47-59 Gear Ratio

[Lowell G]

I am looking for some advise on this please. Just purchased 1959 GMC Model 100. Still pretty much original. Some time in the 1960's Owner was having issues with the engine so he changed it to 327 & put in a manual 4 speed. 1st gear you are gong about 15 MPH before needing to shift into 2nd gear. Too High Speed for me. Any recommendation for rear end gear ratio would be helpful.

[dsraven]

on most 4 speed transmissions 4th gear is a straight through connection, like the input is directly linked to the output, and all the speeds before 4th would be a ratio less than 1:1, so it makes me think that a previous owner has swapped out the rear axle to get a faster road speed. if you could take a few pics of the transmission and the rear axle that would help us indentify what you have to start with and possibly figure out the possible gear rations inside the trans and also the rear axle. does the trans have a floor shifter that goes into the top of the trans like a truck transmission would have, or possibly the shifter is bolted to the floor or the side of the trans and there are rods from there to small levers on the side of the trans in oreder to select a gear?
usually a pic of the rear axle pan cover showing the shape and the number of bolts that hold it on can be compared to an online chart to show what you have. it may have a tag on it that will show the gear ratio or it may be stamped into the axle tube if it is a chevy axle. are your wheels on the rear axle still 6 lug or have they been changed along with the axle?
here is an rpm calculator you could use
https://spicerparts.com/calculators/...rpm-calculator

[dsraven]

the size of your tires, outside diameter, is also important to decide on a final gear ratio. it would also be good to know if the engine is stock and what it has for carburation etc because some engines are put together for higher rpm power and torque while others are built more for lower rpm grunt to tow trailers or whatever.

[dsraven]

here is a tire size comparison chart which has tabs that show tire diameters by wheel size etc. it may help figure out your tire height by your tire size so you don't have to measure and approximate a number.
https://tiresize.com/comparison/

[dsraven]

it's always best to first figure out what you have for driveline and then you know if anything has been changed from original and what you ACTUALLY have. then you can figure out what to change to get what you really want or need.

[mr48chev]

One thing! What 4 speed does it have in in now?

SM 420 SM465 or a Muncie or Saginaw car 4 speed with the shift handle back just in front of the seat? All 4 speeds are not created equal in either strength, weight or how they work gear ratio wise and while 15 might be pushing it with an SM420 it is just comfortably getting ready for you to shift out of first with the Car transmisison.

Now please advise us on what transmission you are working with. Normally with the truck transmission you never start out in first unless you need the compound gears to pull a load or a grade or both.

[dsraven]

yep, thats why I was askig for a few pics so we can see what you have under there. also, what is your rpm at cruising speed and, of course, what is your cruising speed?

[Lowell G]

Thanks for the Help with this. Here is what I have for now. The rear end is original with a 10 bolt cover. Tire size is 235/75/15. The 4 speed manual transmission has GM 0201 on one side & on the other side it looks like ?? 62877889 GM on it. This was done in the mid to late 1960's

[mr48chev]

Is the shfter up in the middle of the transmisison hump or is it back at the seat? Guys always post those casting numbers and they don't mean a freaking thing when trying to decode a transmission but if it has a shift tower in the middle of the hump it is an SM 420 or SM 465. If it is smaller, slimmer and longer and has shift rods on the side and the floor shift fastens to the tail shaft it is a Muncie M 20, 21 or 22 or most likley A Saginaw both being out of cars. They won't pull many stumps but unlike the 7 something granny gear have a 2 or 3 something low gear. 15 mph is nothing with one of those as it is about the same gears as a 3 speed column shift.

[mr48chev]

I'm beginning to wonder If I am reading what you wrote in your first post correctly.

Are you saying that you can only get up to 15 before you have to shift out of low gear ?

Or are you saying that the truck easily runs up to 15 and above before you need to shift out of first gear?

Most of the time with a truck 4 speed with the 7 to 1 low gear we don't start out in first unless we are in a situation where we need the 7 to one reduction. Pulling an incline, pulling a load or both. I will in my 77 with the SM 465 because with the flatbed and hoist and maybe 800 lbs of extra metal on the frame it weights over 8 K empty .

[dsraven]

what you need to do, if you want some help, is post a few pics of what you have. a pic of the inside of the truck showing the actual shifter and it's location relative to the seat and dash, then one of the underside where you could show a pic of the side view of the actual transmission from the driver's side so we can see the outline of the trans and also the location of any levers and rods on the trans if it is not a designated "truck" trans that has the shifter going into the top of the transmission.
here is what a "truck" trans looks like. notice the shifter handle goes right into the top of the transmission
https://www.jeepfan.com/tech/jeeptec...D-gm-sm420.htm
here is what a muncie car trans looks like, I know, its not an actual transmission but you can see the difference between a truck trans and a car trans pretty well and also how the two different shifter arrangements make it easy to figure out what style of trans you have
https://www.flatpyramid.com/3d-model...-transmission/

[dsraven]

so, now the shoes is on the other foot and you have either figured out what you may have or you have grabbed a few pics to show us what you have for a little feedback.

[mr48chev]

I don't know if they will help this time but here are a couple of speed, rpm, gear ratio tire size calculators to play with.

The first is the Advanced EV calculator that you can plug in three knows and get the fourth. https://www.advanced-ev.com/Calculators/TireSize/

The second one is the Pur Performance that only gives rpm when you put in the other factors but will let you see the difference between tire sizes and or gear ratios in both the trans and the rear end.

The last is the tire size dot com page that lets you compare two tire and wheel combos side by side. It will tell you the speed differences between two tires such as a 225-60-15 and a 225-75-15. Throwing a set of low profile tires on a rig can mess with your speed a lot.