take a guess at this rear end ratio... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gabet40 · 03-04-2005, 03:33 AM

just got the blazer back from the tuning shop. they tuned my commander 950 injection system and put it on the dyno for me.
in the process they usually get ur top speed as well but get this; he took it up to 3500 RPMs in third gear (w/a turbo 350) and she was doing 125 mph!
ive got 31" tires on there right now and unless theres a higher geared rear end than a 3.08 that exists, thats gotta be whats in there.

ac102nv · 03-04-2005, 03:50 AM

Here's the formula I found for gear ratios:
GEAR RATIO(?) = TIRE RADIUS(15.5) x RPM(3500) ÷ 168 ÷ MPH(125) (assuming 1:1 thru trans)

According to that you have a 2.58 rear gear...sounds wierd to me. Based on same numbers, you should be about 104pmh at 3500rpm with 31" tire and 3.08 gears. But then again it's late and my calculator could be off....

Longhorn Man · 03-04-2005, 08:24 AM

Sounds about right...this is a 350/TH350 truck with 28 inch tall tires (275/60 R15's) and a 3.07.

gabet40 · 03-04-2005, 12:20 PM

so those 3 inches on the wheels will make a 35mph difference?

71cadc10 · 03-04-2005, 01:20 PM

From the calculator I use you should be going 104.8mph at 3500 rpm with a 31" tire. Maybe there's something wrong with their dyno or the way they set it up???? Either that or somehow your rear gear is around 2.50-2.60 range like ac102nv said.

LUV2XCLR8 · 03-04-2005, 04:35 PM

CAN'T HELP YA, BUT IT'S NICE TO SEE SOMEONE ELSE USING MAGNAFLOW
EXHAUST, I'M SO SICK OF EVERYONE BRAGGING ABOUT FLOWMASTERS.

71Chev · 03-04-2005, 05:41 PM

Was it their speedo or the one in the truck? With 31" tires, and nothing done to correct the speedo for the tire height increase. The faster the wheels turn, the farther off the speedo will read. For example at 60mph your speedo is probably reading right at about 10 percent slower than the truck. Find mile markers that are as close to a mile a part as you can find(it's harder than ya think, there aren't many 1.0 miles a part).
Hold it at 60mph, marker to marker, then clock it with the trucks odometer. IF the odometer only clocks 0.9 miles in a set 1 mile distance, the speedo is reading 10% slow. 0.8 = 20% slow. If the odometer clocks over a mile, same thing. 1.1 speedo is 10% fast.

69 longhorn · 03-04-2005, 05:52 PM

I get right close to 105@3500 also. Try this site for speedo calibration.... http://www.novaresource.org/speedo.htm 69L

67ChevyRedneck · 03-04-2005, 06:14 PM

[QUOTE=71Chev]Was it their speedo or the one in the truck? QUOTE]

I would have to think that it was their speedo cause our trucks speedo's only go to 1oo, right? On a dyno, the truck is not a big brick in the wind and has no wind resisitance on it, it's not even "pushing" it's own weight. It's just sitting on two big barrels spinning the rear wheels. I'm no expert, but could this possibly be the reason the rpm's were lower? If you take it out on the highway that same 125mph might be at 4K rpm becuase the motor is now "pushing" the weight of the truck and it's fighting its own aeorydynamics

Just a thought.....

cableguy0 · 03-04-2005, 06:21 PM

the dyno does provide some resistance thats how it gets it reading cant duplicate wind resistance without actually moving but i doubt it would be 20 mph worth of resistance.with a given gear set up tire size and rpm theres only one speed you can go. it doesnt vary by the weight of the vehicle or anything else. the power it takes to turn that many rpm and in turn go a certain speed is the factor here if your engine wont push the vehicle through the wind at 125 you will never reach the rpm needed to go 125 mph. i hope what i said makes sense

67ChevyRedneck · 03-04-2005, 06:27 PM

Sure does, I've only ever seen a dyno on TV, so I believe ya. Do a thread search and there should be a few threads on how to determine gear ratios by turning the tires and driveshaft. You can also go to GMC pauls site to find the rear axle code number. It should be stamped somewhere on the passenger side tubing of the rear axle. The first numbers will be the date it was stamped and the letters will be the axle code.

cableguy0 · 03-04-2005, 06:31 PM

just pull the cover and count the teeth on the gears thats the most accurate way. probably needs new fluid anyways

junkyardjohn · 03-04-2005, 06:33 PM

WHY DON'T YOU JUST PULL OFF THE COVER & LOOK ON THE EDJE OF THE RING GEAR TO SEE WHAT RATIO IS STAMPED THERE???? THEN YOU'LL KNOW FOR SURE & YOU WON'T HAVE 9 DIFFERENT PEOPLE GUESSING.

JOHN

THANK YOU CABLEGUY.. YOU MUST BE A FASTER TYPER THAN ME

69 longhorn · 03-04-2005, 07:38 PM

Quote:

Originally Posted by junkyardjohn

WHY DON'T YOU JUST PULL OFF THE COVER & LOOK ON THE EDJE OF THE RING GEAR TO SEE WHAT RATIO IS STAMPED THERE???? THEN YOU'LL KNOW FOR SURE & YOU WON'T HAVE 9 DIFFERENT PEOPLE GUESSING.

JOHN

THANK YOU CABLEGUY.. YOU MUST BE A FASTER TYPER THAN ME

Good call guys(both of you!)......guessin is "goofin"

L

**pjmoreland** · 03-05-2005, 01:10 AM

With a TH350 automatic transmission, there will be slippage in the torque converter, so the engine will be spinning a bit faster than the driveshaft.

If you lift one rear wheel off the ground, release your parking brake and shift out of gear, you can spin the wheel by hand and count the revolutions of the driveshaft. I've done this before with somebody helping me. I spun the wheel 100 times, and my friend counted the revolutions of the driveshaft. Divide the number of driveshaft revolutions by the number of wheel revolutions and divide by two. This will give you the gear ratio. If you have posi-trac, lift both wheels off the ground, and just divide the number of driveshaft revolutions by the number of wheel revolutions.

gabet40 · 03-05-2005, 03:10 AM

ok sorry been busy. ill start first to last.

The speed was from the dyno, i know that my speedo is off (need to get a new cable). and id like to doubt their dyno but they do dozens of measurements per week and have nothin but the latest equipment available.

I took the numbers stamped on the pass side of the axle down to the GM dealer and he couldnt make and sence of em. can u guys? if so ill go back and get em (its a little late and im tired right now, lol)

nice suggestion cableguy, just wish i wouldnt of ASSumed that the ratio was a 373 when a pulled the cover, changed the fluid, and replaced the gasket 3 months ago (searcing for the happy face slamming his head on the desk)

LOL love the suggestion junkjohn. i just thought id let u guys know what the dyno gave me and see if there was any rear end higher than 308.

THE NEW INFO: while i was cruisin the freeway at 2100rpms, i was at 65 (coming from my bro drivin my 98 honda behind me). that puts me a little closer to a 308 rear end according to the equation. but im open the d*mn thing up and let yal know sometime this week

71Chev · 03-05-2005, 05:49 AM

72 Axle codes. I didn't list the 4.11 codes.
1/2-Ton RAA 3.73:1
1/2-Ton RHA 3.07:1
1/2-Ton RHB 3.07:1
1/2-Ton RHC 3.73:1
1/2-Ton RHS 3.07:1
1/2-Ton RHT 3.07:1
1/2-Ton RHW 3.73:1
1/2-Ton RHX 3.73:1
1/2-Ton RRR 3.07:1
1/2-Ton RRS 3.07:1
1/2-Ton RRT 3.07:1
1/2-Ton RRW 3.07:1

gabet40 · 03-05-2005, 01:07 PM

ya i have the sheet that breaks down all of those codes but there is no code even close to those on my end

03-04-2005, 03:33 AM	#1
gabet40 Registered User Join Date: Dec 1999 Location: Modesto, CA Posts: 565	take a guess at this rear end ratio... just got the blazer back from the tuning shop. they tuned my commander 950 injection system and put it on the dyno for me. in the process they usually get ur top speed as well but get this; he took it up to 3500 RPMs in third gear (w/a turbo 350) and she was doing 125 mph! ive got 31" tires on there right now and unless theres a higher geared rear end than a 3.08 that exists, thats gotta be whats in there. __________________ Current owner of 2016 2500 Silverado Midnight Edition. Former owner of: 69 1/2 ton, change from long/fleet to short/fleet, shaved door handles, shaved tailgate, 4/6 drop, power windows, 700r4 72 blazer, 350 with Holley Commander 950 DFI, crane energizer cam, 350 tranny, dual sanderson blockhuggers/magnaflow exhaust

03-04-2005, 03:50 AM	#2
ac102nv Registered User Join Date: Jul 2004 Location: Northern Nevada Posts: 156	ratio formula Here's the formula I found for gear ratios: GEAR RATIO(?) = TIRE RADIUS(15.5) x RPM(3500) ÷ 168 ÷ MPH(125) (assuming 1:1 thru trans) According to that you have a 2.58 rear gear...sounds wierd to me. Based on same numbers, you should be about 104pmh at 3500rpm with 31" tire and 3.08 gears. But then again it's late and my calculator could be off.... __________________ '72 C10 lwb 454/TH400

03-04-2005, 12:20 PM	#4
gabet40 Registered User Join Date: Dec 1999 Location: Modesto, CA Posts: 565	so those 3 inches on the wheels will make a 35mph difference? __________________ Current owner of 2016 2500 Silverado Midnight Edition. Former owner of: 69 1/2 ton, change from long/fleet to short/fleet, shaved door handles, shaved tailgate, 4/6 drop, power windows, 700r4 72 blazer, 350 with Holley Commander 950 DFI, crane energizer cam, 350 tranny, dual sanderson blockhuggers/magnaflow exhaust

03-04-2005, 01:20 PM	#5
71cadc10 Registered User Join Date: Apr 2004 Location: Boiling Springs, PA Posts: 435	From the calculator I use you should be going 104.8mph at 3500 rpm with a 31" tire. Maybe there's something wrong with their dyno or the way they set it up???? Either that or somehow your rear gear is around 2.50-2.60 range like ac102nv said. __________________ 71 C10 long bed, 2003 6.0L, 06GTO E40, T56, 12bolt 4.10 rear

03-04-2005, 04:35 PM	#6
LUV2XCLR8 The LuvShack Garage Join Date: Jan 2004 Location: Maple Grove, TN (West Side) Posts: 30,475	CAN'T HELP YA, BUT IT'S NICE TO SEE SOMEONE ELSE USING MAGNAFLOW EXHAUST, I'M SO SICK OF EVERYONE BRAGGING ABOUT FLOWMASTERS. __________________ Owner/Op: "TN Classic Transport Carriers" The Toy: "Square Vette" 72 Hybrid Blazer Toy Barn: "LuvShack" 40 x 60 x 20 Shop Big Piggy: "Goliath" 07 Kodiak C4500 Lil Piggy:"Maddy" 88 Silverado 3500 Hauler: "Feathers" 14 Aluma 8218T

03-04-2005, 08:24 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Sounds about right...this is a 350/TH350 truck with 28 inch tall tires (275/60 R15's) and a 3.07.

03-04-2005, 05:41 PM	#7
71Chev Senior Member Join Date: Dec 2000 Location: Indiana Posts: 3,074	Was it their speedo or the one in the truck? With 31" tires, and nothing done to correct the speedo for the tire height increase. The faster the wheels turn, the farther off the speedo will read. For example at 60mph your speedo is probably reading right at about 10 percent slower than the truck. Find mile markers that are as close to a mile a part as you can find(it's harder than ya think, there aren't many 1.0 miles a part). Hold it at 60mph, marker to marker, then clock it with the trucks odometer. IF the odometer only clocks 0.9 miles in a set 1 mile distance, the speedo is reading 10% slow. 0.8 = 20% slow. If the odometer clocks over a mile, same thing. 1.1 speedo is 10% fast. __________________ 71 Chevy CST Long Fleet 350/350/3.07 leaf rear.

03-04-2005, 05:52 PM	#8
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	I get right close to 105@3500 also. Try this site for speedo calibration.... http://www.novaresource.org/speedo.htm 69L __________________ http://community.webshots.com/user/hotrodhorn

03-04-2005, 06:14 PM	#9
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	[QUOTE=71Chev]Was it their speedo or the one in the truck? QUOTE] I would have to think that it was their speedo cause our trucks speedo's only go to 1oo, right? On a dyno, the truck is not a big brick in the wind and has no wind resisitance on it, it's not even "pushing" it's own weight. It's just sitting on two big barrels spinning the rear wheels. I'm no expert, but could this possibly be the reason the rpm's were lower? If you take it out on the highway that same 125mph might be at 4K rpm becuase the motor is now "pushing" the weight of the truck and it's fighting its own aeorydynamics Just a thought..... __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications!

03-04-2005, 06:21 PM	#10
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	the dyno does provide some resistance thats how it gets it reading cant duplicate wind resistance without actually moving but i doubt it would be 20 mph worth of resistance.with a given gear set up tire size and rpm theres only one speed you can go. it doesnt vary by the weight of the vehicle or anything else. the power it takes to turn that many rpm and in turn go a certain speed is the factor here if your engine wont push the vehicle through the wind at 125 you will never reach the rpm needed to go 125 mph. i hope what i said makes sense __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

03-04-2005, 06:27 PM	#11
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Sure does, I've only ever seen a dyno on TV, so I believe ya. Do a thread search and there should be a few threads on how to determine gear ratios by turning the tires and driveshaft. You can also go to GMC pauls site to find the rear axle code number. It should be stamped somewhere on the passenger side tubing of the rear axle. The first numbers will be the date it was stamped and the letters will be the axle code. __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications!

03-04-2005, 06:31 PM	#12
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	just pull the cover and count the teeth on the gears thats the most accurate way. probably needs new fluid anyways __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

03-04-2005, 06:33 PM	#13
junkyardjohn Registered User Join Date: Aug 2001 Location: SOMERSET KY. Posts: 6,427	WHY DON'T YOU JUST PULL OFF THE COVER & LOOK ON THE EDJE OF THE RING GEAR TO SEE WHAT RATIO IS STAMPED THERE???? THEN YOU'LL KNOW FOR SURE & YOU WON'T HAVE 9 DIFFERENT PEOPLE GUESSING. JOHN THANK YOU CABLEGUY.. YOU MUST BE A FASTER TYPER THAN ME __________________ junkyardjohn 69 1 TON TOW TRUCK // 84 4WD CUCV BLAZER// 85 1 TON 4WD STAKE TRUCK// 86 M1031 5/4 TON 4WD CUCV// ALOT OF OLD TRUCKS FOR ONE OLD MAN TO DRIVE. THERES ROOM FOR ALL OF GODS CREATURES RIGHT NEXT TO MY MASHED POTATOES// LIFE MEMBER OF P.E.T.A (PEOPLE EATING TASTY ANIMALS) DON'T RENT U-HAUL ALWAYS TELL THE TRUTH IT WILL AMAZE PART OF THE PEOPLE & ASTONISH THE REST

03-05-2005, 01:10 AM	#15
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,412	With a TH350 automatic transmission, there will be slippage in the torque converter, so the engine will be spinning a bit faster than the driveshaft. If you lift one rear wheel off the ground, release your parking brake and shift out of gear, you can spin the wheel by hand and count the revolutions of the driveshaft. I've done this before with somebody helping me. I spun the wheel 100 times, and my friend counted the revolutions of the driveshaft. Divide the number of driveshaft revolutions by the number of wheel revolutions and divide by two. This will give you the gear ratio. If you have posi-trac, lift both wheels off the ground, and just divide the number of driveshaft revolutions by the number of wheel revolutions.

03-05-2005, 03:10 AM	#16
gabet40 Registered User Join Date: Dec 1999 Location: Modesto, CA Posts: 565	ok sorry been busy. ill start first to last. The speed was from the dyno, i know that my speedo is off (need to get a new cable). and id like to doubt their dyno but they do dozens of measurements per week and have nothin but the latest equipment available. I took the numbers stamped on the pass side of the axle down to the GM dealer and he couldnt make and sence of em. can u guys? if so ill go back and get em (its a little late and im tired right now, lol) nice suggestion cableguy, just wish i wouldnt of ASSumed that the ratio was a 373 when a pulled the cover, changed the fluid, and replaced the gasket 3 months ago (searcing for the happy face slamming his head on the desk) LOL love the suggestion junkjohn. i just thought id let u guys know what the dyno gave me and see if there was any rear end higher than 308. THE NEW INFO: while i was cruisin the freeway at 2100rpms, i was at 65 (coming from my bro drivin my 98 honda behind me). that puts me a little closer to a 308 rear end according to the equation. but im open the d*mn thing up and let yal know sometime this week __________________ Current owner of 2016 2500 Silverado Midnight Edition. Former owner of: 69 1/2 ton, change from long/fleet to short/fleet, shaved door handles, shaved tailgate, 4/6 drop, power windows, 700r4 72 blazer, 350 with Holley Commander 950 DFI, crane energizer cam, 350 tranny, dual sanderson blockhuggers/magnaflow exhaust



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03-05-2005, 05:49 AM	#17
71Chev Senior Member Join Date: Dec 2000 Location: Indiana Posts: 3,074	72 Axle codes. I didn't list the 4.11 codes. 1/2-Ton RAA 3.73:1 1/2-Ton RHA 3.07:1 1/2-Ton RHB 3.07:1 1/2-Ton RHC 3.73:1 1/2-Ton RHS 3.07:1 1/2-Ton RHT 3.07:1 1/2-Ton RHW 3.73:1 1/2-Ton RHX 3.73:1 1/2-Ton RRR 3.07:1 1/2-Ton RRS 3.07:1 1/2-Ton RRT 3.07:1 1/2-Ton RRW 3.07:1 __________________ 71 Chevy CST Long Fleet 350/350/3.07 leaf rear.

03-05-2005, 01:07 PM	#18
gabet40 Registered User Join Date: Dec 1999 Location: Modesto, CA Posts: 565	ya i have the sheet that breaks down all of those codes but there is no code even close to those on my end __________________ Current owner of 2016 2500 Silverado Midnight Edition. Former owner of: 69 1/2 ton, change from long/fleet to short/fleet, shaved door handles, shaved tailgate, 4/6 drop, power windows, 700r4 72 blazer, 350 with Holley Commander 950 DFI, crane energizer cam, 350 tranny, dual sanderson blockhuggers/magnaflow exhaust

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