Changing tire size /speedo issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Danny D · 11-06-2005, 10:21 AM

I have an 87 4wd that came with the optional 31 x10.50 x 15 tires from the factory. I want to put on a smaller tire with a smaller overall diameter. P235/75/15, the stock size tire size.

How do I get the speedometer/odometer to now read properly? Is it a speedometer gear change in the trans, or a program change in the computer?

Thanks for any help..

SSC's76 · 11-06-2005, 11:01 AM

Ive never found one for manual transmissions. TCI lists several automatic choices. You can use the tooth count and order the gears form GM. http://www.tciauto.com/tech_info/speedo_gears.htm

piecesparts · 11-06-2005, 06:53 PM

If you go to the GM parts counter, they can go back into the parts books for the 87 trucks and gove you the different gear combinations available for the different transmissions. I have copied their books and have a whole listing of the setups and what reducers and stepup convertors would be needed. You need to calculate what you have and what you will go to in tire size and then look at these graphs and decide where to start. I got my truck to within a .05 difference. Most manufactureers can't get any closer.

**N2TRUX** · 11-06-2005, 07:21 PM

Take a look at Bowtie Overdrives site. They have a page there that will tell you everything you need to know in full detail. They selll all the parts for the conversion too. Be sure to bookmark the site because it has tons of great info on it....

Danny D · 11-06-2005, 08:06 PM

Thanks guys. The web link is awesome. Found just what I needed.
Thanks again.
Dan

11-06-2005, 10:21 AM	#1
Danny D Registered User Join Date: Aug 2005 Location: NJ Posts: 18	Changing tire size /speedo issue I have an 87 4wd that came with the optional 31 x10.50 x 15 tires from the factory. I want to put on a smaller tire with a smaller overall diameter. P235/75/15, the stock size tire size. How do I get the speedometer/odometer to now read properly? Is it a speedometer gear change in the trans, or a program change in the computer? Thanks for any help..

11-06-2005, 11:01 AM	#2
SSC's76 No No Join Date: Jun 2005 Location: Pueblo Colorado Posts: 1,781	Re: Changing tire size /speedo issue Ive never found one for manual transmissions. TCI lists several automatic choices. You can use the tooth count and order the gears form GM. http://www.tciauto.com/tech_info/speedo_gears.htm __________________ 1976 Custom Deluxe SWB Med FMB.3/4 Drop. Gen VI 454, Muncie HD 3 speed With Modified OE Hurst Shifter, 3.73 12 bolt, GMPP HO roller cam, Hooker headers, Performer intake with modified Q-jet, Accel distributor. Best 1/4 12.3 @108. Funnest run 19.84 @88 full 1/4 mile burn out. 1997 K1500 ext cab 5.7 stock except muffler

11-06-2005, 06:53 PM	#3
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Changing tire size /speedo issue If you go to the GM parts counter, they can go back into the parts books for the 87 trucks and gove you the different gear combinations available for the different transmissions. I have copied their books and have a whole listing of the setups and what reducers and stepup convertors would be needed. You need to calculate what you have and what you will go to in tire size and then look at these graphs and decide where to start. I got my truck to within a .05 difference. Most manufactureers can't get any closer.

11-06-2005, 07:21 PM	#4
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,021	Re: Changing tire size /speedo issue Take a look at Bowtie Overdrives site. They have a page there that will tell you everything you need to know in full detail. They selll all the parts for the conversion too. Be sure to bookmark the site because it has tons of great info on it.... __________________ Follow me on Facebook and Instagram @N2trux.com Articles- "Jake" the 84 to 74 crewcab "Elwood" the77_Remix 85 GMC Sierra "Scarlett" "Refining Sierra"

11-06-2005, 08:06 PM	#5
Danny D Registered User Join Date: Aug 2005 Location: NJ Posts: 18	Re: Changing tire size /speedo issue Thanks guys. The web link is awesome. Found just what I needed. Thanks again. Dan



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