What should I expect with 3.91 ratio on the freeway?

[bull]

Is the ratio too low for a 300-mile cruise on the freeway? Will I be turning 4,000 rpms at 55 mph and going nuts? Basically I just want to know if this thing will be something I want to (or have to) stick to the side streets in. Is there a calculator out there I can refer to?

Mild build 383 (maybe 400 hp) with a stock 4 speed, 8 3/4 and 295-50/15 Magnums with Coopers.

[A383Wing]

http://vexer.com/automotive-tools/speed-rpm-calculator

[Troy]

My Challenger with 275/60-15 rear tires (approx. 28" tall) and 3.91 gears would turn about 3000 rpm at 61 mph. Any more than that and the outboard carbs (Six Pack) would open and fuel mileage would suffer greatly. Plus, I got passed by a Smart Car! They lasted about 8 days (including 3 trips to the Mopar Nationals) before I chucked them for a set of 3.23s. Now I can cruise at 75 again! Lots of fun from a stoplight but you'll hate them if you have to spend any amount of time on the highway (and you don't drive like an old woman).

Troy

[RallyeMike]

I agree with Troy that you won't like this combo on the highway. Of course, the 383 can handle the higher revs, but also your tires are not tall at about 26" (2 inches shorter than Troy's). After gears, tire diameter makes a big difference.

If you want to get the feel of steep gears and still be able to take reasonable trips on the highway, I would go with 3.55, though I still believe that 3.23 is probably the best all-around gear for a mostly-street car.

[68neverlate]

For what it's worth Bull... I have 323s in my car and am really happy about being able to travel distance without having grannies pass me in their 4 cylinder toyotas on the highway and not having to stop every 100 miles to re-fill the tank!  I agree with Troy and RM... 323s are a great all round gear.  They pull a nice RPM on the highway to give you a bit of mean through the pipes and they allow you to DRIVE the car distance. 

My thought is that the 3.91s will initially be a thrill off the line... but once you've been there and done that for a while, you'll regret putting them in.  Unless you're intending on driving short distances only...   

[Back N Black]

+1 for 323's.

[471_Magnum]

3.23s or 3.55s.

Even with a small block 3.91s are painful over 60 mph. Anything that has you buzzing over 3500 rpm will just wear you out.

I'm the previous owner of Troy's Challenger and I'll vouch for his statement. That sucker got wicked thirsty once the outboards opened up.

I had 2.76s in the same car for a while. Cruised effortlessly at freeway speeds and got 16 mpg. Got real thirsty above 90 though. LOL

[Dino]

For just the little driving I have done with 2.94's and 215-70/14 tires I love it, excellent cruising around town and on the freeway with an auto.  The 3.55's I pulled were too much for me, 3000 rpm+ @ 55 is not my idea of cruising.  Quick off the line though!  Would be a great ratio with an overdrive.

[bull]

My thought is that the 3.91s will initially be a thrill off the line... but once you've been there and done that for a while, you'll regret putting them in.  Unless you're intending on driving short distances only...    

I pretty much agree with you but I'm going to try them for a while. And for a while I probably won't be going too far from home but eventually I will probably want to take a longer trip up to Seattle or maybe Eastern Oregon.

How much does it generally cost to switch gear sets? I don't really want one of those overdrive kits.

[Troy]

Just get a second pumpkin and swap them out when you feel like a change. If you don't need a Sure-Grip (most don't) then a complete extra setup isn't too terribly expensive. I have a guy locally who will install/setup gears for about $100 but I believe that's really cheap. Many times you can buy a complete pumpkin for about that much and you don't need any special tools to put it in.

Troy

[Dino]

Just get a second pumpkin and swap them out when you feel like a change. If you don't need a Sure-Grip (most don't) then a complete extra setup isn't too terribly expensive. I have a guy locally who will install/setup gears for about $100 but I believe that's really cheap. Many times you can buy a complete pumpkin for about that much and you don't need any special tools to put it in.

Troy

That's right, just don't let me do it because it'll take a week and you end up with grey hair and a noisy axle!   

[flyinlow]

Hey guys,I just heard about this new invention. Overdrive transmissions!    Otherwise you just leaving way to much performance on the table.

Hey forget what I just said.......Is your 3.91 a 742, clutch SG, 7290 flange , quiet, and for sale? 

[jb666]

I ran those gears on the highway.. That was what convinced me to get a Gear Vendor's OD unit....  A bit too much for highway cruising speeds...

[bakerhillpins]

I have to agree with what everyone has said up to this point. I have 3.91s in my new charger and freeway speeds are nuts. My car was shipped with 3.23s and I will end up going back to them eventually. Next summer in the heat will be an interesting experience. Car stays nice and cool now but its 60 out.

Lots of fun from a standstill though.