'99 Tahoe 2WD Rear Gearing ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Ironhorse** · 12-30-2022, 08:17 AM

My new to me 2 WD '99 Tahoe has the option codes of GU6 and GU80 on the glove box RPO sticker. From looking on the 'net it codes as the GU6 being a 3:42 gear set and the GU80 codes as a '' locking differential '' or Posi-trac rear end.

I have the original size tires for the Tahoe per the door code decal which are 235/75/15s. Running down the road the speedometer needle which is rock solid with no vibration or movement shows per GPS phone app I am running 10 mph SLOWER than the indicated speed. When the speedometer shows 45 by my GPS app I am doing closer to 35?

Anyone have some thoughts to share? Has my rear gear set been changed? If so, what is the most economical way to rectify the situation to align the indicated speed and the true ground speed. Having just purchased this Tahoe I want to get things correct but do not want to spend a TON of money doing so, LOL.

**Palf70Step** · 12-30-2022, 08:46 AM

My trucks were always off 5 MPH is or so even from factory. I wonder if rear gear may be different that what your SPID says. I know going to a taller tire will correct your situation, and probably help ride handling. a 265/70 will be an inch taller, but I think you might be better off with a 31 x10.50 tire to get your speedo closer. If you are staying with 7" wide factory rims, the 31x10.50 might be a bit big. Either would be fabulous on an 8" rim. I ran the 31x10.50 on my 96 GMC and loved it both for looks, ride and speedo. The 31s changed my speed around 10 MPH from the stock tires

b454rat · 12-30-2022, 08:56 AM

The cluster may have been swapped. When I had my 2wd tahoe I swapped to an Escalade cluster, and was off by quite a bit. I got the ECM tuned, told them tire size and ratio and was still off. Think the only way is to put it on a chassis dyno...dunno.

**Hart_Rod** · 12-30-2022, 10:03 AM

What transmission?

**Ironhorse** · 12-30-2022, 02:25 PM

I was out running errands this morning and stopped in to the local Chevrolet dealership. The parts counter guy is a friend of mine and after giving him the VIN # from the 99 Tahoe he was able to give me a run sheet of the RPO options this Tahoe was born with. Looks as if I do have the GU6 3:42 rear gearing and the G80 locker or limited slip differential. Will have to check into seeing if the the gears or speedo have been changed. The RPO run sheet tells me I have an M30 four speed w/OD 4L60E transmission originally. Not sure if that has been changed, don't think so but not at all familiar with the newer model vehicle transmissions.

1976gmc20 · 12-30-2022, 04:55 PM

Check to see if there is an adapter between the speedometer cable and the transmission?

One of my old Suburbans (different vintage) was off like that, and I crawled under and found this gear adapter and took it out and it was closer to correct with the cable screwed directly into the transmission as normal. I have no clue if that was factory or somebody added it for some strange reason?

OTOH, your Tahoe will get GREAT gas mileage like that! LOL

Just call me Sean · 12-30-2022, 07:10 PM

There is no cable, the speedometer is electric. My speedometer was off on my 93 and I corrected it, but it isn't the same as a 99. It was way off with everything seemingly stock, and it was still off setting it to the calculated setting. I think the 99 needs to be corrected in the computer, reburned.

sntrym95 · 01-02-2023, 10:34 AM

Jack the rear up. Mark the driveshaft and a tire, and spin the tire once. Count the rotations on the driveshaft to get an estimate on the gearing. It sounds like someone may have swapped in a 3.73 or 4.10. For ODBII, all you need is to have the gear ratio programmed into the computer. I believe the DRAC that controlled the gauge speedo was gone after 1995.

The 4L60E uses a speedo sensor in the tailshaft on the driver's side.

ChevyTech · 01-02-2023, 11:35 AM

Quote:

Originally Posted by Just call me Sean

... 99 needs to be corrected in the computer, reburned.

I agree with this reply.

**Ironhorse** · 01-02-2023, 04:44 PM

'' 99 needs to be corrected in the computer, reburned '' ????

Not familiar with this? Some way to hook the speedometer cluster to a machine and have it "re-learn" the correct speeds for the movement of the vehicle? Can you elaborate a bit on the procedure? Sounds like the answer to the problem rather than replacement of the cluster for sure.

ChevyTech · 01-02-2023, 08:58 PM

Quote:

Originally Posted by Ironhorse

'' 99 needs to be corrected in the computer, reburned '' ????

Not familiar with this? Some way to hook the speedometer cluster to a machine and have it "re-learn" the correct speeds for the movement of the vehicle? Can you elaborate a bit on the procedure? Sounds like the answer to the problem rather than replacement of the cluster for sure.

Starting in model year 1996 on a gas powered light trucks the separate DRAC is no longer used and recalibration is done by a reprogramming through the OBD2 port to change what is in the PCM. Most high end scan tools should be able to do it and I would think many tire shops would have something capable of doing the change.

**Ironhorse** · 01-03-2023, 08:44 AM

ChevyTech, thanks for the reply but not at all sure about the ''DRAC'' reference. There is a speedometer repair shop in Wichita, will stop and visit with them about the indicated speed reading higher than the actual speed per GPS.

ChevyTech · 01-03-2023, 10:00 AM

The DRAC (Digital ratio adapter controller) is also referred to as the “vehicle speed sensor buffer”.

The DRAC conditions the alternating currant signal from the vehicle speed sensor to a square wave signal so it can be used by the vehicle electronics.

The DRAC also compensates for axle and tire size variations so the speed signal is accurate.

1995 model was the last year for a separate DRAC, in 1996-up it's built into the PCM.

**Joe Pass** · 01-04-2023, 03:25 PM

On my 1997 K2500, 5.7 Silverado
I used this device after a gear ratio change from 373 to 411 and then a tire size change from 245/75/16 to 265/75/16....
https://www.summitracing.com/parts/h...2500/year/1997
Also it didn't alter any emission data to red flag my smog tests.....

12-30-2022, 08:17 AM	#1
Ironhorse Senior Member Join Date: Jul 2001 Location: Wellington, Ks Posts: 10,045	'99 Tahoe 2WD Rear Gearing ? My new to me 2 WD '99 Tahoe has the option codes of GU6 and GU80 on the glove box RPO sticker. From looking on the 'net it codes as the GU6 being a 3:42 gear set and the GU80 codes as a '' locking differential '' or Posi-trac rear end. I have the original size tires for the Tahoe per the door code decal which are 235/75/15s. Running down the road the speedometer needle which is rock solid with no vibration or movement shows per GPS phone app I am running 10 mph SLOWER than the indicated speed. When the speedometer shows 45 by my GPS app I am doing closer to 35? Anyone have some thoughts to share? Has my rear gear set been changed? If so, what is the most economical way to rectify the situation to align the indicated speed and the true ground speed. Having just purchased this Tahoe I want to get things correct but do not want to spend a TON of money doing so, LOL. __________________ 1971 Chevy 1/2T 4x4 LWB 350 V-8 Was sm465-now 350TH/np205 Tilt, Tach, Towhooks Original Truck AM/FM Wellington, Ks

12-30-2022, 08:46 AM	#2
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 46,665	Re: '99 Tahoe 2WD Rear Gearing ? My trucks were always off 5 MPH is or so even from factory. I wonder if rear gear may be different that what your SPID says. I know going to a taller tire will correct your situation, and probably help ride handling. a 265/70 will be an inch taller, but I think you might be better off with a 31 x10.50 tire to get your speedo closer. If you are staying with 7" wide factory rims, the 31x10.50 might be a bit big. Either would be fabulous on an 8" rim. I ran the 31x10.50 on my 96 GMC and loved it both for looks, ride and speedo. The 31s changed my speed around 10 MPH from the stock tires __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!

12-30-2022, 08:56 AM	#3
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,453	Re: '99 Tahoe 2WD Rear Gearing ? The cluster may have been swapped. When I had my 2wd tahoe I swapped to an Escalade cluster, and was off by quite a bit. I got the ECM tuned, told them tire size and ratio and was still off. Think the only way is to put it on a chassis dyno...dunno. __________________ 2000 GMC CCSB 454/4L80 6” lift SAS 1999 Chevy CCSB 454/NV4500 6” lift 1999 GMC Yukon 350/4L60 4" SAS

12-30-2022, 10:03 AM	#4
Hart_Rod ************* Join Date: Apr 2005 Location: Pensacola, FL Posts: 17,829	Re: '99 Tahoe 2WD Rear Gearing ? What transmission? __________________ Rob - https://www.instagram.com/hart_rod_c10 As Iron Sharpens Iron, So One Man Sharpens Another. Proverbs 27:17 FOR SALE: DBW pedal bracket - http://67-72chevytrucks.com/vboard/s...d.php?t=651123 FOR SALE: Hood Brackets http://67-72chevytrucks.com/vboard/s...d.php?t=679945 1987 Silverado SWB - 34.5K original miles http://67-72chevytrucks.com/vboard/s...d.php?t=801834 1969 SuperBurb - http://67-72chevytrucks.com/vboard/s...d.php?t=200387 1968 Farm truck - http://67-72chevytrucks.com/vboard/s...d.php?t=358692 1968 SWB - http://67-72chevytrucks.com/vboard/s...d.php?t=551258 1948 Chevy - http://67-72chevytrucks.com/vboard/s...=122164&page=3

12-30-2022, 02:25 PM	#5
Ironhorse Senior Member Join Date: Jul 2001 Location: Wellington, Ks Posts: 10,045	Re: '99 Tahoe 2WD Rear Gearing ? I was out running errands this morning and stopped in to the local Chevrolet dealership. The parts counter guy is a friend of mine and after giving him the VIN # from the 99 Tahoe he was able to give me a run sheet of the RPO options this Tahoe was born with. Looks as if I do have the GU6 3:42 rear gearing and the G80 locker or limited slip differential. Will have to check into seeing if the the gears or speedo have been changed. The RPO run sheet tells me I have an M30 four speed w/OD 4L60E transmission originally. Not sure if that has been changed, don't think so but not at all familiar with the newer model vehicle transmissions. __________________ 1971 Chevy 1/2T 4x4 LWB 350 V-8 Was sm465-now 350TH/np205 Tilt, Tach, Towhooks Original Truck AM/FM Wellington, Ks

12-30-2022, 04:55 PM	#6
1976gmc20 Registered User Join Date: May 2015 Location: Montana Posts: 3,696	Re: '99 Tahoe 2WD Rear Gearing ? Check to see if there is an adapter between the speedometer cable and the transmission? One of my old Suburbans (different vintage) was off like that, and I crawled under and found this gear adapter and took it out and it was closer to correct with the cable screwed directly into the transmission as normal. I have no clue if that was factory or somebody added it for some strange reason? OTOH, your Tahoe will get GREAT gas mileage like that! LOL __________________ Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner

01-02-2023, 10:34 AM	#8
sntrym95 Registered User Join Date: Jan 2006 Location: Big Pine Key, FL Posts: 745	Re: '99 Tahoe 2WD Rear Gearing ? Jack the rear up. Mark the driveshaft and a tire, and spin the tire once. Count the rotations on the driveshaft to get an estimate on the gearing. It sounds like someone may have swapped in a 3.73 or 4.10. For ODBII, all you need is to have the gear ratio programmed into the computer. I believe the DRAC that controlled the gauge speedo was gone after 1995. The 4L60E uses a speedo sensor in the tailshaft on the driver's side. __________________ Mike

01-02-2023, 04:44 PM	#10
Ironhorse Senior Member Join Date: Jul 2001 Location: Wellington, Ks Posts: 10,045	Re: '99 Tahoe 2WD Rear Gearing ? '' 99 needs to be corrected in the computer, reburned '' ???? Not familiar with this? Some way to hook the speedometer cluster to a machine and have it "re-learn" the correct speeds for the movement of the vehicle? Can you elaborate a bit on the procedure? Sounds like the answer to the problem rather than replacement of the cluster for sure. __________________ 1971 Chevy 1/2T 4x4 LWB 350 V-8 Was sm465-now 350TH/np205 Tilt, Tach, Towhooks Original Truck AM/FM Wellington, Ks

01-03-2023, 08:44 AM	#12
Ironhorse Senior Member Join Date: Jul 2001 Location: Wellington, Ks Posts: 10,045	Re: '99 Tahoe 2WD Rear Gearing ? ChevyTech, thanks for the reply but not at all sure about the ''DRAC'' reference. There is a speedometer repair shop in Wichita, will stop and visit with them about the indicated speed reading higher than the actual speed per GPS. __________________ 1971 Chevy 1/2T 4x4 LWB 350 V-8 Was sm465-now 350TH/np205 Tilt, Tach, Towhooks Original Truck AM/FM Wellington, Ks

01-03-2023, 10:00 AM	#13
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,620	Re: '99 Tahoe 2WD Rear Gearing ? The DRAC (Digital ratio adapter controller) is also referred to as the “vehicle speed sensor buffer”. The DRAC conditions the alternating currant signal from the vehicle speed sensor to a square wave signal so it can be used by the vehicle electronics. The DRAC also compensates for axle and tire size variations so the speed signal is accurate. 1995 model was the last year for a separate DRAC, in 1996-up it's built into the PCM. __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too.

01-04-2023, 03:25 PM	#14
Joe Pass Senior Member Join Date: Aug 2013 Location: Windsor, California Posts: 1,042	Re: '99 Tahoe 2WD Rear Gearing ? On my 1997 K2500, 5.7 Silverado I used this device after a gear ratio change from 373 to 411 and then a tire size change from 245/75/16 to 265/75/16.... https://www.summitracing.com/parts/h...2500/year/1997 Also it didn't alter any emission data to red flag my smog tests..... __________________ 1962 build album http://67-72chevytrucks.com/vboard/a...p?albumid=2066 http://67-72chevytrucks.com/vboard/a...p?albumid=2066 http://67-72chevytrucks.com/vboard/a...p?albumid=2067 1956 http://67-72chevytrucks.com/vboard/a...p?albumid=2086



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