I have a 68 Dart 270 with a /six 225. Currently doing a super six set-up to get a little better performance. I need to change gears. I'm wondering what gear option to go with. No drag use just street/highway 60/40. 3:73 was suggested....to high RPM on hwy? I've read 3:55 is a good balance.

Still has original rear end with drum brakes but I plan to do a disc brake conversion with 15" tires. Set up for 14" SBP right now.

Thanks and look forward to comments

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1968 Dodge Dart 270

With a stock cam I'd use 3.23s with a manual trans and 2.93 with an automatic.

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Joshua

I forgot to mention..It's an automatic

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1968 Dodge Dart 270

Guess i need to install a tach, but my car isnt anything but basically stock (IE no upgrades @ all ) and it had been converted from a stick to auto ( No I didnt do it ) and it has the 3:23's in it, and its pretty snappy.... I would it rather have the manual but thats a whole 'nuther story... But having said that, my combo @ 70 mph, it SOUNDS kinda busy.... JMO..

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MRO....

Cheap, Fast, Reliable.... Pick 2...

Are you sure? The super six upgrade itself will give you some more pep. I have a 4bbl on my car and, when I did the slider disc brake upgrade, I went from 2.93:1 to 2.76:1 gears. I find that my car is still quick enough and my engine revs around 2500 RPM at 100 km/h with P195/75R14 tires.

As you have a slant with an automatic, you might have the 2:93 gears. If you like driving your car, you'll probably enjoy the better fuel economy of those gears (or 2.76's with disc brake upgrade) more often than the occasional better burst of speed.

3.55 is too much gear; the engine will scream and the car will guzzle gasoline on the highway.

What size tires are you running?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

currently have 14" but want to change to 15". Seems to be alot more options for 15" to do a disc brake conversion and get tires

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1968 Dodge Dart 270

2.9 for a light car with little to no towing or heavy loads; 3.23 for a heavy car/truck/van or a light car that sees moderate towing or load carrying.

When Dan asked about tire size, I suspect he was trying to determine the overall rear tire diameter. 14 or 15 inch rim size doesn't matter in this discussion since we are concerned with rear tire RPMs per mile and 14 and 15 inch rims can have tires of the same overall diameter.

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Casually looking for a Clifford hyperpak intake for cheap.

rustys ring and pinion has chart to plot tyre size ,ring ratio ,rpm then you will be able to make educated guess at what you are after.....

keep on roddin'

I'd like to have 205-60R15 all around with front disc brakes

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1968 Dodge Dart 270

With a 2.76:1 axle, your speedometer will be more accurate with P215/60R-15 or P225/60R-15 tires if you have the 29 tooth speedo pinion.

But you will either have to be VERY careful with your rim width and backspacing or roll the front fender lip to get either of those tires to fit on the front of a 68 Dart.

My brother's Duster is currently sporting 67 Dart front fenders and 15x7 (might be 15x8, I need to double check) inch slotted mag rims with 3.75 inch backspacing. The 205-65-15 tires currently on the front do not clear the fenders unless the wheels are perfectly straight. In order to clear the fenders on a 67-69 (at least) A body with 15x7 inch front rims you need roughly 4 inches of backspacing or 205-50-15 or 205-55-15 tires.

Original equipment tire size on the 67-69 Darts was D70x14 which is equivalent to 185-70-14.

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Casually looking for a Clifford hyperpak intake for cheap.

That's a big tire on an A-body; they'll very likely rub the insides of the quarter panels, catch the corner of the front fender, and rub the front frame rail. Check fitment carefully. But let's go ahead and use that size to run through the math so you can see how it's done and pick your gears and tires according to reality rather than just guessing.

Engine RPM at any given road speed can be calculated:

First, figure out the tire diameter. A 205/60R15 tire has a 205mm width, and a height 60% of that width. 60% of 205 is 123mm.

Multiply this × 2 because the tire extends outward from the whole rim, not just half of it, so 2 × 123 = 246 mm.

Convert that figure to inches. There are 25.4 mm per inch, so 246 ÷ 25.4 = 9.685 inches.

Add the diameter of the wheel, 15 inches in this case, so 15 + 9.685 = 24.685 inches.

Tire diameter × π = tire circumference, so 24.685 × 3.14159 = 77.55 inches.

There are 63,360 inches in a mile, so
(63360 ÷ tire circumference) × (rear axle ratio) = engine rpm @ 60 mph in direct gear — that is 3rd gear with an automatic, a 3-speed manual, or an overdrive 4-speed manual; it's 4th gear in a non-overdrive 4-speed.

So, (63360 ÷ 77.55) × 3.55 = 2900 rpm.

The figure with a 3.23 rear axle is 2640 rpm.

The figure with a 2.93 rear axle is 2390 rpm.

The figure with a 2.76 rear axle is 2255 rpm.

To figure out the engine speed at highway road speed in a lower-than-direct gear (for example in 2nd gear kickdown on an automatic while passing), multiply the road-speed, direct-gear engine RPM by the lower-than-direct gear ratio (for example, an ordinary automatic has a 1.45 second gear; a wide-ratio automatic has 1.54). This is an important calculation to do, because if you overgear the car you'll run out of revs and be unable to use the "passing gear" kickdown effectively on the highway; you'll be stuck choosing between 3rd gear which doesn't get you enough oomph to pass, and 2nd gear which has the engine out of breath without enough oomph to pass. No fun. For example, suppose you want to pass someone while you're going 60 mph on the highway. You floor the gas, the transmission kicks down to 2nd, and now you multiply that 2900 rpm (with 3.55 rear gears) &# 1.45 to get 4205 rpm, which is well into "outta breath" territory for a slant-6 with anything like a stockish configuration, even with a 2bbl.

Multiply any of these result numbers by 0.83 to get engine rpm @ 50 mph, by 1.17 for 70 mph, by 1.33 for 80 mph, by 1.5 for 90 mph, and by (n ÷ 60) for n mph.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I'm not sure that P205/60R-15 tires are too big for an A-body. DonC1965 has a set mounted on 15x7 Wheel Vintiques and they seem to fit his 65 Barracuda well. I thought the '67 & up A-bodies had a bit more room under their wheel wells than the earlier cars.
15x7 wheels- fit 65 Cuda?

As for the 225 mm wide tire, Doc has P225/50R-15 tires on 15x7 rims for his 66 "Dacuda". I tried fitting a set of P225/70R-14s on 14x7 Cragar rims and they were tight on the back and rubbed on the front. I not sure of the backspacing on the Cragars but the Mopar rims probably have better offsets than my set of Cragars. That Cragar rim & tire combination fit well on our 74 Barracuda.

Probably marginal for the 1960-1966 A-body... marginal on the 1967-1969 cars (more than likely on the 4 doors)....no problem on the Duster/Demon/Dart Sport bodies (I just had a set of 275/50R15 Nitto Drag Radials on the back of the Fatpack duster and they are even with the upper swell of the wheel well)...Chase's duster had LBP Kragers with 225/60R15's and they looked about right front and back...again these sizes may be marginal on not do-able on the coupes especially the rear wheel well.

Suspension adjustments and not so sunk leaf springs can affect what might fit, but that doesn't necessarily say that it will work...

Personal experience in heavy late cars and driving other early/lighter cars tells me that 2.94 in an early car has good street manners for daily driving and gump to pass on a stock motor...if you want more 'fun' 3.23 is the next choice...without an OD box 3.55/3.73/3.91/4.10 are just not good daily driver choices. (Case and point: 1976 Duster with 10:1 compression motor 3.55 and OD gear box 205/70R14 tires averaged 21 mpg highway short hops, 24 mpg 'long hauls'....Same motor in my 1973 Duster same 3.55 rear mated to a 3 speed tranny at best got 18-19 mpg...).

-D.Idiot