Need help with speedo gear after rear end gear change - The 1947 - Present Chevrolet & GMC Truck Message Board Network

hdff · 01-16-2015, 05:33 PM

Can somebody help me with changing the gear on the speedo cable at the Transmission after a rear end gear change. The truck as a 700r4 and I had a 3.07 rear end and this weekend I had it swapped to a 3.73posi and my speedometer is now about 10 miles an hour off.what is the best way to figure out what gear I need to change to on the transmission

Mike C · 01-16-2015, 06:22 PM

Here is a link to TCI's page. They walk you through it pretty much, but you have to figure it out based on what you have and how you want to change it.

http://www.tciauto.com/tc/speedometer-gear-calculator/

Keith Seymore · 01-16-2015, 08:26 PM

I usually just add an external mounted ratio adapter of the percentage required, like these: http://www.partshp.com/speedometer_gears.htm

It "skips to the end", bypassing the math and any detail measurement errors regarding tire rolling radius and such.

When I switched out the "Slim Jim" for a TH400 in my '63 Grand Prix the adapter I needed was already laying out on the workbench.

K

**Ironhorse** · 01-16-2015, 08:58 PM

X2 on what Keith S. said with the gear ratio adapter. He guided me to the same website when I had a similar problem going from a sm465 granny 4spd to a TH350 auto. Had a 3:07 rear axle ratio in my '71 4x4 and I ordered an "off the rack" ratio adapter speeding up my speedometer. Easy install, 15 minutes while I drank a cold Bud. Speedometer is dead on exact using our Garmin GPS. HIGHLY recommend looking into the same fix.

Thanks again for your help, Keith.

Custom/Ten · 01-16-2015, 09:24 PM

Just throwing this out there as a quick fix. I have a portable garmin gps, it's got a speedo built in that runs off satellites. Costs around 100 bucks and there's no monthly fee. Might keep you from getting a ticket till you can get this figured out.

01-16-2015, 05:33 PM	#1
hdff Registered User Join Date: Apr 2007 Location: pasadena,TEXAS Posts: 880	Need help with speedo gear after rear end gear change Can somebody help me with changing the gear on the speedo cable at the Transmission after a rear end gear change. The truck as a 700r4 and I had a 3.07 rear end and this weekend I had it swapped to a 3.73posi and my speedometer is now about 10 miles an hour off.what is the best way to figure out what gear I need to change to on the transmission __________________ 1971 Cheyenne SWB 402BB, 700R4, 3.73 posi, PS, PB, A/C, Tilt, Tach "Wanda" Don't argue with idiots, People watching may not be able to tell the difference... There's someone in my head........But it's not me...... cheap tricks thread great info here!! http://67-72chevytrucks.com/vboard/s...d.php?t=489394

01-16-2015, 06:22 PM	#2
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,731	Re: Need help with speedo gear after rear end gear change Here is a link to TCI's page. They walk you through it pretty much, but you have to figure it out based on what you have and how you want to change it. http://www.tciauto.com/tc/speedometer-gear-calculator/ __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

01-16-2015, 08:26 PM	#3
Keith Seymore Registered User Join Date: May 2009 Location: Motor City Posts: 9,245	Re: Need help with speedo gear after rear end gear change I usually just add an external mounted ratio adapter of the percentage required, like these: http://www.partshp.com/speedometer_gears.htm It "skips to the end", bypassing the math and any detail measurement errors regarding tire rolling radius and such. When I switched out the "Slim Jim" for a TH400 in my '63 Grand Prix the adapter I needed was already laying out on the workbench. K __________________ Chevrolet Flint Assembly 1979-1986 GM Full Size Truck Engineering 1986 - 2019 Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926 My Pontiac story: http://forums.maxperformanceinc.com/...d.php?t=560524 Chevelle intro: http://www.superchevy.com/features/s...hevy-chevelle/

01-16-2015, 08:58 PM	#4
Ironhorse Senior Member Join Date: Jul 2001 Location: Wellington, Ks Posts: 10,302	Re: Need help with speedo gear after rear end gear change X2 on what Keith S. said with the gear ratio adapter. He guided me to the same website when I had a similar problem going from a sm465 granny 4spd to a TH350 auto. Had a 3:07 rear axle ratio in my '71 4x4 and I ordered an "off the rack" ratio adapter speeding up my speedometer. Easy install, 15 minutes while I drank a cold Bud. Speedometer is dead on exact using our Garmin GPS. HIGHLY recommend looking into the same fix. Thanks again for your help, Keith. __________________ 1971 Chevy 1/2T 4x4 LWB 350 V-8 Was sm465-now 350TH/np205 Tilt, Tach, Towhooks Original Truck AM/FM Wellington, Ks

01-16-2015, 09:24 PM	#5
Custom/Ten Registered User Join Date: May 2014 Location: Mobile, AL Posts: 422	Re: Need help with speedo gear after rear end gear change Just throwing this out there as a quick fix. I have a portable garmin gps, it's got a speedo built in that runs off satellites. Costs around 100 bucks and there's no monthly fee. Might keep you from getting a ticket till you can get this figured out.



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