Speedometer Change has me puzzled

[02pewterz28]

I'm still very new to the hptuners thing and I'm having a small issue here. Recently I went from the stock size tire of 245/50/16 which is a 25.64" tire to a 295/35/18 which is a 26.1" tire. If I go into hptuners for my camaro it lists the stock tire as being 24.91?? So I'm rather confused on what this means should I simply add 1/2" to the calculated tire in hptuners or what? Also will it automatically adjust my shift point if I have the auto scale feature checked off? Finally if for whatever reason I left my speedo stock will it have any real affects on shift points? After all the speedo is only off 1.1 mph at 60.
Thanks

[8850]

Best to use a GPS to adjust your tire diameter if you want your speedometer correct. Changing the tire diameter does change your shift points. When you adjust automatically your shift points do correct in your file.

[02pewterz28]

Thanks man my tuner was telling me not to do it that way because it will also alter the mph in the tranny. He said he finds the percent error that the new tire is and adjusts it to the way the cluster reads? Any idea what he's talkin about? Also one other thing should I just play around with the diameter until I get the correct speed?

[8850]

I have an 08 Corvette and don't know what you have but yes, it will change shift points when changing tire diameter. I don't think it's going to be that much for you assuming your speedometer was almost correct with the smaller original tire. All you have to do then is get your speedometer the same. If you use your GPS and it's difference then the shifts will be different. You will probably need to adjust your shift points for max et anyway. But yes you tuner is correct. However, I would not adjust for the difference only in tire diameters unless your speedometer is already correct. Mine was wrong from the factory and adjusted with my GPS first.

[02pewterz28]

The car is an 02 z28 camaro with an ls6 crate swap its set to shift at 6600. Would you agree though that any wot shift change it would make is negligible because th difference in the diamater is so small? The stock calculated tire in hp tuners is 24.91, I know the difference to the new tire is 1.84% larger and would be 1.87% reading slower. So I went in and made the tire diameter 1.8& larger in the calculated area and came up with 25.37 as the new calculated tire. This upped my wot mph shifts by 1. Assuming my speedo is correct tomorrow when I test it out is that all that would need to be done as far as tuning the shift points and speed? If for whatever reason its still off would I simply just play around with it till it's accurate and the autscale feature should take care of the shifts?
Thanks again

[02pewterz28]

One other thing I forgot to mention is that today while I was driving I used the gps in my iphone to get me a speed. At 50 mph the gps said I was actually going 52. This leads me to belive that my speedo is 3.5% off I figured that it was a combination of being off from factory and having the bigger tire. So in addition to my above post I'll start with the 1.84% and jump to here if its still off but once again adjusting with this and using the autoscale should adjust my shift points at wot for me correct?

[8850]

If the GPS is reading high then you want to increase your tire size. And yes, with autoscale your shift points will change too.

[ms38w]

I've had this happen to me on more than one occasion. I bought a tune from a performance shop and he emails me the file to load in using my HPTuners. Every once in a while, a new tune will make the speedo not work...at all...sits below 0 MPH and doesn't move. With that, the ABS & Parking break dash lights come on and stay on. In the past, when I revert to the previous tune or get another tune, the speedo works again and the ABS/break lights go out and all is right with the world. So I have loaded in a tune file that didn't quite suit my needs at one time, but do now...and the speedo isn't working, ABS/parking break dash lights are on . I looked at the previous tune to see if I could see something different that might cause this, but I see nothing. I even put the previous tune back in and the speedo still isn't working. I must be looking in the wrong place to correct this or am overlooking something. What would cause the speedo to not register ? I am assuming it is all in the "speedo" field or in the "trans" field. I hope you can point me in the right direction.

Thanks

[ms38w]

fixed it. thanks anyway.

[Shrek]

fixed it. thanks anyway.

Always helpful when people post the details of how the fix came about - others may be experiencing the same issue.

Depending on the vehicle, a mismatch in the VIN between the various modules (PCM / BCM / SIR (airbag) / EBCM) can often cause these types of issues - dash lights flashing, some of the gauges / message centre not working, etc.