Tune Help!

Bowtie619 · February 28, 2016

Hey guys my speedo is off about 6mph when I get above 40mph (GPS has me going +6mph faster than speedo). I got my hypertech tuner for multiple reasons but one was that I was hoping to correct speedo calibration. I didn't do enough reading beforehand and it looks like the only way to fix this is through changing the tire size. The problem is I have the stock tires! So I figured what the heck I put in 31" for the size after measuring diameter and sure enough it didn't fix anything. My questions is...

Can I increase tire size until my speedo is properly calibrated or would that throw something else off and create a potential problem?

Any advice would be appreciated

Bowtie619 · February 28, 2016

I just wanted to add that I know the GPS signal is good. I tested on 2 other vehicles and was within 1mph of speedo. Also tested different phone on my truck and showed same 6mph difference.

Bowtie619 · February 28, 2016

Did a bit more testing. I'm at 35.5 diameter and speedo looks to be matching GPS now. Actual diameter is 31.5. I'll contact hypertech on Monday to ask if doing this can cause potential issues. I'll report back in case someone else has this problem in the future.

Bowtie619 · March 1, 2016

Just an update, I had it set nearly perfect but Hypertech said that was a bad idea and to change it back to stock. I have a dealer appointment to get it calibrated properly.

TJay74 · March 1, 2016

What size tires are you running, if you are running anything other than a stock size tire there is nothing the dealer can do to correct this in the programming. GM uses the same overall height for all 4 of their wheel sizes, the tire size in the factory programming is the same for all trucks.

Bowtie619 · March 1, 2016

What size tires are you running, if you are running anything other than a stock size tire there is nothing the dealer can do to correct this in the programming. GM uses the same overall height for all 4 of their wheel sizes, the tire size in the factory programming is the same for all trucks.

The tires and rims are stock. They are the 20' chrome Chevrolet rims. I used the hypertech and set the setting to stock and it did nothing. I measured the diameter to be 31.5' and it maybe got 1-2 mph closer. Then I tinkered until I got it almost exact at 35.5". I have no idea what the previous owner did or if he had tuned the truck in anyway. Hypertech seems to think he did since the stock setting is off so much and then he never set it back to stock when he sold it to me. So I will have Chevrolet reflash to factory stock and if that doesn't work I will find out how much they will charge me to replace. Not sure I have any other options in this situation.

Jon A · March 1, 2016

It would be best if you could look at the tune and see what the problem is. To me it sounds as if your truck has different gears than it thinks it does. Have you tried leaving the tire size stock and changing the gear ratio with the Hypertech? If your truck has 3.08's but thinks it has 3.42's, that's about the same difference as there is between 31.5" tires and 35.5" tires.

Bowtie619 · March 2, 2016

It would be best if you could look at the tune and see what the problem is. To me it sounds as if your truck has different gears than it thinks it does. Have you tried leaving the tire size stock and changing the gear ratio with the Hypertech? If your truck has 3.08's but thinks it has 3.42's, that's about the same difference as there is between 31.5" tires and 35.5" tires.

I've been at chevy 2 hours now. They reflashed everything possible and problem still persists. They charged me 150$...What a joke. He suggested a custom speedo tune at a local shop. He said the only thing they can do is replace Guage cluster. He even said adjusting the tire diameter shouldn't mess anything up.

Bowtie619 · March 2, 2016

So I got back in contact with Hypertech. He said adjusting the tire size should not be an issue as long as the speedometer is reading correctly after I make the changes. So the original guy I talked to gave me incorrect information. I took his advice and took it into Chevy and all they did was tell me it was already stock and reflashed it as stock. Cost me 150$. Pretty pissed about that but I don't know enough about tuning.

I'm still not sure why my speedo is off by 10% since it is all electronics controlling the speedo but I will recalibrate back to 35.5 diameter so it's corrected.

Bowtie619 · March 2, 2016

It would be best if you could look at the tune and see what the problem is. To me it sounds as if your truck has different gears than it thinks it does. Have you tried leaving the tire size stock and changing the gear ratio with the Hypertech? If your truck has 3.08's but thinks it has 3.42's, that's about the same difference as there is between 31.5" tires and 35.5" tires.

I'm definitely going to play with this as well. Do you know what the stock gear setting should be?

Also, as you increase the gear ratio will my speedo increase in the same way that I increase the tire diameter it increases my speedo?

morrislee · March 4, 2016

I'm definitely going to play with this as well. Do you know what the stock gear setting should be?

Also, as you increase the gear ratio will my speedo increase in the same way that I increase the tire diameter it increases my speedo?

Did some quick math, looks like it is thinking it is on 3.08 axle ration probably from the percentage difference.

Check your RPO codes and find out what axle ration your truck is, advice your tuner the number and have the tune the transmission and the tire size according to that.

Every truck comes with different "stock gear ratio" or called axle ratio, this is depended on the package or what the truck was ordered it to be.

TJay74 · March 4, 2016

you can also jack up the rear of the truck, secure it on jackstands and put in neutral. Then rotate the rear tires. Count how many times the driveshaft rotates for 1 rotation of the rear wheels. The 3.08 gears will be close to 3 turns of the driveshaft for 1 turn of the rear tires. Once you know your factory rear end gear ratio you can use this test to confirm the gears where never changed out.

Have you visually looked at the tires to confirm the tire size? What brand are they? I have never heard of a factory speedo calibration being off by more than 2-3%.

Jon A · March 4, 2016

I'm definitely going to play with this as well. Do you know what the stock gear setting should be?

Also, as you increase the gear ratio will my speedo increase in the same way that I increase the tire diameter it increases my speedo?

From the sound of it, you probably have 3.08 gears, so try that along with the stock tire size and see if it works. As you decrease the numerical ratio of the gears, the speedo will read higher just the same as increasing tire diameter.

Bowtie619 · March 5, 2016

From the sound of it, you probably have 3.08 gears, so try that along with the stock tire size and see if it works. As you decrease the numerical ratio of the gears, the speedo will read higher just the same as increasing tire diameter.

I will definitely try that, right now I just have the tire diameter set higher and my speedo is on point but I will set it back to stock and set my gears to 3.08.