Is there something magical in correcting an '08 Corvette speedometer calibration???

[JRHAWK9]

I sent my '08 Vette ECM (6 speed manual) to a very well known and respected tuner who has tried twice now to get my C6 speedometer to read 2.5% slower than what it's currently reading (stock calibration) to compensate for my 305/30/19 rear tires...doesn't seem too tough, right?

Well, I sent my ECM to this well known tuner to have their mail order tune done and have the speedometer calibrated. I gave him my tire size along with the other mods. Well, I get the ECM back, install it and take it for a ride. Low and behold I'm still off the IDENTICAL 2.5% I was before. I call them and he looks up my file and it shows that he did change it to my new tire size, but for whatever reason it didn't work. He told me to send it back to him and he will input it manually and that I have to just get him the speed I am actually going as well as the speed the speedometer thinks I am going. I do this and send the ECM back. He does this and sends it back to me once again. I just got back from taking it for a ride after the second try and it's -STILL- off the same exact amount it was before. Whatever is being changed is not doing a thing. What is the magic combination of check boxes and numbers in order to change a late model C6 speedometer calibration?

So, with my current wheels/tires installed, running the stock calibration = 2.5% fast, first attempt at changing the calibration = 2.5% fast, second attempt = 2.5% fast. If I were to put my stock wheels back on the speedometer would once again be accurate. Whatever is getting changed isn't affecting my speedometer reading.

All speeds were monitored using the same GPS in all three comparisons on the same stretch of FLAT road.

Any help would be greatly appreciated!
Paul

[WeathermanShawn]

I have always used the 'Speedo Calculator' provided in the EFILive Tune Tool. I believe your year has one included. That calculates a new VSS that should match your actual speed.

Its tough to get it completely accurate when you change tire size. Plus I believe your tire size changes at various speeds and loads. I also compare with GPS. I try to get it within 1-2 mph, with the speedo reading a little high (speed limit laws, etc..).

If you don't have access to the 'Calculator', I am sure someone could run the numbers for you. There are quite a number of variables, so you might review them if you need to re-post.

Good luck..

[JRHAWK9]

that's essentially what I am doing when driving 100mph (as indicated by the speedometer) and comparing it to the GPS speed (which, in my case, reads right around 97.5 mph, +/- 0.1mph).

Which, according to -HERE- (http://www.miata.net/garage/tirecalc.html) and -HERE- (http://www.1010tires.com/tiresizecalculator.asp) is exactly what they should be off.


Regardless of the math involved, my calibration hasn't changed a bit from the stock calibration in two attempts of trying to change it. This leads me to believe something isn't being changed in EFI Live that should be getting changed. Now if his changes were actually showing up in my speedometer readings, then we know the changes are actually taking and then we could discuss the math involved in dialing in accurate speedometer readings. At this point I would just like to see ANY kind of change to what what speedometer reads...lol If it goes from reading 2.5% too fast to reading 1% too slow at least we know whatever's being changed in EFI Live is actually working.

[WeathermanShawn]

I take it you are going through a Vendor, or do you have EFILive?

If you do not, then I would download a tune from their site (for your year) and attempt to duplicate the necessary changes. You won't be able to flash a new tune, but at least you will have more information than you do now.

Good luck..

[JRHAWK9]

I am going through a vendor, correct. I guess I would rather have the vendor I am using know how to do this than me, as it's useless for me seeing I can't flash it to see what works and what doesn't.

Where can I get this tune you are referring to? I could try installing EFI Live and opening up a tune and check out what's all involved.

[joecar]

What was your OEM tire size...?

Your tire size is now 305/30R19...?

[JRHAWK9]

OEM was 285/35/19, I am now running a 305/30/19.

[WeathermanShawn]

I take it that is not a standard 'Plus Size'? I.E., its a different Tire Diameter than OEM?

If that is the case, it can be tough fine-tuning a speedo. I would imagine the actual tire diameter being utilized in the VSS calculation is off. You can manipulate the speedo in EFILive and get close, but you almost need the program to do it right.

I spent a month on mine, and I am happy getting it within 1-2 mph. It is all in the tire diameter. At 40 mph you can nail the speedo reading, and then it is off at 20 mph and/or 100 mph.

I think it would get frustrating to keep sending an ECM out for such a small tweak. Sounds like it is time to get EFILive..:)

[L31Sleeper]

No offense Paul, but is this post really about 2.5% ?? I mean I would consider that a good "safety margin" not
to get a ticket but at 100 I don't think you are worried about that.
I'm sure you have a nice car hell it's an 08 Vette !! At 100 you still have 90 mph left !!
Focus on DRIVING, don't kill anyone or yourself and have fun with the car !!

[JRHAWK9]

what I am trying to get at is that any changes being made, regardless of the amount, should actually be showing up. In my case it wouldn't matter if it was off 25% or 2.5%, as the changes being made are not showing up. It's a CONSISTENT 2.5% it's off no matter what is "changed" in EFI Live. This tells me that the changes being made are not affecting the calibration in any way, shape or form. It just seems like something is not being changed in EFI that should be changed in order to apply the changes.

I have an '00 Firehawk with different gears -AND- different sized tires and I never had any issues with getting that speedometer correctly calibrated.

[WeathermanShawn]

Paul:

I think what we are trying to tell you is that it is not EFILive per se at fault.

It is probably a 99% chance that the information obtained via published or through an online Tire Calculator's is incorrect. You have to actually measure the tire's diameter or measure the revolutions per mile to ever get it deadly accurate. The tune is probably coming back the same because each time the tire size provided is the same.

At this point there is really not much you can do. Working through the Vendor is the best bet. As you said you can not flash in a new tune, so he only way you can resolve it is through the Vendor you chose.

Ultimately having the capability yourself is the best option.

That is all I can think of at this point. Good luck..

[joecar]

OEM was 285/35/19, I am now running a 305/30/19.
I calculate:
285/35R19 = 26.85" diameter
305/30R19 = 26.20" diameter

However, do measure your tire diameter (measure it's rolling circumference and divide by pi), since some tires don't quite match up to their spec'd values.

I looked in a stock 2008 C6 file and the tunetool speedo calculator says tire diameter is 26.12"... i.e. the Vette came stock with a -2.8% error built in somewhere when compared to the calculated tire diameter (hmmm... what was the actual measured tire diameter...?)

This is what I think is happening: your tuner entered 305/30R19 and the tunetool speedo calculator is calculating 26.20" and saving the value... 26.20" is only 0.3% error from 26.12" so your speedo will read the same...

What you need to do is increase one of the speedo parameters (I'm not sure which) by the error percent you see.

[JRHAWK9]

I calculate:
285/35R19 = 26.85" diameter
305/30R19 = 26.20" diameter

However, do measure your tire diameter (measure it's rolling circumference and divide by pi), since some tires don't quite match up to their spec'd values.

I looked in a stock 2008 C6 file and the tunetool speedo calculator says tire diameter is 26.12"... i.e. the Vette came stock with a -2.8% error built in somewhere when compared to the calculated tire diameter (hmmm... what was the actual measured tire diameter...?)

This is what I think is happening: your tuner entered 305/30R19 and the tunetool speedo calculator is calculating 26.20" and saving the value... 26.20" is only 0.3% error from 26.12" so your speedo will read the same...

What you need to do is increase one of the speedo parameters (I'm not sure which) by the error percent you see.

thank you! This could be the issue, I will keep this thread updated.

[gmperformancecentre]

open the tune and open up the speedometer section and click on speedo calculator and then click on speedo fine tune and add the speedo speed and the actual speed in and then click apply.
THIS SHOULD HELP YOU OUT HOPEFULLY

[L31Sleeper]

Paul EFI Live has a speedo calculator (course adjustment) and a MPH adjustment (fine adjustment)
If you tuner uses the calculator each time you send your PCM in the number is going to be calculated
the same each time, based on your tire size. The fine adjustment allows you to type in Displayed speed
and Actual speed. I used this to get mine right on the money.

[jstncse]

The fine adjustment calculator did not work on my 09 LMM. It would not update the revolutions per mile, I manually changed that value by the percentage that I was off.
My 285/70-17 tires are actually 32.5 inches tall, a tire calculator comes up with 32.8 inches, and the value that corrected the speedometer was 32.00 inches or 630.3 rev per mile.
Just goes to show that actual tire size does not correlate to a correct speedometer.

I would take your current rev per mile and increase that number by 2.5%.

Hope this helps