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07-06-2004, 05:13 AM
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#1 |
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Registered User
Join Date: Mar 2000
Location: San Diego, CA USA
Posts: 344
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Tach over 3K at 65MPH
Ok, I have my new motor, tires,gears in the rear end,new tested tach and the speedo dynoed. However, I'm running a tad over 3000 rpm at 65mph. I have 32inch tires with 3.73 gears. How much do you guys and gals think I will drop in rpms if I go with 33 inch tires? My poor new motor needs a break.
thanks for any suggestions, James
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--------- 72 SWB K10 San Diego CA |
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07-06-2004, 06:48 AM
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#2 |
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Senior Member
Join Date: Jul 2001
Location: Muskegon,MI,USA
Posts: 6,026
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If you look at the distance traveled per rev, (pi x D), the 32" tire travels 100.53 inches and the 33" tire travels 103.67". Percentage wise this works out to 3 percent. If you recalibrate your speedometer, at 3,000 rpm you will be going 66.95 MPH with the 33" tires on your truck.
At 65 MPH, your engine will be turning 2910 RPM. It appears that going to 33" tires will not reduce RPM by very much. Jim |
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07-06-2004, 06:55 AM
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#3 |
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Registered User
Join Date: Mar 2000
Location: San Diego, CA USA
Posts: 344
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thanks Jim, I guess I should have kept my 308 gears.....
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--------- 72 SWB K10 San Diego CA |
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07-06-2004, 09:07 AM
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#4 |
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Registered User
Join Date: Apr 2003
Location: NB
Posts: 3,367
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Man I need the 3:08 cure too!
Any one want to trade? 3:73 posi for 3:08 posi?
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
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07-06-2004, 09:36 AM
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#5 |
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Senior Member
Join Date: Jan 2003
Location: daytonabeach
Posts: 22,956
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overdrive tranny :the best of both worlds
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71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY |
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07-06-2004, 10:40 AM
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#6 |
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Registered User
Join Date: Apr 2003
Location: NB
Posts: 3,367
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Couldn't give up the Muncie!
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
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07-06-2004, 11:01 AM
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#7 |
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Registered User
Join Date: Jan 2000
Location: North Dakota
Posts: 351
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Install a lift and 35's
 I have 3.73 and about 2500 at 65MPH.
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1972 K-10 Longbox,6" lift w/35's/350/SM465/NP205/3.73 1972 C-10 Longbox,350/350 1998 Pontiac Grand Prix GTP FOR SALE: 1971 K-10 Longbox,350/SM465/NP205 |
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07-06-2004, 11:15 AM
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#8 |
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user # 2756
 
Join Date: Jun 2001
Location: Chesapeake, Virginia
Posts: 4,612
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Maybe you tach is set for a 6 cyl?
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1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm 
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07-06-2004, 04:16 PM
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#9 |
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Senior Member
Join Date: Dec 2000
Location: Indiana
Posts: 3,074
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Changing tire size will calculate to roughly 100rpms for every 1" change in height.
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71 Chevy CST Long Fleet 350/350/3.07 leaf rear. |
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07-06-2004, 04:22 PM
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#10 |
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Registered User
Join Date: Jan 2002
Location: Sahuarita, AZ USA
Posts: 2,033
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My 1 Ton has 4:11 gears. I went from truck 4 speed to a 700R4 and helped it SOME. Now 75 MPH is about 3300 RPM in overdrive. In Drive, 55 MPH is about the same. I have the formula for gear ratios, RPM, etc, all taped on the wall in the shop at home. If you want them, let me know and I'll post them later.
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Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com |
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07-06-2004, 04:41 PM
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#11 |
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The oddest Todd around
Join Date: May 2004
Location: Miami, Florida
Posts: 1,418
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Busterwivell, I would love to see that list.
67chevemall, I don't have posi but I have 3.08's. I would love some 3.73's and I don't care about the raise in rpm's. Too bad you're so far away. If not I'd say we could just trade ring and pinion's. |
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07-06-2004, 05:55 PM
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#12 |
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Registered User
Join Date: Jul 2003
Location: Cypress, Tx
Posts: 4,005
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Shoot, I got 4.11's in my truck and it's a daily driver, approx 60 miles a day, mostly highway. Talk about screamin engine all the time. I really need to get some better gears!
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1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE |
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07-06-2004, 06:33 PM
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#13 |
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Registered User
Join Date: Mar 2000
Location: San Diego, CA USA
Posts: 344
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thanks for all of the replies. Guess I better start towing something....
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--------- 72 SWB K10 San Diego CA |
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07-06-2004, 06:52 PM
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#14 |
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Registered User
Join Date: Jan 2004
Location: NH
Posts: 2,266
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I was running 26.5 in tires with a 4sp and 3.73 keeping up with traffic my engine was screaming 3800rpm, dropped in a 700r4 and now my cruise speed is around 2200rpm. the OD really is sweet with the super low first, I havent really been able to check out my new gears 4.10 with the 700r4 on the highway, but I dont figure it really will change a whole lot.
Here is a calculator to figure RPMS with tire size and gear ratio, etc. http://www.angelfire.com/fl/procrastination/rear.html
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1967 C10 - SWBSS Pro Street 427sbc, 700R4 & 4.10 Gears 1948 Chevy FleetMaster Coupe, LS1, (almost done) 1950 Chevy StyleLine Coupe 250/6 3x2 Dueces 2009 Toyota Tundra 1996 Harley RoadKing "I maybe getting old, but I can still burn rubber with the best of them" |
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07-07-2004, 09:16 AM
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#15 |
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Registered User
Join Date: Jan 2002
Location: Sahuarita, AZ USA
Posts: 2,033
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Todd, and others, here's the formulas:
MPH = RPM x tire diameter divided by rear gear ratio x 336 RPM = MPH x gear ratiox336 divided by tire diameter gear ratio = RPMxtire diameter divided by MPH x 336 tire diameter = MPH x gear ratio x 336 divided by RPM Hope this helps, I keep this posted on shop wall (along with SBC and BBC firing order)
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Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com |
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