speedo not right

[Cowboy_Customs]

my speedo, starting at about 35+ is off. 73MPH is really 65 (GPS checked) 60 is 55, 55 is 51 and so on, but accurate at 35 and below.
I do have the wrong tire size 8.75 16.5 now, should have 7.50 16's. but according to this calculator http://www.1010tires.com/TireSizeCalculator.asp
it should not be this bad.

The rear end gears are stock, 4.10  Im assuming the tranny has either been rebuilt or just a shift kit. 4L80E Transmission.
I know of the older 350 and 400 its just a simple speedo gear change, been there done that, but what about this one?

[Captkaos]

None of these were that accurate, the 4L80E uses a VSS for the Speedo.  I will have to look up on the correction proceedure

[Cowboy_Customs]

Thank you Capt.! Itd be nice if it was somewhat close ya know?

[Lt.Del]

I read somewhere that 90 and 91 models used an electronic speedo, different than any other years, which used the cable/gear. 

My 91 GMC Sub speedo messed up about a month ago (reads buried) and I have to go get it fixed....the cruise and odometer work fine.  Go figure. 

[Captkaos]

Andy,
that is what I meant by VSS for the speedo,

[zieg85]

my speedo, starting at about 35+ is off. 73MPH is really 65 (GPS checked) 60 is 55, 55 is 51 and so on, but accurate at 35 and below.
I do have the wrong tire size 8.75 16.5 now, should have 7.50 16's. but according to this calculator http://www.1010tires.com/TireSizeCalculator.asp
it should not be this bad.

The rear end gears are stock, 4.10  Im assuming the tranny has either been rebuilt or just a shift kit. 4L80E Transmission.
I know of the older 350 and 400 its just a simple speedo gear change, been there done that, but what about this one?

One thing to keep in mind is if your accurate at 35 or below, how is the odometer at the higher speeds?  I have had some the acted like yours but the odometer was dead nuts mile after mile.  Mine turned out to be the speedometer itself.  Just my $.02

[Cowboy_Customs]

Hmm... good point. I will have to check it, fortunatly there is a "measured mile" deal on the highway like 2 miles from here. Thanx!

[Cowboy_Customs]

OK, finally did the measured mile, and the Odometer is off exactly 1/10th per mile traveled.
1 mile = 1 1/10th mile recorded by the odometer.

Trip reset doesnt work either, but I would assume thats a whole other issue with the reset spring.