I know I'm beating a dead horse with this topic, but I've done a search and the only thing I've definitively been able to confirm is that I'm even dumber than I thought. I tried Julie's gear calculator, but the output doesn't seem to make sense or, more likely, I'm interpreting it wrong. Anyway, my truck is going to the shop on Wednesday to have the gears in the rear end changed. I want to be able to do 70mph on the freeway without blowing up. The truck is a 1960 F100, 223-I6, 3-spd manual. Searching old threads, I found the following gear ratios for my transmission: 1=2.78;2=1.62 and 3=1.00 (don't know 100% that that's correct). My tires are approximately 31 3/8" tall on 17" rims. Given this info, what gear ratio would you recommend and how will it affect starting out in 1st gear? With the current setup, 1st feels almost like a granny gear, so I'm hoping regearing will not only give me more speed on the freeway at lower RPM's, but will make all my gears a little more usable. This is a reputable shop, but I just want to see how the Forum's advice compares to what he will recommend. Thanks.

 

It would be helpful to know what your current rear gear ratio is, what rpm your turning currently at 70, and what you want it to do. If your engine hasn't blown up by now, it's unlikely that it will. We have merely become spoiled by the quietness of modern cars with overdrive transmissions slowing engines down to barely above idle while driving on the highway. It was not unusual for engines to spin around 3000 all day long, "back in the day", without issue.

 

Really?................Really?

I have basically the same setup as the OP with a 3.92:1 rear end and I will be using stock tire sizes--but I just want to be able to go 60mph...is this easy on the engine?

 

Wayne, the metal tag on the differential says 3.70. I don't have a tach and have never gotten the truck up to 70. 60 is about as far as I'm comfortable winding it out to.

 

RPM / Speed / Gear Ratio Visual Calc

With your current setup in top gear 1:1 drive @ 70mph with 31" tires your turning 2831 rpms. Yes it will effect your take off. do you plan to do a lot of highway driving? Something to think about with your "granny tranny" with a higher ratio you may end up in a position where your 2nd gear is to high to take off in and your first is to low.. Do you know what tranny you have? Looks like a 3.30ish for cruising and a 3.50ish for towing would be your best bet. With you current setup 60mps is about 2400rpms. that's about right in my book.

 

Don't forget to consider what your power will be with a higher ratio.
If you have any hills or do any towing or hauling the 223 may be a little hard pressed to maintain speed at the lower rpm's in high gear and a down shift to second might be a little slow.

 

Yes, really! Engines don't last as long if you're cruising on the interstate all day like that, but nothing is going to come flying apart.

I think the point about 2nd gear is a good one; you need to be able to go around corners in town in 2nd and not need to slip the clutch. 3.50 is a good compromise with those tall tires.

See why T5 conversions are so popular?!

 

Here's a third calculator that may be helpful. Stu

Novak Conversions - Gearing & Gearing Math for Jeeps

 

Howdy,

Try a little bit of 8th grade math!


I tried that "calculator" too and it gave me zeros in several of the spaces.

It's not rocket science though.

Since you know that a mile is 5280 feet, and 1 mile per min (60 mph) is 5280'/min,

You can easily figure out what your rear wheel RPM is at 60 miles per hour.

Once you know your rear wheel RPM, you just multiply it (times) whatever rear ratio you want to try and you'll get engine RPM at 60 with that axle ratio......


To get rear wheel RPM, you have to know the circumference (the total outside distance around the tire!) IN FEET...........


Circumference is defined as the the diameter TIMES pi

pi = 3.14159265

(Just use 3.14)

You said your diameter 31 3/8" tall....... (31.375) INCHES...........

divide that by 12 to get feet -----------> 31.375 / 12 = 2.61458333333 (Just use 2.61)!!

(by the way, if your tire has a lot of "squash" then get the approx "rolling" radius from the center of the tire to the ground and multiply that number by 2 for the effective diameter)






Now you know it's 2.61 FEET diameter............... Multiply 2.61 by 3.14 and you get 8.1954 ( round up....... just use 8.2)


8.2' is the circumference. I.E. the truck rolls 8.2 FEET forward for every rear wheel revolution.


Now, if you're going 60 miles per hour (or ONE mile per min) you're also going 5280 FEET per min!


divide 5280 by 8.2 and you get 643.902439024 or 644 RPM


The rear wheel is turning 644 RPM at 60mph with those tires!


NOW multiply 644 by what ever axle ratio that gives you the desired engine RPM you want.


644 x ratio = engine RPM. Or, if you want a particular RPM (@60) use Ratio = RPM divided by 644 (Ratio = RPM/644)

So if you want 2350 RPM at 60mph, the ratio = 2350/644 = 3.65 etc.....


If you don't have a calculator, use the one in windows!



Cheers,


Rick

 

Thanks for the replies. Lots of good things to condider.

Bob, my tranny is not real a 4-speed "granny tranny". I have the stock Ford 3-speed column shift model. I was just saying that 1st feels almost like a granny...it is not synchronized and once I get it going it will crawl along with the gears whinig away like a granny. I probably get up to 15mph before shifting to 2nd. I have started out in 2nd, but it's not as easy as with, for instance, my old NP435 4-speed (that was in a different truck). Are you saying you would leave everything as is?

Hauling/Towing. Not a lot of hills around town and I won't be towing with this truck. The hills on the freeways are gradual grades.

2nd gear around town. This one definitely concerns me a little. The hope was to extend the range of 1st gear a little as a side-result of the swap, but if I'm coming up on a corner and 2nd is lugging badly, I'd have to come basically to a stop to engage unsynchro'd 1st anyway...hmmm,T-5 huh?

A couple of you guys have recommended going from 3.70 to 3.50. Will I see that much of a difference given the cash outlay? I don't think I'm going to do this unless I go down to the 3.3ish ratio that will significantly lower RPM's at high speed. The freeways are the quickest, most convenient way to get around town here, especially going back and forth to work. With the proper gearing, I would do a lot more freeway driving.

I would also like to hear first-hand if anyone has done this with my setup and how it worked out...thanks!

 

Rick--excellent post--it answered another one of my questions: "Am I dumber than an 8th grader?" I think you know the answer...

 

using the T4 with 4.03 2.37 1.50 1.00 w/2.75 gears

______1500rpm_____ 2000rpm ______2500rpm

1st _____11_________15___________19
2nd______19________ 26 ________ 32
3rd_______30 _______ 40 ________ 51
4th______45 _________61 ________ 76



using the SROD with 3.07, 1.72, 1.00, 0.70 w/3.75 gears



_____1500rpm______2000rpm______2500rpm

1st___11________ 14 ________ 18
2nd___19________ 26 ________ 32
3rd ___33 ________ 44 ________ 56
4th ___48 ________ 63 ________ 79




Here is the calculator I used.
Gear Ratio Calculator

Almost looks like a wash, Either would work, the overlap, how does that look?



Here's the factory 3speed with 2.78, 1.62, 1.00, w/4.25 gears



_____1500rpm______2000rpm______2500rpm

1st___11________ 14 ________ 18
2nd___18________ 24 ________ 30
3rd ___29 ________ 39 ________ 49



Here's the factory 3speed with 2.78, 1.62, 1.00, w/3.75 gears



_____1500rpm______2000rpm______2500rpm

1st___12________ 16 ________ 20
2nd___21________ 27 ________ 34
3rd ___33 _________ 44 ________ 56


Here's the factory 3speed with 2.78, 1.62, 1.00, w/3.50 gears



_____1500rpm______2000rpm______2500rpm

1st___13________ 17 ________ 21
2nd___22________ 29 ________ 37
3rd ___36 _________ 48 ________ 60


Here's the factory 3speed with 2.78, 1.62, 1.00, w/3.25 gears



_____1500rpm______2000rpm______2500rpm

1st___14________ 18 ________ 23
2nd___24________ 32 ________ 40
3rd ___38 _________ 51 ________ 64
__________________

I went through a lot of this;

https://www.ford-trucks.com/forums/9...rivetrain.html

 

Nah! I don't think so!!!



I had to do it my way because those "calculators" kept giving me zeros or errors when I put in my 6.80:1 and 5.83:1 axle ratios!!!

And my 5.83 is a 2-speed that has an 8.11:1 low range!!


I also have nearly 40" diameter (9.00-20) tires and that seemed to screw it up too! So I needed to come up with a way to get some ball-park numbers.


I also have an automatic transmission that has an OD (0.71:1) that I had to throw in there.....


My rear wheel RPM (40" tires) comes in at 504 RPM (at 60) and if I stay with the 6.80 axle, I get a little less than 3500 RPM!!

I wanted to be able to drive on the freeway (comfortably) at 60 mph or so and with the 5.83 x 0.71 x 504 I get 2086........or about 2100 rpm.

I think a Y-block will run pretty nice & quiet at 2100 RPM and still have plenty of torque to run on the flat (empty) with the torque converter locked up!


I think I might even get "good" gas mileage!!! (if that's even possible with an F-600!!!)

 

Bob--thanks a lot of for that...gives me a good comparison!

Rick--that sounds like a beast! I'll have to look in your gallery.

 

Wow lots of Math!

I'm not quite sure what you did with the gear ratio calculator, but I plugged in the numbers you posted here and posted the results below - looks right to me!

The .82 would be a 20% drop overdrive - that's the last column.



From this information you would be turning 2739.13 rpm to be doing 65 mph in
3rd gear. With a 20% Overdrive you will be turning 2241.38 rpm to do 65 mph.

This chart is much more accurate when you measure the roll out distance of the tire (math conversion is not accurate enough), by marking the tire and rolling the truck to complete one revolution of the tire and measuring the distance it travelled.

To accurately measure "Roll-out Distance," See this thread, Post #6:

https://www.ford-trucks.com/forums/8...-look-for.html