Speedometer Corrections for 245/45/R17 Tires?

[GeforceXtreme]

Hey guys I own a 2000 Buick Regal GSE and stock my car has 225/60/R16 tires.

Well I got new 17" rims and General Exclaim UHP 245/45/R17 Tires.

I am not exactly sure what numbers I put into the Speedometer correction page in HP tuners.

This is currently what is in my car now:


My cars transmission has a 3.29 gear ratio and I must keep the sprocket ratio at 1.121 and calculated gear as 3.69 in HP tuners or I get an incorrect gear ratio error code.

As of right now everything works fine however I am not exactly sure what number I put in as the "New" tire size. According to the companies website my tire has an overall diameter of 25.7". Should I put 25.7" into the new tire size and also 3.69 as the gear ratio?

Also should I let it "Auto Scale Transmission tables" or keep that off?

[dlagrua]

I believe that you must put in the new tire size (diameter value) and leave everything else the same. Then press the Commit button. Now monitor your speed with a GPS and see if they match. If the speedometer reads higher than the actual speed then increase the tire size on the chart by the same % difference. If it reads lower then decrease the tire size by the same percentage difference.

[Ivan]

On the VCM upper menu there is a Unit Conversion Option under Tools. This also have the utility where you can write the numbers of your tire and it calculates the tire height.

Ivan

[GeforceXtreme]

I put 25.7 in as my new tire size and I think the speedometer is off just a little bit. I guess I will have to redo it with the number being a little higher/lower then 25.7.

However when I did this I let it commit the transmission shift changes and man how much lower it dropped the mph shifts. Because of this my car shifts into 2nd gear even before I can go WOT. I am going to have to make changes to the table so over 50% TPS it isn't looking to shift at 37mph... too low!

[Ivan]

When you get a higher rim, the car acceleration goes low (wheels turns slower), its like changing the gear to a numerically low one. Anyway, HPT automatically changes the shifting points very nice acording to your settings, but you can use the shift points settings your car have before the tire change (remember after a change save the file with a new name) copying from your backup file.

Ivan