Ok, since I have been racking up the miles on my truck lately and really prefer driving it over my new one..lol I am thinking of swapping the 4.10's out for something a little better on the hwy. Currently I am running about 1950rpm at 70mph with the newer lock up trans and 35" tires.. I am getting between 17 and 19.5 depending on the wind!! What would you guess the rpm's would drop by swapping to a 3.73.. I was thinking maybe 1750-1800 and maybe a 1-2 mpg.. Is there a factor chart that anybody knows off hand..

 

Curious, what torque converter are you running? Maybe the lockup clutch is slipping if its stock.

1950 sounds high...

 

Going from a 4:10 to a 3:73 is a 9% drop in gears. Wouldn't that also be a 9% drop in RPM? If so your RPMs would be what 1775? I think my math is correct but check it anyways. Good luck with your swap.

 

I checked around a little on prices for gear sets, the rear from Motive gear is like 450 bucks The front is more resonable at 225.. Just don't know if it will be worth it.. I think it would helm mpg a little and be quiter in the cab, I can tell a big difference inside from 1700 to 1900..

ON the converter, it is stock and probably slips a little, mainly under heavy accel when locked.. But locked up just cruising I doubt it is slipping very much.. I think 1950 rmp at 70 is pretty good with 4.10 gears locked up, I'm sure it would be like 2300 unlocked.. Any more input??

 

I don't think you are going to notice much difference MPG-wise at 70mph with those tires, maybe even lose a little. 34-35" tires seems to be around where the MPG tradeoffs at cruise occur, as far as I can tell. Any fuel you save at speed is used up getting there. What you need at that point is just another higher top gear (6th).

I actually lost 1mpg (average) going from 33s to 34s, in mixed driving, with no gear change. Highway mileage may have went up just slightly, but the slightly higher overall gearing uses more fuel in town and accelerating to more than cancel that out. It's worth it on long, mostly highway trips, tho...

 

speed= tire diameter / 2 / 168 x rpm / (axle ratio x trans gear ratio)

speed=35 / 2 / 168 = .104 x 1700 = 176.8 / (3.73 x .70 = 2.611) = 67.71mph

 

JQmile that would be a good "renewed" article in your magazine. All the old hotrod formulas, hotrod many years ago would put them in from time to time. I had them at one time but over the years have lost them. How project rust bucket coming along?

 

One step ahead of ya....we did an article about gearing calculations, quarter mile calculators, and more, a few months ago....

http://www.dieselpowermag.com/tech/0...ons/index.html

RB is doing ok, got a wastegate on it now, and I'm going to try spraying a five million shot of nitrous to see what happens

 

I am trying to figure out why you are only turning 1950 with 4:10 and I am turning 2300 at about 110k (67mph) with the same gears

 

He's got an overdrive trans with a lockup converter (I think), and huge tires.

 

You don't have 4.10's?

If your truck was originally equiped with a 3 speed auto it has 3.07 gears no matter what. Look on the tag hanging off the diff cover.

 

Ya, I have a newer style trans with lock up and O/D.. Without lockup at 70mph, I'm at about 2200-2250.. It is nice!!

 

With the 5spd (see sig) I am turning about 2000 rpm so your not to far off. Only thing is my truck gets about 12-14mpg if I am on the hwy at all.
DS79

 

Just swapped 391's for 430's in a Barracuda with 28.5 tires and gained 2mph and .10 in the quarter. No noticeable difference in driveability or engine speed. I would think you would need to go taller to see any difference.

 

I just made the change with gears in my 4 door and wow what a difference.

Of course I went from 4.56 with 33's
to 3.55 with 33's

when I hasd the 4.56 it was great off the line, but on the highway the rpms were so loud I could not even talk to the person next to me.. I could not go over 65mph.

Now That I changed gears and have 3.55 I notice the truck is a little slower off the line but not a huge difference but at 65mph it just purrs along.. and I love that.. I am going to be pulling about 16,000lb this weekend so I will see how it does with that.. if it has issues then I am gonna step up to 3.73 and stay with that..

I used to have 4.10 in my std cab '92 and that was loud on highway also.. so I am really hoping 3.55 while loaded down works out..