Can anyone tell me the top speed of a stock '70-'72 Valiant with 225/904 and 2.76 gears out back? and how it would be affected with the addition of 3.23, 3.55 and 3.91 gears ? Thanks.

Just a little math problem. The way I do it is to calculate rpm per mile, and make a proportion to figure rpm or speed.

A few known values:
rpm @ 60 mph = [rear gear] x [mile in inches / circumference of tire x 3.14]

60 mph = one mile per minute.
One mile = 5,280 ft or 12 inches/ft x 5,280 ft = 63,360 inches
Circumference = diameter x pie


Top speed mph = {[red line rpm] / [rpm @ 60 mph] }x 60 mph

For example:

My car has a 3.55 rear ratio, and tires are 24.5 inch in diameter, find rpm @ 60 mph, and red line mph.

Rpm @ 60 mph =[ 3.55 rpm x 63,360â€￾] / [24.5â€￾ x 3.14] = 2923 rpm

MPH @ red line = [5500 rpm (my red line) / 2923 rpm] x 60 mph = 112.9 mph.

With a 2.76 gear:

Rpm @ 60 = [2.76 x 63360â€￾] / [ 24.5â€￾ x 3.14] = 2273 rpm

MPH @ red line = [5500 rpm / 2273 rpm] x 60 = 145 mph that would be with no wind resistance or 450 hp with wind…. LOL

A stock slant six ain’t making much power to over come wind resistance, and it ain’t going to turn up to 5500 rpm. So some engines may yield different results because they have different power output, and red line rpm.

So to answer your question; the taller (2.76) the gear, the slower or fewer engine revelations per given distance.

In the other direction; the lower (3.91) the gear, the faster or more turns the engine has to make to go the same distance as the tall gear.

So if you have 4.11 gears, your engine will be turning 1.489* times for every revolution of the rear tire that a 2.76 gear equipped car would have to go the same distance, or speed.

*4.11 / 2.76 = 1.489

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

If you have gears that spin the engine above it's power peak before reaching top speed you will limit the top speed. Same is true with gears that don't allow the engine to reach peak power. You want to reach peak power RPM at the peak road speed.

With a stock 225 and 2.76 gears you'll run out of horsepower before running out of RPM. I doubt you'd hit 110 MPH, but you might come close. 4500 RPM with 2.76 gears and a 26" tall tire is 126 MPH with no slippage. A stock 225 doesn't have enough power to push a '70s Valiant 126 MPH so you'll never get to 4500 RPM.

4500 RPM and 26" tall tire with 2.94s is 118 MPH.
4500 RPM and 26" tall tire with 3.23s is 108 MPH.

Your 225 will have to be in pretty good shape to make peak power at 4500 RPM. 4000 might be more realistic.

4200 RPM, 26" tall tire with 2.94 gears is right about 110 MPH.
4000 RPM, 26" tall tire with 2.76 gears is right about 112 MPH.

Try this calculator: http://wahiduddin.net/calc/calc_speed_rpm.htm

So if you have the HP to run 110 MPH and make peak power at 4000 RPM you'll want 2.76s. If the engine makes peak power at 4200 RPM you'd want 2.94s. More power with the same RPM potential requires a numerically lower gear ratio to hit the maximum potential speed. Higher RPM needs numerically higher gears.

Gearing for top speed is usually a waste of time because the potential to actually drive at that speed is practically nil even on a race track. If you really want something fun and flexible run 3.55s with a 5-speed having a .75-.80 5th gear.

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Joshua

Josh is correct for real world applications vs. my mathematical manipulations. It all has to do with the engine’s usable power band, and how you are going to use your car. Is going to be an uppity, snotty, high strung, full race car trailer queen, or a grocery getter putting along urban roads in stop and go traffic never seeing 45 mph more than once a week?

Generally speaking, the factory engineers equipped these six cylinder cars for everyday driving on typical US roads & highways. These cars will easily keep up with late sixties speed limits, 70 tops, in most locals in the day, and handle in town traffic with ease. Don’t expect to run with the high end Euro Trash & 300 C’s in the brake-down lane on the Mass Pike with a stocker; it ain’t go’na happen… LOL

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

My best scientific guess is that I'll need 3.73s to hit the peak power RPM at peak 'road' speed with my '66 Valiant. Of course I'm trying to run 150 MPH at 8000 RPM.

With a mild 225 and relaxed highway driving I wouldn't go numerically higher than 3.23. If you have an engine that likes higher RPM you can run numerically higher hears, but you better have a high tolerance for noise.

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Joshua

.......thats my car!

............and I can still go 105mph....in 4th or 5th......its about aerodynamics not gears.

My 65 will go faster than my 69 with the same setup because the 65 has better aerodynamics.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Great rundowns there, boyz.

I would say 105 MPH on level ground and no wind, regardless of what your tail gears are.

Also, never believe your speedo unless it is carefully calibrated, or just do timing over a mile and calculate.

Lou

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Home of Slant6-powered fun machines since 1988

105 MPH seems slow.

Is it sad that my Neon is so much faster than any of my stock slant six A bodies and gets 50% more miles on a gallon of gas or just a clear sign of progress? BTW, the speed limiter in a standard Neon is set at 118 MPH. I have not been able to hit the limiter at Portland International Raceway. The poor tires on the car limit the speed I can carry onto the front straight.

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Joshua

Thanks guys,
My '72 Valiant 225/904/2.76 has seen maybe 95MPH a couple times and it felt like it could go more but that was relying on the stock speedo and it was for a short distance. Main concern is I want to keep this car a one barrel and want to know if it would worth the switch to say 3.55 or 3.91's for mostly city duty. I want to see an increase acceleration from stop light to stop light. Thanks for the responses. nico