply’s and links, I tested the gps in another vehicle and a 1mph gps reading error does exist, that would leave me with a 3 mph error at my test speeds of 75 on gps reading 71 on dash bumping up to 76 gps reading showed 72 on dash then minus the 1 mph gps error leaves me with a 3 mph error in both speeds. So being the reading comes from the rear tires yes that calculator gets me to the 3 mph and looking at that calculator reading of 50 actually being 52.8 and it’s rounding up to 53 on the dash now makes sense to me. Initially while driving and saw that large disparity I was concerned and TBH I never thought it be off by as much as the 3 I did change brands from the stock Eagle F1’s, As for the tire pressures it’s 35 all around, i didn’t see 19’s in the latest stock look 6th gen style so I stayed with the 20’s based on wheel style I was looking for. So there appears to be no way to correct unless going to a full blown tuner, I guess 3 will be a number I’ll get used to while driving. Here’s the new look, from the black wheels to these gunmetal gray. Attached Images __________________ 2023 3LT RS Shadow Gray 2015 1LT R/S Blue Velvet Coupe. Previous: 2015 1LT Ashen Grey 2016 1LT Blue Velvet My entire life has been 'A Midlife Crisis' Last edited by Rob99; 10-13-2023 at 04:24 PM.

10-13-2023, 04:23 PM	#8
Rob99 I’m Magik a physco dog Drives: '15 1LT/RS & '23 3LT/RS Join Date: May 2016 Location: N.J. Posts: 251	I checked the speedo with speed signs in my neighborhood with cruise set to 25 the sign showed 27, 30 showed 32 same with gps Comparing to the calculator the gps, speed sign and dash are rounding up. __________________ 2023 3LT RS Shadow Gray 2015 1LT R/S Blue Velvet Coupe. Previous: 2015 1LT Ashen Grey 2016 1LT Blue Velvet My entire life has been 'A Midlife Crisis'

10-12-2023, 10:10 AM	#4
WhiteMale Drives: 23 ZL1 A10 Rapid Blue Join Date: Aug 2020 Location: MO Posts: 2,014	That's a rather small change in aspect ratio It shouldn't be off my 4mph.

10-12-2023, 11:02 AM	#5
ctrlz Drives: 2017 2SS, 50th pkg, M6, MRC, NPP Join Date: Jan 2016 Location: Ocean City, NJ Posts: 3,216	Any possibility new tires are overinflated? Tire pressure will have an impact.

10-27-2023, 02:10 PM	#9
chevyssto2ss Drives: 2021 Camaro 2SS Join Date: Oct 2023 Location: NJ Posts: 1	I'm thinking of doing the same setup. Driving crappy roads through Philadelphia to get to work, I can definitely use more sidewall. The stock Eagle F1 ROFs are punishing. I can deal with the speedo being off but any issue with rubbing in the front?

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