i need to know what gears to use on my old stuff. 1990 xlt 4x4 with 3.o8 is at 1350 rpms at 60 in auto od with 33 inch tires would like to see more like 2000 or so truck has 351. also bronco has 351 but am putting in a 450 horse 429. rpms are now at 3000 at 60 i think it has 3.50 in it now but the 429 doesnt need gears to mke power it also has 33's what gears to use to lower rpms to about 2000?

 

The first one's easy:

(2000/1350) x 3.08 = 4.56

I checked the numbers on a gear spread sheet and they looked right. You may want to go to 4.11s instead, you'd be pulling about 1800 RPM at 60. Check with your rear end shop, I've heard there are some installation issues with ratios higher than 4.11.

The second one is giving me a problem. I'm assuming from your high RPMs that you don't have an OD tranny, but it looks like you don't have 3.50s either. With 4.56 gears you would be making something like 2900-3000 RPM at 60 in a 1:1 drive, so that would be my best guess. If that's the case you would want to go to:

(2000/3000) x 4.56 = 3.039 or 3.08

Is this really enough gear, even with a 429? A set of 3.55s would run about 2350 RPM at 60, 3.27s would run about 2150. Either might give you a lot more useful range than 3.08s with no appreciable loss in fuel economy.

If you need any other numbers crunched, let me know

 

thanx strangeranger how did you get these figures? how do you know for sure how many rpm's you'll turn at 60 in od. 4-11's for the f-150 sound good and i know they make a 4-10 for the front axle.. how many rpms at 60 in od with 373"s? also bronco is an 83 so no od c6 is 3 gear auto an it does have 3.50 and it is at about 3000 at 60. how did you get 2800 with 4.56? cause im tellin ya its at 3000 with 3.50......... thanks for the help what about 3.27? and will 3.08 gears swap from one 8.8 to another?

 

Les,
are you sure the bronco is shifting out of 2'nd gear? Anyhow, unless the bronc has a 351 C or M, you'll have to change the trans with the 429 & be sure it matches the transfer case.

------------------
97 Lariet-sc-4.6-4x off road-auto-3.55 LS-
superchip-K&N-Poweraid-3"
pvc air box mod-Granetelli MAF

 

les 1,
I know from manufacturers published specs that 275/60-17s which are 30" in dia roll at about 695 revs/mile. From that I estimated your 33" tires at 640 revs/mile. At 60 MPH, that's 640 tire revs/min.

3000 engine RPM / 640 tire RPM = 4.69

or close enough to 4.56, since the tire revs/mile was an educated guess and speedometers aren't exactly accurate anyway.

If you really had 3.50 gears, the numbers would look like:

640 tire revs/min x 3.50 = 2240 approx. engine RPM at 60 MPH

If Redlariat is right and you're still in second at 60, then your revs would be:

640 x 3.50 x 1.46 2nd gear ratio = 3270

so that doesn't quite match your tach either. Have you tried jacking the rear of your truck up, putting it in neutral and turning the driveshaft by hand and counting shaft revs per wheel rev? That would settle it for sure.

For your late model with the OD trans, there's another step involved:

640 tire RPM x 3.08 final drive x .7 OD ratio = 1380 RPM, again close enough.

Running the numbers with the new gears:

640 x 4.11 x .7 = 1841 and

640 x 4.56 x .71 = 2042.

If you check your tire manufacturer's web site, they probably list revs/mile for your tire. If so, you can sharpen up the numbers a little.

[This message has been edited by StrangeRanger (edited 05-23-2000).]

 

hey guys thanks its gonna be 4-11 for the f-150, and not sure on bronco yet and YES its out of second gear im not a woman....also bronco is getting a detroit locker,and the c-6 should bolt up if not i will get big block tranny. those 4-11 gears should be a huge improvement over 3-08,s huh. thanks again