Axle Gear Ratio and Mechanic Recommendation

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kezamora

10 Year
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Location
Corrales, NM
I have installed a 4" lift kit and 35" tires on my 1982 FJ40. I need to change the axle gear ratio, I am guessing to 4.88 but would greatly appreciate a recommendation on what would be the optimum ratio. The 40 has the original 2F motor in it which is fairy gutless with the OEM ratio even with headers, HEI distributor and weber carburetor. I have a rebuilt SBC 350 ci motor that I may install later if the 2F doesn't have the hp and torque to be suitable for offroading and highway driving. I am also hoping to get a recommendation on a good mechanic to perform this work.
Thanks,
Ken
 
kezamora said:
I have installed a 4" lift kit and 35" tires on my 1982 FJ40. I need to change the axle gear ratio, I am guessing to 4.88 but would greatly appreciate a recommendation on what would be the optimum ratio. The 40 has the original 2F motor in it which is fairy gutless with the OEM ratio even with headers, HEI distributor and weber carburetor. I have a rebuilt SBC 350 ci motor that I may install later if the 2F doesn't have the hp and torque to be suitable for offroading and highway driving. I am also hoping to get a recommendation on a good mechanic to perform this work.
Thanks,
Ken
Click to expand...

What's the original ratio and tire size?
 
I believe you have 3.73:1 gears? If running the 4 speed I'd probably suggest 4.11's. 4.88's would leave highway RPM's on the high side.

For reference, my old 60 had 4:11's with 33's and the 5 speed which worked well with the truck being a bit under geared. I could have ran 35's with that setup.
 
I believe you have 3.73:1 gears? If running the 4 speed I'd probably suggest 4.11's. 4.88's would leave highway RPM's on the high side.

For reference, my old 60 had 4:11's with 33's and the 5 speed which worked well with the truck being a bit under geared. I could have ran 35's with that setup. However, being under geared would have left highway revs too high I think for most people.
 
Also, try RPM Garage in Monrovia. They do a lot of Toyota’s and have a good reputation.

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The original tires were H78-15, which is comparable to a 205/75R15 LT and is approximately 27" diameter. I agree with you on keeping highway RPM down. Will the 4.11 gears be low enough for off roading with the 2F engine? Thank you for your recommendations on gearing and a mechanic.
 
For most of the offloading that we do in our club, do I need lockers? From what I have read, Limited Slip only works when both tires are planted, but would be better overall than floating axles.
 
I have found limited slips to be worthless (have tried them). They let you down right when you need them most.

I don't know how hard your club wheels but 40's are darn capable especially on 35's.

Now the reality. The 4 speed doesn't have the gear split to cover modern highway speeds and give you stock crawl performance you'd get with a deeper 1st gear and OD 5th. My 60 turned around 2500 RPM with stock gears and tires at 55 MPH. That meant it revved a tad high for driving comfort at 65+. Going to 31's helped because that had me 14% overgeared (31/27). Interestingly the H55F has a .85:1 OD. Of course that hurt off road performance.

Going from 27's to 35 is a 30% difference (35/27) as far as speed in a given gear. That is 30% less thrust pushing the vehicle. Going from 3.7's to 4.11's is an -10% difference (3.7/4.11) as far as speed in a given gear. Add them together and you will be about 20% overgeared. That might be nice for the highway but you will feel that on the trail.

3.7/4.88 is a -25% difference which will pretty much offset the +30% difference of the 35's. You will probably rev around 2500 RPM at 55 again and will have to live with a rather high revving engine at typical highway speeds of 65-70MPH.

So.... you might think about 4.56's? That will be a -19% difference and against the 35's +30% which will leave you about 10% overgeared. That might be the best compromise.

If you ever plan to go to an H55F, I'd say go 4.56 with 35's and a 2F. If you had or intend to swap to a more powerful engine and an H55F I'd suggest 4.11's to get the freeway RPM's down.

Frank
 
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BTW, one thing you can do to help yourself on the trail when over geared is air down more. That effectively reduces your final ratio to whatever the distance is from the hub center to the ground. So, if you wanted to 4.11's and have a nice OD effect on the highway you could air down on the trail and reduce that 20% value. Otherwise, if you want to keep the truck 4 speed/2F forever, it sounds like 4.56's are the best compromise.

Frank
 
Wow, thank you. That was extremely informative. I would like a lower first gear and overdrive, so the H55F 5 spd would be a great upgrade to my 40. I will have to research swapping out the 4 speed, I'll need to locate all the correct Toyota parts for the swap. Up first will be the correct axle gear ratio and then seeing how it performs to determine if I really need lockers. With the current gear ratio it can't get out of it's own way. I was initially leaning toward the 4.88 gear ratio, I don't mind keeping highway speed down but want to have the necessary thrust when crawling. 4.56's may be the best all around compromise.
 
I think 4.56's, H55F, and a 2F pushing 35's will make for a very capable rig that can keep up on the highway. If you think you might someday ditch the 2F, you might go 4.11. My had upgraded my 60 to a 3FE and H55f and it did well enough with 4.11's and 33's that I would not have hesitated to go to 35's.

For me the challenge crawling withe a manual is your minimum speed. An auto basically has an infinite crawl ratio. A manual has a min speed before the engine lugs/stalls. The H55f has a lower 1st gear and OD so it lets you have the best of all worlds.

Frank
 
In my builds I have run the following gear combinations:
33" 4.30
35" 4.56
37" 4.88
38"+ 5.29
 
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