Both of those equations are the same.

 

Whoops!!!

Your right!

Tire diameter in inches or in metric?

 

inches because it's in MPH.

The gear ratio would be just it's number, like 4.10 or 3.55.

 

ok, sounds good. Y'all rock.

My desired RPM will be through a SMOD 4 spd. So, to get cruise RPM in 4th (OD), it would be

MPH x gear ratio x Final Drive Ratio x 336 / tire size = RPM

50 x 0.81 x 3.73 x 336 / 28.8 = 1762
70 x 0.81 x 3.73 x 336 / 28.8 = 2467

OR: MPH x Final Drive x 336/Tire size x gear ratio = RPM

50 x 3.73 x 336 / 28.8 = 2175 x 0.81 = 1762
70 x 3.73 x 336 / 28.8 = 3046 x 0.81 = 2467

I like the second set of numbers, they tell me what my revs will be before and after the swap

Thanks again, this has to be the best site on the web!!!!!!!!

 

OK, so the .81 is your transmission's overdrive gear ratio? If so, you're right. I didn't think you had overdrive, so I didn't include that in the formula (i'd of given you the 2nd formula instead, didn't want to confuse you).

I don't understand what you mean by liking the second set of numbers, they're the same as the first...

 

but the way I did the math, it gives you third AND fourth gear numbers. Both are important, since I will drive the 3 spd until I can figure out how to hook up the reverse. I am having trouble finding my grandfathers GMC, so it looks like I have to figure this one out without any visual clues.

My grandpa did his swap for the same reason I am, the stick gets in the way. How come no one can help me with the linkage. Seems easy enough, and the payoffs for a column shift 4 speed are plenty, add OD, and seems like there would be plenty of people doing it.

I understand the SMOD isnt as strong as a true toploader, but I am not throwing 750 horses at it, maybe like 450 later on, but probably below 350 for quite a while. I want something I can drive on the cheap stuff (87), maybe midgrade (91) in the summer. I am still contenplating rebuilding the 78 windsor, but I have to get my LTD somewhere to take out the engine/trans.

did the gear ratio above sound ok? I could go a bit deeper to use OD sooner if 1750 will be too low for the engine. I will not be going over 85, and most of my driving will be in the 45-52 range. How much would it affect fuel milage not to use OD until over 55?

I try to keep my engines between 2-3k, 2000-2500 when possible, but I havent driven a v8 since I was 14. can they run lower than these 4 bangers, or do I need to build it to run a little higher up? My goal is a peppy truck which should last 200k miles, I want a small 4 bbl, 12-18 miles to the gallon, fairly quiet exhaust, (no AC, so I need to think with the windows down) and nothing too fancy. No chrome, no loud paint, not low rider, just your basic farmyard Ford. Towing is optional, but it will be my backup DD. Probably running 235/75 15s, N/A, Iron heads-possible upgrade later, if not FRPP crate; comp cam, Ede performer RPM air gap, thinking about 8.5-9.0 compression. 195* thermostat, Duraspark II ignition, Blue Oval dash kit w/auto meter guages

 

In my opinion, for your senario I think you'd want the RPMs to be about 2300 (give or take) at 70 in the final gear.

I still don't know what you did with the math. There is nothing in that which takes into account 3rd gear. You have 2 groups of equations there, slightly different but ultimately the same equation, and then say you like the second one better....but they're the same....so I'm confused by that from what you're saying.


My truck has a 5 spd manual. It's 4th gear is the 1:1 and the 5th gear is the overdrive (.80). The 3rd gear is 1.50ish, the 2nd gear is 2.40ish i think, and the first gear is 3.60ish I believe. The formula I gave you only involves the 1:1 gear. I didn't add in the part about overdrive because I didn't think you had overdrive, apparently you do .

I'll use my truck as an example...

70 x 4.10 x 336 / 35" = 2755 - That's my RPM in 4th gear (1:1) at 70 MPH.

Now if I want to find out what RPM's for my 5th gear at that speed, I just multiply the 2755 by .80 to get 2204. I run 2200 rpm at 70 mph in 5th gear (overdrive).

I could be worng but I get the feeling you thought you were involving 3rd gear somewhere in the formula, but you weren't, you were involving a overdrive gear ratio of .81. Keep in mind an automatic tranny will have some slippage, so allow 100-300 rpm into you're thoughts when trying to determine RPM. That forumla doesn't take into account slippage.

 

doing it the first way (gear ratio b4 final drive), I could not SEE 1:1 gearing, the second set, (1:1 cruise RPM x gear ratio), you get both numbers, this way you can set up the truck to use OD only when needed, and see where direct drive will put you for gear down situations.

 

I don't understand where your getting this "final drive" ratio...I said nothing about it...where did you get this information to perform that? I don't think what you're doing makes sense (atleast it doesn't to me). The .81 is your overdrive ratio, not your final drive ratio. The final drive ratio takes in account, tire size and transmission gearing as well, to get the final gearing at the wheel.

You havn't clarified but it appears that you have a 4 spd automatic, 3rd is 1:1 and 4th is overdrive w/ a ratio of .81.

In that case...

Your truck will run 3046 rpm at 70 mph in 3rd gear with 3.73s and 28.8" tires. It will run 2467 rpm in overdrive. It will run 2175 rpm in 3rd gear at 50 mph, upshift to 4th and that'll drop you down to 1762 rpm at 50mph.


To determine gear ratios, you generally pay attention to what the RPM will be in 3rd and 4th, like I've shown you. Now you can change the MPH to see how the RPMs change at different speeds, but thats how you do it. The 3046, is you "seeing" 1:1 as you put it.

I wouldn't use a 3.73 gear, it appears to be a little high. A 3.55 would be more up your alley.