Tuner/tire size/speedometer questions

[PaHumBug]

I recently bought this H2 with a lift and brand new 37 inch tires on it. The speedometer was reading 8 mph faster than I was travelling (according to my Garmin), which makes me think the previous owner had larger tires than what is on there now. So I just bought a Superchips Cortex programmer so I could adjust the tire size. When I plugged it in, it saved my "stock" settings. However I don't think my stock settings were actually stock. I changed the tire size to 37, and the speedometer was still reading 8 mph too fast. So had to reduce the tire size all the way down to 30 inches to get the speedometer to read accurately. Does anyone see a problem with this? Will it hurt my truck? I just did it and haven't done anything but driven up the street to verify the speedometer.

There is a $100 program I can buy from Superchips that you are supposed to use if you failed to return your vehicle to stock and took it to the dealer. Maybe that is what I need but if I have the mileage acurate then maybe I can save my $100??? What do you guys think?
Thanks!

[guruerror]

That doesn't sound right...and you may be on to something with your logic.

Others may think differently...but I've had real good experiences with my local dealer and after-market mods...I would at least ask the dealer what they'd charge to reset your truck to default; then you can put the Cortex back on it.

Once you get it all worked out, push the Cortex 'Performance' program down to it...me like!

[twr]

Hi do you know if the other owner had changed rear gears? If he did and never had the speedo recal that could be why the speedo reads fast. Also alot of guys change gears when they change to bigs tires because the trucks don't have any balls. Check the gear ratio first if your truck is 2003 to 2007 it came with 4.10 hope this helps.

[HUMMERICH23]

So in 08 they switched rear ends to 3.73 then? Why didn?t they just keep the 4.10? So if you put 37's on an 08 or 09 is re-gear like a mandatory upgrade then?

[PaHumBug]

Originally Posted by twr

Hi do you know if the other owner had changed rear gears? If he did and never had the speedo recal that could be why the speedo reads fast. Also alot of guys change gears when they change to bigs tires because the trucks don't have any balls. Check the gear ratio first if your truck is 2003 to 2007 it came with 4.10 hope this helps.

I don't know if the gears were changed, but wouldn't re-gearing make the speedo read slower instead of faster? Or is my thinking backwards...

I can adjust gear ratio with the programmer too, maybe I should play with that???

[guruerror]

Originally Posted by HUMMERICH23

So in 08 they switched rear ends to 3.73 then? Why didn?t they just keep the 4.10? So if you put 37's on an 08 or 09 is re-gear like a mandatory upgrade then?

May not be mandatory but it would help keep you in your power band...

Check out this calculator: http://www.4lo.com/calc/geartire.htm

Looks like you may want to move to 4.10s.