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427 RPM at 60 mile per hour
Hi
With my 631/2 427 along with my 4speed toploader from a 67 mustang, i am at 3000 rpms going 60 miles an hour. I have a 9 inch rear end with 3.55 gears. Is something wrong there or expected with my setup. Automan |
What size tires are on the rear?
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Sounds about right...only variable missing to compute it exactly is tire diameter
There are tons of RPM/gear ratio calculators out there online. I did a quick search and came up with this nice one... http://www.idavette.net/tech/ratioc.htm |
My 427 Galaxie with a close ratio toploader , 3.89 L.S. rear runs about 3400 rpm at 65 mph . Tires are P 225 70/15 `s. 70 mph gets me up to about 3550 rpm .... and a whopping 7 mpg , which considering I`m pulling a 4100 lb car ain`t too bad ?? :eek:?? Don`t ask what happens when I bring all 8 barrels open :eek::eek:
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Christ, that can't be fun at all. :(
3:55s w/T-5 and 275/50/15s gives me 70 at about 2,100. |
100 octane was 31 cents a gallon, Kennedy was in the White House, and 1:1 final for transmissions was the current trend............HOW THINGS CHANGE!
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Assuming a rear tire diameter of 26.5" (approx. 295/R50-15), a 9" rear with 3.50 gearing and a toploader in 4th gear around 2650 RPM at 65 mph is about right. Swap in a close ratio T-5 with a 0.68 OD and drop it into 5th gear and now you are at 1750 RPM at 65 mph. That's the specs I got when I swapped trannies last year.
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Automan,
What you're seeing is exactly why I swapped my 3.54 gears for 3.0 gears last year. I have a Richmond 5 speed with a 1:1 5th gear and I was turning high rpm's at normal hiway speeds (75 - 85) and burning beaucoup gas. I haven't checked my mileage this year but the rpm's have dropped to around 3,000 avg at those speeds. Feels much better and not like I'm ringing it's neck on the freeway. I also get to use first gear. The Doug Nash has 3.27, 2.13, 1.57, 1.24, and 1:1 gear ratios I think. That first gear 3.27 winds it up pretty quick with a 3.55 rear ratio. Much better with 3.0 rear. |
You could easily run a 273s which would make your gear spacing better and with the torque of your 427 in a 2,500 pound car it should scoot nicly. It's much easier and cheaper to swap the ring and pinion or use taller third member which you could swap in an hour.
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Seems a little high to me (for my preference) but if you have a tall tire maybe not. Based on your numbers you are in the 30 1/2" rear tire range. The motor makes no difference it's all in the math that gets you to a final drive ratio. I have a 3.54:1 rear and I see 2800-2900 with a 1:1 fourth gear (toploader) and a 315 35 17 (26") rear tire.
Is it possible your rear is lower than 3.55:1? The number you list put your tire in the 275 70 15- 305 65 15 range. |
I have a Doug Nash 5 spd with a 1:1 fifth gear. I just swapped to a 2:36 and it is great. At 2100 RPM, I am doing 63 and with the 3:27 1st gear...I can use it. There is plenty torque in any gear. The car really drives nice and I can still break the 3:15's loose anytime I want. All these low geared cars must be using 5th gear overdrives in the .84 or .62 range.
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3:31 rear gear, top loader, about 2800 rpm at 60 mph, your right in the ball park. Word of WARNING if you go with a high rear gear or an OD trans. Can your engine camshaft profile LIVE with 1800 rpm? Many cam profiles in a big block just don't like running that low.
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Thanks everyone for your help. I went with the size tires that were in the AC assembly manual. I dont remember but i'm thinking that they are 295 15r but will confirm tonight.
So it looks like its might just be normal after all. Thanks now for the next question during my break in period. Automan |
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