Ok , this is the set-up; flathead 175-200 HP, mated to 4 speed tranny with the following gears; 4.02, 2.41, 1.41, and 1.0. So the question is anyone have similar gearing, if so what's your rearend gear ratio? What's your RPM at 60 MPH? Share your experience, what do you think would be the best gear ratio, Thanks.

 

julie? where are you, julie? measure or calculate the roll out of your particular tire, julie will be asking. dick r

 

You need to know tire diameter to figure this.

You can put in your own info here, and change ratio/tire size etc as you wish.

Best ratio I would think would bring you to about 2200 rpm at 60, maybe 2400.

http://www.novak-adapt.com/knowledge/gearing.htm

 

I've got a calculator, what I'm looking for is real world experience. Maybe another way to put this is what's going to be the best all around gear?
Tires 28.9

 

3.0 or 3.25 if freeways are in the pic.

3.50 or 3.55 if you never go over 60.

YMMV

 

With that size wheels/tires, and the little extra horse power/4sdp, 3.25 would be ideal.

3.0 is ok if you never tow or seldom haul anything, and as mentioned 3.5 will do it if you don't drive 60 or over for more than about 20 minutes (it'll drive ya nuts - high RPM).

Best thing to do is measure the roll out distance of the back wheels, not the diameter.

This calculator is exactly right on when using roll out:

http://www.et-studios.com/motorsports/gears/gears.html

The highest sustained (over 30 minutes) rpm you will want is 2500. With the four speed, hauling will not be a problem as you will probably haul only a short distance and can do it in 3rd. And with the granny, starting hills with the higher gearing will not be a problem.

So I would not go lower than 3.25. 3.50 will be too low - 3.0 will be good too

 

[quote=Julies Cool F1;7878508]With that size wheels/tires, and the little extra horse power/4sdp, 3.25 would be ideal.

3.0 is ok if you never tow or seldom haul anything, and as mentioned 3.5 will do it if you don't drive 60 or over for more than about 20 minutes (it'll drive ya nuts - high RPM).
Well currently I have the following gears 6.4, 3.09, 1.69, 1.0 and a 4.27 diff. Anything will be an improvement.
From the calculator that I have I came to the same conclusion, but it's all theory for me. Thanks Julie. Anyone else run anything close to what Julie has suggested? Any comments on over all drivability?

 

as soon as i get round to the swap im going with a 3.08 geared 8.8 { gotta have the axles redrilled as i went with truck rotors up front not the vics and i want to use the set of oe 15's i got tucked away } with a stock flatty and a t-98 four gear . im hoping with 245 70's on the rear , for a cruise rpm of 1800 - 2000 , although as you said anything is an improvement over what i got now ........................

 

Thanks for the responses. I have to say I am surprised by the number of comments. I guess most everyone here has changed to the Chevy small block and the automatic transmission. humm?

 

not me !! if you see a chevy small block in mine call the law as someone stole it !!!!!!!!!!!!!!!!!!!!!!! now i can't say that i haven't had evil thoughts 'bout that 4.6 in the bird in one of these , or that i haven't looked at putting an auto behind my flatty .........................

 

I am under the weather this evening and wide awake because my doctor gave me an injection of b-12, steroid, anti-histamine and anti-biotic he calls a 'red zinger'.

Having time on my hands, a lot of energy and nothing better to do, I removed a rear wheel from my truck (because there are no front wheels on it and it would have been REAAALLL hard to push) and I measured the roll out on it. It came to 88.25" rolling it. (I think there is some error there in trying to be precise marking where the tire started it's roll and where it ended it's roll and trying to roll it perfectly straight.

Since I had the wheel off the truck, I then measured the circumference. It came to 88".

Next, I computed the circumference using the 275/60x15 printed on the side of the tire and arrived at 87.9399" (using Microsofts value for PI to 14 decimal places).

On a side note, I measured the diameter (with the tire laying on it's side using 2 straight edges) at 28" and it computed to be 27.9921" using the sidewall markings. A neglible difference there.

With all due respect to Julie, the roll out method proved to be the most work and a real PITA.

Of these methods, the most accurate and most precise methods seem to be the circumference measurement and the computation by the side wall specs.

Then I punched my numbers from my rear tire (88 and 87.93 for rollout and 28 and 27.99 for diameter) with your trans and diff specs into the calculator Julie specified. It made 1 mph difference in 4th gear in the 7k, 5k, 3k (58 and 59 mph) and 1k rpm range and in 2nd gear in the 8k rpm (like your flat head will ever see that) range without changing any other numbers in any other rpm ranges or gears.

Maybe it's my 26 years working in the Surveying frield and with computers but I look for accuracy and precision. I was once told that accuracy is a 6 inch shotgun pattern, precision is a 6 inch shotgun pattern on target.

Work smarter, not harder.

Nope, Ford 289 and C4 that I might replace with a Ford 460 and C6.

 

The accurate roll-out diameter is measured under load of the truck. I've found a difference in effective diameter of about 1/2" on high-profile (75 series) "balloon" tires, compared to using siidewall calcs. When a car/truck is assembled, it is very quick and easy to do the roll-out method.

 

Years ago I restored a 51 F-1 that had 4.27 gear and a flattie.......way to slow (like 50 mph). So I swapped in a 9 inch with 3.25 gears...........man what a difference!! It ran nice at 60-65 mph, though a little scary with skinny bias ply tires..............T/M

 

Well thanks to all that have commented. Based on the gear calculators and a number of options I am going with the 3.25 gearing. Today I was finally able to mach-up the T-19 4 speed with the flattie. Everything went to gather like they were made for each other. The next real obstacle is the tranny crossmember, but that still a little down the road.

 

And doesn't it make you feel fantastic when a great plan starts coming together?!