Hello everyone,

Trying to use a rpm calculator to figure out new rear end gears for my 9" swap. I am not sure and would love some advice as to a good, safe rpm for a 1951 flathead V8 to crusie at around 65mph down the highway. I am not sure if 3,000 rpm is too high and I should shoot for 2500 or possibly even 2000. I dont know much about flatheads and dont want to overwork or over rev this engine driving it down the highway.

I am thinking about a 3.25 or possibly even 3.00 gear ratio depending on what you guys recommend for cruising rpm.

 

Based on the published specs several places (here's one: 1948 Ford 226 Inline Six Engine Specs) the max horsepower is 3300 rpm, and a few places somewhere I've seen the redline listed at 4300. That said when I bought mine and drove it home I was doing 43mph the whole way cause it was so loud I figured I had to be at redline (no gauges).
A lot of guys went with 3.50 rear end gears in order to avoid hard starting from dead stop and also not sacrifice their low end pulling power.

I personally went with 3.00 rear end gears because my choices at the junk yard didn't include any 3.25. My goal when I went to the yard was a 3.25, but I said I would settle for anything from 3.00 to 3.50. (The 3.0 happened to both be the very newest possible, 1972, and had complete brakes and everything looked good on it).

If you insulate the firewall and carpet, then the noise issue will be less but I would advise you to do like I did - be willing to go anywhere between xx and yy with a target of zz.

Note, I now use my granny gear as 1st - my shift pattern is 10/20/30 on a crash box.
Do you have a 4 speed spur gear (crash box), 3 speed synchro, 4 speed synchro or some non-original tranny?

[edit] My bad, I read it twice and still missed the V8... disregard the specs for redline and such obviously that is for the 226 I6.
Here's your engine if original: http://www.secondchancegarage.com/public/402.cfm

 

A Flatmotor MAY turn 3300RPM but it won't be happy for long
It CAN cruise all day at 2500 RPM
It will be VERY happy between 1800-2100

 

I have a 3 speed synchro from a 48 panel truck. It has the floor shifter and not the column shifter. I am just afraid of over revving the engine if I have to drive an hour away and blowing the engine up. I dont have overdrive so my final drive ratio is 1:1. I just wasnt sure what rpm to program into the calculator to get the best possible gear solution. I would be happy with a 3.0 or a 3.25, I am going to listen to your advice about being flexible.

 

Thanks Dick, I was waiting for you to chime in, I knew you would know the answer!

 

I should note that later I figured out I "must" have been doing about 2800 rpm on that drive home when it was screaming like a banshee - the PO put smaller 15" tires on it, and my original rear end was 4.27 - using the apex garage gear ratio calculated I come up with that estimate. I'd hate to sit next to a flattie doing 3000 rpm. Fuel economy (ha like that matters with these guzzlers), the lower the rpm the less fuel as long as you aren't lugging the engine.... I haven't put many miles on mine yet (doing carpet right now), but I am very happy with the 3.00 so far.

The 239 V8 specs: http://www.secondchancegarage.com/public/402.cfm

 

Are you happy with the 3.00 ratio? I dont have the low first gear ratio so I am not sure if that would make much of a difference or not. Also, I am thinking I am going to be running a tire that is about 28" tall. I dont want to run a tire that is too small and not fill the fender opening but I dont want to run a tire too tall, I would like thwe truck to sit a little lower.

 

I am switching back to 16" rims, and the closest "realistically available" size tire to stock 225/70/16 - still the same 16" rim, and still 28.4" though significantly wider.
I have driven it on a country road at 45, and you can hold a completely normal conversation with the passenger at that speed in top gear (1:1) still on the smaller 15" tires (which the calculator says is 1700 rpm)... unfortunately I have not had it up on the freeway yet. I am still putting upholstery in and will have it out on the road no sooner than 2 weekends from now. I'm very happy with reverse (peeled the tires making a T turn), and forward low speeds - it's not impossible to get it started from a dead stop, and still has enough power to yank a tree stump. I just don't have the highway answer for you yet, sorry.

 

Are you happy with the 28" tall tires? Do they look right on the truck? I was thinking of running a 245 50 18 and that comes out to right under 28". I figured 28" is a good size without being too small or too tall.

 

The difference between the PO tires and the stock size are pretty small - 26.85 v. 28.4.... I can't really tell much difference at all in the feel. The looks though, it raised the truck a 1/2 inch and fills the wheel well correctly - the 28.4" doesn't look odd, like someone put too small a tire in there. The PO's tires looked wrong to me (all the pics I have are PO 15" ers).

 

Dave for you terrain down there I wouldn't go below 3.25 - You would be doing a lot of rowing the gearbox off the thruways

 

I couldn't find the gear ratios for the 3 speed light duty off hand after a couple googles - what are your gears in that tranny anyhow?

The more you debate this and discuss it the worse it gets I know - been there
Tires, TRANSMISSION, and differential all play together... change one variable and you may have to change the other.

In my case I wanted to stay as true to stock as possible appearance - that meant stock looking tires for sure. .... 1 variable changed.
I have no issues double clutching and clutchless shifting (I drove a Kenworth for a while - got that down pat). My transmission works, no need to change it, but if it died I will not fix it - I would replace with a T5 or possibly another if we come up with a good solid modern replacement choice.

When I got the 3.00 I said to myself IF I ever have problems with it, if I find I really need to put a locker in it, if if... I probably will change the center section out with a 3.25 locker (there are no old cars for me to scab 9" off at the pull-n-pay)

I do drive in the mountains, I do drive on the freeway (speedlimit 75, but everyone is going 85), and I do not haul 5,000# (max I haul with this short a wheelbase is a boat or jetski - not pulling any 5th wheels here).

Some mega factors for you to decide - what do you want to do with the vehicle when it is done, all play, all work? What do you want it to look like - stock tires, bit beefier, bit more low ride?

Being mine is a bit showier and less of a mule now I have no problem with the lower gears, but it isn't all pretty - it is my ONLY truck, so it will fill in for anything requiring a truck.

I probably would prefer the 3.25 - aint nobody driving a solid front axle at 85mph and if they are I am not riding with em!

 

1948 3spd LD
1 - 2.81:1
2 - 1.60:1
3 - 1.00:1
R - 3.62:1

1951 3spd LD
1 - 2.78:1
2 - 1.62:1
3 - 1.00:1
R - 3.63:1

 

ahhh, and if my googling was right mine are:
1948 crash box
6.40:1 granny
3.09:1 1st
1.60:1 2nd
1.00:1 3rd

I have no problems launching from a dead start and the granny still gives me low low power. With your 1st being that I would not do a 3.00 myself, 3.25 would be the bottom end "if it were me". A lot of guys have done the 3.5 - for a reason... These old gals aren't ever going to give you great fuel economy, might as well keep the power close to stock.

All my consideration of the T5 has been with a nagging itch in the back of my mind that I do not want to do the T5 without going up to the 3.25 simultaneously.

The decision for me finally came when I solidified other parts I did it in this order
1) tires, 2) transmission, 3) driving needs and style... after you have those 3 then the 4th one (rear end ratio) is pretty much dictated and not a subjective decision:

Tires, Gears, MPH and RPM Relationship Between

 

The transmission not mentioned yet:
3 speed heavy duty
3.71:1 1st
1.87:1 2nd
1.00:1 3rd

The problem with using a 3.00 axle ratio with a light duty three speed is the abuse to the clutch you have to do to get your truck moving in first. The solution to this is to use a heavy duty three speed. This can be seen when comparing the total gear reduction.

rear axle ratio____first gear ratio____total gear reduction
3.91____________2.82____________11.03
3.00____________2.82_____________8.46
3.00____________3.71____________11.13

Please pardon the strange underlines, it is what I had to do to get the table to read correctly.