4.11's iposi rear. what front ratio
- Thread starter 7dhamper
- Start date
V6 Kompressor
New member
Actually, you need to change the gear in the front gear housing to a 4:11 also, and keep the circumfrence/diameter the same for both front and rear tires to keep from screwing up the transmission/transfercase/tires/driveshafts/ect..... Actually, I would change the gear in the t/a rear-end to a 3:73 so you wouldn't have to worry about the high cost of front gears, As a matter of fact, the gear out of your rear end should fit the t/a's as long as it is also a 7.5/7.625... As cheap as the gears are, I would put a new set in the t/a's...
This has been covered before, look at:
http://syty.net/phpBB2/viewtopic.php?t=13467&highlight=
http://syty.net/phpBB2/viewtopic.php?t=13467&highlight=
Re: 4.11's iposi rear. what front ratio
Do you have 3.42's in the front?
if you have 3.42's a 21.27" tire will give you a 4.12 effective diff ratio
assuming you have 4.11 in the rear and are running a 25.65" 4.11 effective diff ratio
with 4.11's you will only be hitting 90mph at 5,000 rpms in 3rd
7dhamper said:
Do you have 3.42's in the front?
if you have 3.42's a 21.27" tire will give you a 4.12 effective diff ratio
assuming you have 4.11 in the rear and are running a 25.65" 4.11 effective diff ratio
with 4.11's you will only be hitting 90mph at 5,000 rpms in 3rd
this may help you
http://www.syty.org/old/calculator.html
http://www.syty.org/old/calculator.html
7dhamper
New member
Thanks U guyz! I knew it had been covered before but search was comming up with all wrong post. I have an '89 GMC Jimmy which came stock with 3.73's and 235/75R15's but no posi! I really only want the wider rear vs. the skinny front, but the taller rear and shorter front will be a nice touch. My main goal is to retain 4X4 set up and swap to Bravado AWD. I'm not to concerned with topping out into overdrive. I just want my 0-60 to be under 5 sec. After that I'll be getting ticket so who cares. This thing is mainly for putting rice in their place on country back roads, not for running QM times. I'll eventually be going to an all aluminum bored and stroked 383 ci. LT1 block running twin turbos 3 inch in and out. Once that is pefected I'll be stripping this rust bucket and swapping everything to a late model S-10 Quad cab with a long bed and dually flares. The Jimmy is just a test platform for the components for what I really want: a daily driver center fold cover truck in what ever zine will have me. Thanks again! Big help!
7dhamper
New member
Allright, here's what I've come up with: BFGoodrich Radial TA 295/50SR16 a nice fat 12.2" section width, 9.5" contact patch
But it's 27.6" tall! What to do?
27.6" / 4.11 rear = "X" / 3.73 front Right? "X" = 25.048"
The closest I can get to that is the factory 245/50R16 @ 25.7" tall
or a 225/5016 @ 24.9" tall if I stay with the BFG Radial TA line.
Will either one of these do? What is my tolerance limitation? If it's greater than 0.148" I'll use the 225 up front. The factory size 245 is 0.652" too big and would be too big of a difference I think. What would be worse? If the front pulled the rear a little or if the rear pushed the front like it does when you go down a hill faster then your gassing would do? I'd like to use the same brand and line of tire on front and rear for a more predictable handling and I know these BFG's are great winter tires so... Once I figure the tolerence I'll worry about the 18's or dubs for the rear.
But it's 27.6" tall! What to do?
27.6" / 4.11 rear = "X" / 3.73 front Right? "X" = 25.048"
The closest I can get to that is the factory 245/50R16 @ 25.7" tall
or a 225/5016 @ 24.9" tall if I stay with the BFG Radial TA line.
Will either one of these do? What is my tolerance limitation? If it's greater than 0.148" I'll use the 225 up front. The factory size 245 is 0.652" too big and would be too big of a difference I think. What would be worse? If the front pulled the rear a little or if the rear pushed the front like it does when you go down a hill faster then your gassing would do? I'd like to use the same brand and line of tire on front and rear for a more predictable handling and I know these BFG's are great winter tires so... Once I figure the tolerence I'll worry about the 18's or dubs for the rear.
7dhamper
New member
SY732 said:
What, is that equation for effective gear ratio based on differential raio and tire hieght? How do I do that?
SY732 said:
Are you saying that running 3.82's vs. 3.73's yeild similar results on the track using stock wheel height? That's how I took it. Who is running either of those set ups? I'd like to see their stats. I know that all Typhoons are stock 3.42's right? I know a lot of guys are running the 17X11 rear and 17X9 front ZR1 rims. How does it effect your QM times vs. using your stockers? One more thing: All SY/Ty trucks have OD and D on the collum don't they? Cuz I always get post about how I'll hit OD before I clear the traps and I was starting to think that maybe OD was automatic on the Sy/Ty where as my Jimmy has a 3 speed D and the OD (right). I also just found out that I might not have a 700R4 tranny! It could very well be a 40LE60 or 40LE80 with this '89 tow package I have. Where do I find the tranny model #'s? Thanks!
TheObiJuan
Active member
4l60e on newer trucks and cars, 4l80e on newer BIG trucks.
700r4 on older trucks and suv's despite their optional package. unless that tranny was put on 5 years after it was built.
700r4 on older trucks and suv's despite their optional package. unless that tranny was put on 5 years after it was built.
7dhamper said:
I was looking at
http://www.syty.org/old/calculator.html
The 17x11 and 17x9's most people rund are the same height as the stock 245/50/16's
we have D an OD, but most prefer not to shift into OD in the 1/4 as the trans should last a little longer in D for one
all I was saying is that 3.73's are close to 3.82's
Hey all- I was the one that started that previous post about this and unfortunately I still haven't got it figured out yet. I've been up to my elbows in fomulas & equations for the past few weeks, and the closet thing I've come up with so far is to swap in a newer T-truck 8.5" rear (hopefully disc 
) and see what I can come up with from there... Well anyhow for anyone who doesn't know, the formula for computing overall tire diameter is: section width (in mm) x aspect ratio (as percentage) x 2 (tire on both sides of rim) x .0394 (convert from mm to inches) + wheel diameter. So pretty much a 235/40/17 is : 235 x .40 = 94mm x 2 = 188mm x .0394 = 7.4072" + 17" = 24.4072 overall height. Also you want to be accurate in these formulas, so don't just use the nominal gear ratio- you have to divide the actual # of teeth on the ring gear by the pinion teeth. So a stock '3.73' front is actually rounded up from 3.72727... (41ring, 11pinion). A 4.11 front is 37 and 9, a 3.42 is 41 and 12. You can also divide the rear ratio by the front ratio, which will give you the difference ratio you will need between tires: (37/9) / (41/11) = 1.10298103. Multiply that by 24.4072 (235/40/17) = 26.92067859 rear tire diameter. Or vise-versa, divide what tire sizes you want to use on the rear by the front, and multiply by the front gear ratio which will give you what rear ratio you will need. The main problem I've found is that most known ratios for our front/ rear diffs are too limited and the difference between them is too great to accomodate with reasonable tire sizes on 17"s and 18"s, especially with the more desirable lower ratios. Well good luck and if anyone knows where to get custom ratios for either the 7.25" front, 7.5"/7.625" OR 8.5"/8.625" rears PLEASE let me know ~> I gotta get my wheels & tires ordered soon!! L8tr
ps- hey make sure whether that T/A rear is a 4.11 (RPO HO4- 37ring,9pinion) or 4.10 (RPO GT5/GT8- 41ring,10pinion)
) and see what I can come up with from there... Well anyhow for anyone who doesn't know, the formula for computing overall tire diameter is: section width (in mm) x aspect ratio (as percentage) x 2 (tire on both sides of rim) x .0394 (convert from mm to inches) + wheel diameter. So pretty much a 235/40/17 is : 235 x .40 = 94mm x 2 = 188mm x .0394 = 7.4072" + 17" = 24.4072 overall height. Also you want to be accurate in these formulas, so don't just use the nominal gear ratio- you have to divide the actual # of teeth on the ring gear by the pinion teeth. So a stock '3.73' front is actually rounded up from 3.72727... (41ring, 11pinion). A 4.11 front is 37 and 9, a 3.42 is 41 and 12. You can also divide the rear ratio by the front ratio, which will give you the difference ratio you will need between tires: (37/9) / (41/11) = 1.10298103. Multiply that by 24.4072 (235/40/17) = 26.92067859 rear tire diameter. Or vise-versa, divide what tire sizes you want to use on the rear by the front, and multiply by the front gear ratio which will give you what rear ratio you will need. The main problem I've found is that most known ratios for our front/ rear diffs are too limited and the difference between them is too great to accomodate with reasonable tire sizes on 17"s and 18"s, especially with the more desirable lower ratios. Well good luck and if anyone knows where to get custom ratios for either the 7.25" front, 7.5"/7.625" OR 8.5"/8.625" rears PLEASE let me know ~> I gotta get my wheels & tires ordered soon!! L8tr ps- hey make sure whether that T/A rear is a 4.11 (RPO HO4- 37ring,9pinion) or 4.10 (RPO GT5/GT8- 41ring,10pinion)