Need opinions on rear end ratios for flathead w/t5

Hi guys,
I have a close to stock flathead with T5 conversion (3.76 gear set) in my 31 Ford pu. The time has come for me to select a gear ratio for the rear end (Ford 9"). I'll be running 29.5" tall tires in the rear. I know a lot of you have similar setups so I was hoping for some opinions on which ratio would work good. I'm not looking for a ratio that will burn rubber in every gear or set a land speed record, just a good ratio for cruising around town. Thanks for any and all help.

 

My combo is very similar to yours except a modified flatty. I'm happy with 3.55s in mine. Great around town and calms right down on the hiway, with room to gear down if I have to at speed...

 

Thanks MBog, just the kind of info I was looking for.

 

in a light rod with a stock motor the 376 might be good. if you can, run it that way for a while and see what you think

 

I'm running 3.25's in my truck with a flattie and T-5.

 

Thanks oldman41, I wish I had the option of trying out different ratios but I'm hoping to get it close on the first shot, optimistic I know.

Sidevalve8ba, how do you like the 3.25?

 

I like the set up. My rear tires are 29" tall and it cruises great on the highway. What overdrive ratio do you have? More than likely it is .72.

 

4.11:1 will work fine and keep you with decent mileage.

its been cussed and discussed at length so I won't go into it. Tall tires and OD you want a lowish final gear ratio.

 

Sidevalve8ba, I do have a .72 overdrive ratio.

 

Better watch it P&B, that 4.11 stuff is "hot rod" talk, that dont go down well 'round these parts...

 

Everyone has given you the range so far. Figure on where you want your flattie to rev for three hours on the highway (I'm guessing around the 2,500-2,700 rpm range), then do the math factoring in the .72 overdrive. If you don't think you'll spend much more than once a year on a road trip, and you're just going to tool around town most of the time, go with Beaner's numbers so you can feel a little more hole shot and hear that flattie hit some high notes. The 4.11 with an overdrive still puts you on the highway without going deaf.

 

I'm running 3.84 and 4.43 in my quick change and the 3.84 works well with the T5 on the highway, with plenty of spunk on the street.

 

My stock Merc Flatty seemed to have a sweet spot around 2300 rpms calculate it out so you are in that range or close to it at highway speed and you should be good.

 

I'm not a "flatty" guy, but Faye has a 200 4R (which is .72 overdrive), 3:89 posi w/31" tires and she cruised all day long @ 70mph and roughly 2200rpm. I don't know the exact rpms because I have a POS moon eyes tach. With 2 GPS's running simutaniously on I80 to Bonneville, the tach would read 2200rpm between 64 & 73mph. I've since put on 29 1/2" tires and the RPM is down about 750. The nice thing about the T5 is: if OD is too high you can downshift.

 

a friend has a roadster flathead t5 and 4.10's drives on the highway at 120kmh and says that it's perfect

 

This is where those gear ratio calculators are very handy. Most flatheads like to lump along between 2 and 2300 cruising. If you use those rpms and know the tire circumference you can plug in the numbers and determine the 1-1 gear ratio for final drive. Most over drives are a little over 25% so if you take the 1-1 ratio and divide it by 1.25 it will give you an idea of the gear ratio that would accomodate that range.

 

google speed calculator I think if you put in your info you should get real close.

 

So a 3.73 rear gear with a .72 overdrive Trans and a 29" tire puts you right about 2300 RPM's at 75MPH. As for in town cruising, you should be OK with that too I would think with 4 gears to choose from still. A bit higher or lower 3.55 or even a 4.11 as Beaner suggested would not be unreasonable at a bit over 2500 at 75MPH

http://www.csgnetwork.com/multirpmcalc.html

 

Thanks guys, I appreciate all of your input. After playing with the ratio calculator I'm thinking a 3.70 or 3.89 (two choices I have available). I'm thinking this will provide good get up and go in town without sacrificing what limited highway driving I'll be doing. I guess it's nothing time and money can't fix if I decided to change it later.

 

I had a 3.89 gear in mine for a while. First gear in the transmission was useless, too low. It also turned more revs than I was comfortable with on the Interstate. I am still very happy with the 3.25 set-up.

Here's a calculator from Richmond Gear you might find handy.

http://richmondgear.com/index.php/gear-transmission-calculators

 

Sidevalve brings up a good point. When calculating rear end ratio you should not only consider your 5th gear ratio, but also 1st. I've also got a 3.76 1st ratio running through a 3.00 9" turning 29" tall tires and 1st really isn't too tall. But I've got a supercharged 284 cube 8BA. Depending on how stock and which flattie you have somewhere in the 3.25 to 3.50 sounds like what you should be considering. Past 3.50 1st is going to get pretty short.

 

Thanks guys, while using the calculator I kept thinking I was calculating a ratio off of the gear I'll be using the least. With most of my driving being in town I want a ratio that accommodates my lower gears. I'm back to my original thought of around 3.50.

 

My Stewart tach don't work, but the calculator don't lie, 70 mph in fifth 2053 rpm...