Calibration of speedometer in a Vortec truck

[HotWheelsBurban]

Hi all, got what may be a dumb question. '97 Chevy C3500 crew cab, 5.7/4L80E/4.10 gears.
Currently the speedometer under reads a little bit, like it says 30 and we're actually going 25-26. Stock size tires(245/75-16) but the rears are older than the fronts (so may be slightly worn and therefore shorter).
I'm contemplating, when I replace the rear tires, going with a 265/75-16 which is an inch taller and according to the tire calculator, will decrease the RPM about 4% which in theory would make the speedo read even lower?
Is there a way to make it read correctly, that doesn't cost multiple body parts ? We go through several "semi speed trap" communities in our part of Houston and I would like to know if I could get an accurate speedometer.....

 

[99xcss4]

sadly I think the only way is with a tuner and seeing you are 97 that mean 5 plug black box so you would need to get a hold of black bear or other mail order tune service that can do the 96-97 black box if it was a 98 or newer you could go to the same place you went to get your security disabled on your suburban jet dst can do it but it is 400 plus you need a laptop there are hand helds you can get that might be able to but even they are a little expensive it is not like the old days with cable driven speedometers where you could change a gear in the transmission or t-case we have 265s on ours and as far as I know it is pretty accurate even though it came from the factory with 245s it is more or less the same set up crew cab long bed srw 4l80e 4.10:1s only difference is 4x4 and a 6.5 turbo diesel can not do it right but I will look at some of the hand helds some thing like this might work I will get on summit ebay jegs amazon and see what the going rate is

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[Cadillacmak]

Hi all, got what may be a dumb question. '97 Chevy C3500 crew cab, 5.7/4L80E/4.10 gears.
Currently the speedometer under reads a little bit, like it says 30 and we're actually going 25-26. Stock size tires(245/75-16) but the rears are older than the fronts (so may be slightly worn and therefore shorter).
I'm contemplating, when I replace the rear tires, going with a 265/75-16 which is an inch taller and according to the tire calculator, will decrease the RPM about 4% which in theory would make the speedo read even lower?
Is there a way to make it read correctly, that doesn't cost multiple body parts ? We go through several "semi speed trap" communities in our part of Houston and I would like to know if I could get an accurate speedometer.....

I would like to see the answer on this one also. I went down the rabbit trail on it and spent a lot! Ended up with a 411, HP tuners and a new laptop just because the Jet tuner would not talk to my black box.

 

[99xcss4]

well sadly it looks like the hand held that can not do nearly as much as the real deal cost about the same

 

[99xcss4]

think another option could be to to go to the junk yard or get on ebay and find another 96 or 97 pcm and send it to @df2x4 he has jet dst and could do it I use to have a 96-97 pcm but it was for a 305 and would need more tuning on it then just the tire size fix wish I still had it I would gladly send it to you free of charge if I still had it

 

[1998_K1500_Sub]

Currently the speedometer under reads a little bit, like it says 30 and we're actually going 25-26. Stock size tires(245/75-16) but the rears are older than the fronts (so may be slightly worn and therefore shorter).
I'm contemplating, when I replace the rear tires, going with a 265/75-16 which is an inch taller and according to the tire calculator, will decrease the RPM about 4% which in theory would make the speedo read even lower?

Using a relatively larger tire will increase your actual speed relative to your indicated speed.

Since your actual speed is already low, you can compensate for that by going with a slightly larger tire as you suggested.

I would compare the indicated speed to actual speed at a number of points, maybe 25, 45, 65 and 85 miles an hour, and then pick a tire size that would compensate best for the error at the speeds which are most important to you.

So, if nominally your indicated speed is 1.052x of actual, increase your tire diameter by 1.052x to compensate.

In the example you gave, your indicated speed is 30 but your actual is 26. You can compensate for that by increasing the tire diameter by 30/26 = 1.15x

 

[kennythewelder]

I have that Jet programmer. It will reset the speed-o. Problem is, they aren't cheap, if you can still find one. As for its capabilities of programing. It's OK, but not near as effective as having a custom tune done. It does improve the transmission shift sharpness a lot. It helps somewhat with overall performance. Thing is, for around $100 more, you can get the Jet programmer package, that you use on your lap top. This is similar to the way HP tunner works. You can adjust everything with it. Personally, I like the handheld like in the video that was posted. It also is a code reader, that you can reset your CEL with, besides tuning. It also does live data, so you can see what your sensors are doing. It is a good diagnostic tool, along with a being a somewhat limited programmer. The biggest draw back, is that you can only program 1 vehicle with it. It locks onto the VIN number, and you have to remove the tune from that vehicle, to use it, on another vehicle. IMO, it's a little expensive, but worth the money, B/C of what you can do with it. OH also you can adjust the tune on the vehicle is locked to, as many times as you want to. There is no limit to how many times you use it. The biggest caution when using it, is that you need to make sure that your battery is fully charged. If your battery dies, while it is installing your tune, you will have major problems, and your vehicle will not run. It stores the OE tune, in the hand held. If you loose power while it is tuning you will loose everything. I think a dyno shop, that tunes may be able to help, or probably blackbare can sale you a new ECM.

 

[HotWheelsBurban]

Good to know, thanks guys!
Unfortunately this is getting tabled for now.... stupid multi function switch is cuckoo on the crew cab. No blinkers, hazards don't work like they're supposed to either.

 

[99xcss4]

stupid multi function switch is cuckoo on the crew cab. No blinkers, hazards don't work like they're supposed to either.

Multi-Function Combination Switch with Turn Signal For Chevrolet Chevy GMC 95-02 | eBay

1999-2000 Chevrolet Cadillac Escalade. 1996-1999 Chevrolet Astro. 1995-1997 Chevrolet Blazer S10. 1995-1999 Chevrolet C1500 Truck. 1995-2000 Chevrolet C2500 Truck. 1995-2002 Chevrolet C3500 Truck. 1995-1999 Chevrolet K1500 Truck.

www.ebay.com

 

[HotWheelsBurban]

Multi-Function Combination Switch with Turn Signal For Chevrolet Chevy GMC 95-02 | eBay

1999-2000 Chevrolet Cadillac Escalade. 1996-1999 Chevrolet Astro. 1995-1997 Chevrolet Blazer S10. 1995-1999 Chevrolet C1500 Truck. 1995-2000 Chevrolet C2500 Truck. 1995-2002 Chevrolet C3500 Truck. 1995-1999 Chevrolet K1500 Truck.

www.ebay.com

Man I'd be scared to run one that cheap. The one in Burb is the O'Reilly's one year warranty, cause it was $85 with my "commercial" discount, and that was what we could afford then. It works fine, but I try to be gentle with it.....
In all fairness, the switch is the original one....just wasn't financially ready to have it go bad yet.