Speedo Gear...How and where do I get the correct one? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72skytop · 11-30-2004, 10:44 PM

I just changed Rear ends in my 1971 Cheyenne. It is a 402 big block with a 400 turbo hydramatic. It HAD a 3.08 posi-trac that I switched to a 4.10 posi-trac. What I need to know is how to find and buy the correct speedometer gear. I also have a drive line vibration at about 30-40 mph now that was there before the rear end swap but it came in at 40-50 mph. Before I switched rear ends, I have had the engine and trans rebuilt and reinstalled this spring(I have been busy) Main question I have here is where and how do I go about finding out the correct speedometer gear to get the speedometer correct again?

Thanks

Robert1970C20rstbukt · 11-30-2004, 10:55 PM

Go here and enter your tire size and rear end ratio and it will tell you which gears you need.

http://www.700r4.com/speedoCalc/speedocalc.shtml

68w/sbc406 · 11-30-2004, 11:04 PM

thats a cool little site, is it accurate???

Longhorn Man · 11-30-2004, 11:14 PM

And once you know what gears you need, go to your chevy dealer. The gears are dirt cheap.
If you know what you have in there already, you may be able to use one of them.

Robert1970C20rstbukt · 12-01-2004, 12:32 AM

As far as I know it is, I typed in 31" tires and 4.11 gears for mine and it came back with a 15 drive and 40 driven gears. I haven't tried changing the gears yet because I don't have a carb that acutally works right now so driving it would be a real PITA.

68w/sbc406 · 12-01-2004, 12:41 AM

i put in 275/60/15 with 3.73 and got 15/40 too

72skytop · 12-01-2004, 11:29 AM

I put in 275/60R15, 4.11 rear end ratio and got 15 Drive and 40 tooth driven. The only thing that concerns me about that sight is 700R4 written all over the place. A 400 is not an overdrive trans....I need to think about that. Does that make a difference in speedometer gear selection?...hmmm.

Thank you so much for the quick response and resource. I'll give it a try.

ROBERT....Have you actually applied this calculation to a non overdrive transmission on an actual vehicle. If not just say so. No big deal. I have to start somewhere. This is as good as any.

Robert1970C20rstbukt · 12-01-2004, 05:13 PM

No, I haven't used those calculations on a non overdrive tranny.

11-30-2004, 10:44 PM	#1
72skytop Registered User Join Date: May 2003 Location: Evansville, IN Posts: 147	Speedo Gear...How and where do I get the correct one? I just changed Rear ends in my 1971 Cheyenne. It is a 402 big block with a 400 turbo hydramatic. It HAD a 3.08 posi-trac that I switched to a 4.10 posi-trac. What I need to know is how to find and buy the correct speedometer gear. I also have a drive line vibration at about 30-40 mph now that was there before the rear end swap but it came in at 40-50 mph. Before I switched rear ends, I have had the engine and trans rebuilt and reinstalled this spring(I have been busy) Main question I have here is where and how do I go about finding out the correct speedometer gear to get the speedometer correct again? Thanks __________________ Kevin Korff Evansville, IN 47711 1997 Silverado 3500 LWB Vortec 454, headers 4.11 posi. Victory Red WOW! 1976 Oldsmobile 98 Regency four door 42,000 miles original paint. This is still a car! 1990 Oldsmobile 98 Touring Sedan 34,000 miles 1983 Hurst/Olds 30,000 actual miles 2013 Ford Taurus SHO 1972 Hurst/Olds-a work with no progress 1972 Oldsmobile 442 Convertible 455

11-30-2004, 10:55 PM	#2
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	Go here and enter your tire size and rear end ratio and it will tell you which gears you need. http://www.700r4.com/speedoCalc/speedocalc.shtml __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

11-30-2004, 11:04 PM	#3
68w/sbc406 Got Light Emitting Diode? Join Date: Apr 2004 Location: Newberg, OR Posts: 2,485	thats a cool little site, is it accurate??? __________________ 1968 1/2 ton 2wd lwb 6 lug disc and bags up front. Next c notch and rear bags '02 Chevy Silverado LT ext cab short bed 2WD 5/7 drop on 22's(the family car) 1993 Cherokee work ride/weekend wheeler

12-01-2004, 12:32 AM	#5
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	As far as I know it is, I typed in 31" tires and 4.11 gears for mine and it came back with a 15 drive and 40 driven gears. I haven't tried changing the gears yet because I don't have a carb that acutally works right now so driving it would be a real PITA. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

12-01-2004, 12:41 AM	#6
68w/sbc406 Got Light Emitting Diode? Join Date: Apr 2004 Location: Newberg, OR Posts: 2,485	i put in 275/60/15 with 3.73 and got 15/40 too __________________ 1968 1/2 ton 2wd lwb 6 lug disc and bags up front. Next c notch and rear bags '02 Chevy Silverado LT ext cab short bed 2WD 5/7 drop on 22's(the family car) 1993 Cherokee work ride/weekend wheeler

11-30-2004, 11:14 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	And once you know what gears you need, go to your chevy dealer. The gears are dirt cheap. If you know what you have in there already, you may be able to use one of them.

12-01-2004, 11:29 AM	#7
72skytop Registered User Join Date: May 2003 Location: Evansville, IN Posts: 147	I put in 275/60R15, 4.11 rear end ratio and got 15 Drive and 40 tooth driven. The only thing that concerns me about that sight is 700R4 written all over the place. A 400 is not an overdrive trans....I need to think about that. Does that make a difference in speedometer gear selection?...hmmm. Thank you so much for the quick response and resource. I'll give it a try. ROBERT....Have you actually applied this calculation to a non overdrive transmission on an actual vehicle. If not just say so. No big deal. I have to start somewhere. This is as good as any. __________________ Kevin Korff Evansville, IN 47711 1997 Silverado 3500 LWB Vortec 454, headers 4.11 posi. Victory Red WOW! 1976 Oldsmobile 98 Regency four door 42,000 miles original paint. This is still a car! 1990 Oldsmobile 98 Touring Sedan 34,000 miles 1983 Hurst/Olds 30,000 actual miles 2013 Ford Taurus SHO 1972 Hurst/Olds-a work with no progress 1972 Oldsmobile 442 Convertible 455

12-01-2004, 05:13 PM	#8
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	No, I haven't used those calculations on a non overdrive tranny. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

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