1997 gmc speedo recalibration

[68c10owner]

My 2wd truck had 31/10.5/15`s on it when I bought it. I`m about to replace the tires and was thinking of going back to the stock 235/75/15 to get better mileage but I know they are shorter and will throw the speedo off. I googled it and read something about resoldering stuff in the VSSB but there has to be a easier way. I dont have alot of tools available and dont have money for a scanner. I have a $50 unit to check codes but I dont believe it will recalibrate the speedo. What can I do? will driving it with the speedo off effect anything? I mean will the computer think its going a certain speed and do something it shouldnt? I dont know much about these newer trucks so I`m still learning.

[speedygonzales]

What makes you believe it was changed from stock ?

[400sbctruck]

Gearheadefi.com has some info about that stuff.

[68c10owner]

Originally Posted by speedygonzales

What makes you believe it was changed from stock ?

because the tire size was changed for a taller tire and the speedo reads correct now. if it wasnt changed it would read slower.

[68c10owner]

Originally Posted by 400sbctruck

Gearheadefi.com has some info about that stuff.

thanks I'll check it out. was hoping someone here would be able to help me.

[95 S_Trucker]

I would recommend plugging a gps in and seeing what your speedo says vs the gps. Your tire sizes are fairly close. And the difference wouldn't be huge.

[68c10owner]

Originally Posted by 95 S_Trucker

I would recommend plugging a gps in and seeing what your speedo says vs the gps. Your tire sizes are fairly close. And the difference wouldn't be huge.

its about a 2-2.5" difference but I just got them put on and didn't notice if its off or not yet. Only put a couple miles on it. I will try the GPS and see if its off. If not then I'm good otherwise I found on the net that the dealer should be able to recalibrate the speedo if needed. Hopefully the stock size tires will help get some better fuel mileage. still going to do a full tune up Thursday.

[95 S_Trucker]

Most factory speedometers read slow. I put 285 75 16 on my truck where 245 75 16 and the speedo is accurate right now.

[68c10owner]

Originally Posted by 95 S_Trucker

Most factory speedometers read slow. I put 285 75 16 on my truck where 245 75 16 and the speedo is accurate right now.

Every speedo I have GPS's has been off from the factory. When it says 55mph its actually 53mph. We'll see if its off. I heard the speedo being off could effect the transmission shifting, ABS, and the airbags if I'm in a accident. Just don't want any issues.

[speedygonzales]

Originally Posted by 68c10owner

Every speedo I have GPS's has been off from the factory. When it says 55mph its actually 53mph. We'll see if its off. I heard the speedo being off could effect the transmission shifting, ABS, and the airbags if I'm in a accident. Just don't want any issues.

Yes all speedo's are off by design. Slower than reading.

The only way to change the year in question(accurately) is by programming.

[400sbctruck]

On topic related thought-
Would a rheostat or pot on the pulse side act as an attenuator?
The real question is whether it is pulse code or pulse width i assume.
Obviously, this would only work for "slowing" speed if it would work at all.

[68c10owner]

So far the speedo appears to be correct. Guess the difference in diameter wasnt enough to mess with it too much. Rpm`s are up a bit but not too bad. Sure rides a hell of alot better and quieter inside too. Still has what feels like a slight tire out of round but cound be a drivetrain issue I`m working on. Need to rebuild the frontend and get a alignment and could use some new shocks too. Little bit at a time.