Hot Rods Could use some advice on rear end gears

I recently bought a 32 roadster setup with a Ford 302, Richmond 5speed, and 9" rear end. I want to make the car more highway friendly. I haven't had it on a lift to check the rear end but from calculations it appears to be a 4.11 rear gear. (28.9 rear tire, .82 5th gear, at 70mph= 2700 rpm). I've never had a Ford 302 so I'm interested in knowing the optimum rpm cruising for this engine. A 3.50 rear gear will give me about 2330 rpm, a 3.25 about 2165 rpm, and a 3.00 about 2000 rpm. I would still like performance in the other gears so undecided on which is best. The previous owner didnt know much about the car but I think the 302 might have a mild cam and it is running 2-4 barrel carbs. Really quick car off the line but I'm more interested in highway performance without sacrificing too much on the low end. Any thoughts or advice? Thanks.

 

Are you sure your 5 speed has .82 high gear?
Your rpm calculation makes it sound like your high gear is 1:1.

 

Going to a higher gear will be a trade off for "off the line" performance. Those engines like RPM (depending on what heads are on it), and with 2 4V carbs even more so. IMHO I would go the 3.50 gear, even with the light car.

 

Check the rear end gear. Something seems off in the math.

You can do this by rotating the wheels once, while counting the driveshaft turns.

 

my 40 sedan is a 302 5 speed with 235 /70 rear tires when I put it together had a 325 gear in it you had to be on the interstate to use 5th gear. I had a 350 and a 370 gear to choose from . I put the 370 in and was glad I didn't use the 350 that's my opinion of my car.I put the info you had at the top in a calculator it showed a 4.18 rear gear so I would say 411 pretty close. I like my roadster but would like it a lot better with a 302 5 speed

 

It has always been my practice when choosing gears, when they are close, to chose the numerically higher gear.

It is better to turn the engine a little faster, with a lighter load from mechanical advantage, than it is to turn it more slowly, with a heavier load from mechanical disadvantage.

There is a sweet-spot. If you go below that, you will have your throttle blades open more, and will be introducing more fuel, just to maintain the lower RPM.

The only vehicles that have the capability of operating at very low RPM, and still maintain good performance and fuel economy are EFI (which is forbidden here).

Second would be those with a single two barrel carburetor.

Third would be a single small four barrel.

You have the worst of all worlds. You have two four barrels.

Even if they are the smallest ones that are sold, you are already exposing them to a weak off-idle vacuum signal, and poor venturi velocity.

Without going into a full explanation of the functions of a carburetor, they don't put fuel into the engine. They let fuel into the engine. They meter fuel into the air stream going in, in relationship to venturi velocity.

The larger the venturi area, either by bigger ones, or more of them, the poorer the fuel metering will be at lower velocity (lower RPM).

In your case, you need to keep your RPM UP!

 

do all the math calcs and consult tables all you want, but i'll bet in the end you wind up with something between 3:40--3:50 for the gearing. While its not perfect either way (hiway or the line) its the best tradeoff between the two.

 

Richmond 5 speed has an overdrive ratio of .77:1.
70mph / 28.9" tires / 2700rpm / .77 overdrive would calculate to a 4.3:1 rear end ratio.
What rpm would you be happy with at 70mph?
With the Richmond trans and the 28.9" tires, a 3.7:1 rear would run 2319rpm at 70mph.
That would still be quite responsive in the lower gears.......

 

Thanks for all the comments. I think I need to verify the exact Richmond 5 speed I have, .77 or .82 in 5th gear. I’ll also spin the tire and check rear end gear then use the comments to determine the best fix. Thanks again!

 

Definitely check the trans. Didn’t some have 1:1 in 5th
Had a lower first gear
Or am I thinking Doug Nash

 

^^^^^Sounds like Doug Nash .

Tommy

 

My search showed some Richmond five speeds had 1:1 high gear, hence my post above.

 

4.11 gears x.82 = 3.37 With a 28.8 tire at 7o my calculator says 2743 rpm

A 3.73 would give you 2482 rpm at 70.

A 3.55 would bring it down to 2368

3.30 would be 2197.

if we get crazy and run a 3.08 it brings it down to 2051

 

With dual quads on a small cube motor, and an OD trans, any rear gear with a lower numerical number smaller the 3:50 will kill the fun.

The dual quads kill low rpm performance. You will need the 3:50 gear to get up in rpms faster to be much fun.

 

It takes a bigger engine to pull a higher gear. I hope I'm not speaking too simple for you. Lippy

 

We all need to listen to what @L. Eckart wants.
It sounds like a highway cruiser that doesn't beat his ear drums to death.

With his cruising speed at 70 mph , If the goal was 2200 Rpm with those 28.9" tires he would need an overall ratio of 2.70:1
With a 0.78 O/D 5th gear he would need a rear end ratio of 3.46:1

This ^^^^ is lower than a Fox Body Mustang which had a 3.27:1 rear end
The Fox Body had the same engine [302] and weighed 3160 lbs [these went OK for a stock vehicle]

@L. Eckart you need to identify what 5th gear ratio you have.
Obviously transmission tags would be a big help [but these are usually long forgotten]

A quick close enough method is to jack the rear wheels off the ground.
With your car in 5th gear, put a mark on the driveshaft AND a mark on the balancer [or use TDC]
Rotate the engine 1 full turn with a ratchet/socket on the crank pulley. Then observe the mark on the Driveshaft.
If it is 1:1 5th you certainly will know. If it rotates approx 1.3 times it is 0.78:1 O/D

The math is ..... 1 divided by 1.3 = 0.769 [ 0.78 = 1.28 rotations which is near enough to 1.3]

Also if you want a nice Highway cruiser, don't go too big in cam duration.

Then go out there and drive the wheels off it, while all the chest thumping keyboard warriors can't.

 

A far simpler is to plug any three factors into this calculator and get the the fourth. No nonsense of playing math wiz or what not. Just drive down the road at the engine speed the engine likes, take note of the rpm then plug rpm, speed and tire size into the calculator click on "compute ratio and then figure the actual ratio after you calculate in the overdrive ratio. Tire Size, RPM, Speed, and Differential Ratio Calculator (advanced-ev.com)
To do that you divide the ratio the calculator gives by .82 in this case.

 

I have to admit, you guys have done a fair bit of thinking here. Way more thinking than I'm up for. All that math.
But looking at the problem a little differently, our friend wants highway fun with a v8 dual quad 5 speed.

My buddy Dimmer had that motor and trans minus a carb and it was a angry chimp alright. Not to be confused with a raped ape. That was Rob's 54 Olds sbc dual quad cross ram with a mopar 4 speed.
Point I'm trying to make is, where it's making the fun factor happen, is somewhere towards torque and horsepower meeting. What the gearing is going to do is move you from one to another in spread and duration. I'd rather pick up dropping a gear and being on top of a working range, than dropping a gear to quickly shift again. As I see it 5th doesn't play a part. 3rd and 4th. that's where you play.

 

Horsepower and torque meet at 5,252-rpm.

 

Dual quads will need lotsa gear to be useful. Especially in a SBF.
Don't forget you're working with a LIGHT weight vehicle..........so lotsa gear is NOT necessary.
With lotsa gear .........1st gear is gonna be useless. Like driving a truck with a granny gear.
AND you want good manners for highway cruis'in..?!!
3.25:1 seems like a pretty good compromise to 6sally6!!
I'd also put a snotty(er) cam in it too!
Why NOT?!... The dual quads would thank ya....
light vehicle weight and good 3.25gear would thank ya...
The low FIRST gear will make almost any cam/2-4 set-up work so IT would thank ya....
You'll never(well almost never) be using all 8 barrels strolling down the highway anyway so your wallet will thank ya... ALL the guys at the cruise-in will get an erection when you come loping in so they will thank ya...
With 2-4's, snotty cam, light weight, 3.25 gear, and a low first gear,a healthy compression ratio will be EZ to handle so your smiling face will thank ya !!!!
End of discussion.....
Thank ya
6sally6

 

I believe Ford put 2.87s behind there 5.0 five speed Fox bodys for a final drive of 1200 to 1500 RPM.

 

What was the fuel delivery methodology, and for how long did they do that?

 

I don't know my 08 Expedition only runs 1500 rpm down the highway. It's 6000 lbs but has a 5.4 in it.
He will be able to figure out the gear ratios in the transmission but I would think a 3.25 or 3.50 would work just fine.

 

It has complete digital powertrain management.

Much luck to anyone trying to do that with a carburetor.

 

Yep.

My roadster has a 302 , 3.7 rear, no overdrive. Tyres are 31" I think (7.50/16). It spins along fine for hours on end at our max speed limit (110 kph, which is just under 70 mph.) Economy is good, I think (I've never attempted to work it out.) Seems to be riding in the sweet spot for my engine.

 

I run into this question all the time. Guys want to know the best gearset for their Quickchange. The problem in selecting one set is that it depends on many factors. The combination is critical. Several guys have pointed out that certain items will affect the optimum gear ratio.
The good thing about @L. Eckart's situation is that he has a complete running vehicle. And, as @Mr48chev observed, finding the engine's sweet spot, and adjusting the final drive ratio based on that, will give him the result he is looking for.
Gimpy rightly stated that computer managed systems have nothing in common with carbureted engines.

With phone apps to indicate speed (and who knows what else) the math becomes easy. It is still important to know what the transmission and final drive ratios are to begin with.

 

An interesting thing to know would be; what rpm does it currently take to run 70 mph? From there you can play with rear gear ratios to get it where you want it. Like others have said, a 302 with 2-4bbl and a cam might not like cruising at lower rpm.

 

The first question I would ask you is how well does your car hook up with its current set-up? Does it scald the tires and go no-where......or hook up well?

My point is that you need to have room to give up some performance on take-off if you are going to change anything.

Next is the fact that even though you have a light car, a 302 does not have a wide power band with lots of torque. It may be fine in your car however. Add to that you have a hi-po cam of unknown specs and 2x4 carb set-up.......which makes having a decent power band less likely at lower rpms.

Going too far .......making too big a change may get you the nice cruise speed you desire but off the line could be problematic. I'd say go with the 3.70........That said, another option would be to locate a later model 6 speed and bell housing. Then you have two overdrives......and give up nothing in low gear.

 

More info is needed... A quick check on the 'net seems to show two Richmond five-speeds; if it has external linkage there is no OD but the lower gears are much steeper, the internal linkage version has a .77 OD and more 'conventional' gear spreads. If he has a wide-ratio OD version of the latter, he can tolerate a taller rear axle ratio with little loss in performance at lower speeds.

One thing the OP needs to decide is just what 'highway' driving consists of. Strictly interstates? Or maybe some slightly lower speed state highways? My OT DD has a six speed but only sixth is OD. The car came with a 3.23 rear axle, but a 3.55 would have been better. With the 3.23s OD is useless under 60 MPH, making it pretty much an interstate-only option. The 3.55s would have been better for the type of driving I do.

The SBF will tolerate higher RPM just fine, but above about 2500 they do start to sound 'busy'. The 'sweet spot' IMO is right around 2200 for where I live (lots of hills), you could probably drop as low as 2000 living in the flatlands as you do. I'm assuming the OP doesn't have a tach in the car, it may be worth it to put one in if only temporarily to determine just what you have and know just how fast the motor is spinning. And I agree, from the description it sounds like the OP has a non-OD version, if it was an OD it would need a 4.30 gear for that RPM at 70 but only a 3.30 for non-OD.

Personally, I'd drop it to a 3.00. That will give roughly 2450 RPM @ 70, 2100 @ 60 (still usable), and a too-tall 1750 @ 50. Drop down a gear, and you have 3000 RPM @ 70, 2600 @ 60, and 2100 @ 50 (just right). Drop down another gear when you want more zip...

You could go as low as a 2.80, but that will make 5th useless everywhere but the interstate.

 

3.23-3.55 is my vote, you have a light enough car to pull down into the 2.8 area if you wanted to roll 85 everywhere. With 5 speed you wont sacrifice the get up and go much either doing a low 3.something.

My 56 has a 2.92, rolls 3k at 80 mph with no OD. Its a bit more than I want to turn but the overdrive trans selections for mopar kinda suck so I deal with what I got.