Need some guidance. New owner of a 78 FJ40. Would like to set-up the cruiser for improved highway travel (higher top end.) Believe there is a 4:11 rear in the vehicle now, and would appreciate conversion recommendations. Additionally, if the rear is modified, will changes to the front diff also be necessary? Thanks!
78 FJ40 Gear Ratio
- Thread starter Pinhi70
- Start date
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Josie'sLandCruiser
Stop calling it a "FJ."
Hi All:
Ha!
Sorry "Pinhi70" but you should be posting in the 40/45/55 Series section!
That said, where are you at?
If we are speaking of a USA-market 1978 FJ40 (with 2F engine) the axles will have 4.11 ratio gears with the coveted fine-spline pinions. First gear ratio of the H42 four-speed transmission is 3.55, while the low-range ratio of the transfer case is 1.96.
Regards,
Alan
Ha!
Sorry "Pinhi70" but you should be posting in the 40/45/55 Series section!
That said, where are you at?
If we are speaking of a USA-market 1978 FJ40 (with 2F engine) the axles will have 4.11 ratio gears with the coveted fine-spline pinions. First gear ratio of the H42 four-speed transmission is 3.55, while the low-range ratio of the transfer case is 1.96.
Regards,
Alan
if you re-gear the rear then yes you will have to re-gear the front to match or you will destroy a differential or blow the t-case apart .
crushers
post ho
IF it is stock then a North American 1978 40 series will have 3.70 gearing.
check your plate on the firewall and report back the axle code.
since the truck is that old then unless you know the history someone "might" have changed out the center units.
jack up the rear tire one side, block the front tires, mark the driveshaft, put into neutral and turn the rear tire by hand. for each revelution of the tire the driveshaft will turn either just before or just after 4 turns. if before then you have 3.7 if after then 4.11. these are stock LC diffs, if it turns roughly 4 and 1/2 turns then you have 4.56 and if almost 5 complete turns you have 4.88s after market gear sets.
helps?
check your plate on the firewall and report back the axle code.
since the truck is that old then unless you know the history someone "might" have changed out the center units.
jack up the rear tire one side, block the front tires, mark the driveshaft, put into neutral and turn the rear tire by hand. for each revelution of the tire the driveshaft will turn either just before or just after 4 turns. if before then you have 3.7 if after then 4.11. these are stock LC diffs, if it turns roughly 4 and 1/2 turns then you have 4.56 and if almost 5 complete turns you have 4.88s after market gear sets.
helps?
why not just get larger tires for the rubber overdrive, alot easier and cheaper. 40's aren't really the best vehicle for lots of highway driving anyways.
crushers
post ho
personally, i would install the H55F behind the 2F and be done with it...
Your truck should have had 4:11's from the factory. 3:70's were standard in 79 or 80 and up. Cheap overdrive = 33" or larger tires. Driving that old truck at speeds over 65-70 is scary frankly. An h55 5speed swap is more involved than most think. You would have to reconfigure an e-brake on brake calipers or cut the crossmember on the frame, driveshaft mods are also required. If you've got a 2F than its fairly bolt on up front. The later split transfer case is a bonus (you'll need to run this as well as spline counts are different I believe) its stronger and you can drop in lower gears for better wheeling.
If your running a chevy engine then use a ranger torque splitter as an overdrive to replace adapters and use a chevy bellhousing. Very simple install and well worth it, this is the way I'll be going next time.
If your running a chevy engine then use a ranger torque splitter as an overdrive to replace adapters and use a chevy bellhousing. Very simple install and well worth it, this is the way I'll be going next time.
So, if you're looking for improved highway, I'd either put a mild lift and bigger tires (rubber overdrive) or put in 3.70's. You're still not gonna be cranking down the freeway at mach 8, though.....that's just part of owning a 40.....you gotta know the limitations of the vehicle and live within them. 
....interesting TLCA#......but here's the Mud official welcome salute....
....interesting TLCA#......but here's the Mud official welcome salute....
dgangle
total rice
I changed my complete third members to get the 3.70:1 ratios inexpensively. With 33" MT's, 70MPH =~ 2800 RPM. Tires , thirds and disk brakes brought the thing out of the stone age.
I am not concerned about crawl ratio, obviously.
I am not concerned about crawl ratio, obviously.
crushers
post ho
low range is accomplished with lower t/case gearing. (and H55F)
I must be dumber than a brick. I followed the above directions precisely but one complete turn of the rear passenger tire turns the rear driveshaft just slightly over one and three quarter (1 3/4) revolutions. I checked it twice. I'd be willing to bet my life that the cruiser has the original differential. I have a stock 79 FJ40 running P235/75 R15 tires with no lift kit. I'm told these tires are the modern day equivalent to the factory original. This is the transmission and axle code off of the tag on the firewall: H42 K0520-12.
So what gear ratio do I have in my differential?
3.70. With an open differential you turn the tire two revolutions.
Jason
How many revolutions of the driveshaft should I expect to see with 2 revolutions of the tire?
No, it won't.
Correct.
3.70 / 2 = 1.85.
If you leave it on the ground, one complete turn of both rear tires will be 3.7 turns of the driveshaft.Original for a '79.
MOTOV8R
SILVER Star
3.7, but you really don't need to do that since 1 rev was less than 2 you have 3.7
I didn't see the follow up messages before I retested it yesterday. Just to be clear it is a 79 built in February 79. As far as I know the drive train is completely stock.
I jacked up the rear driver's tire. Two (2) revolutions of the tire turned the driveshaft just a little over 3 1/2 revolutions. So, can I safely assume I have a 3.7?
The rear diff needs to be rebuilt. Assuming I don't need or want to make any changes to the front diff do I need to keep the rear diff at the current ratio?
I jacked up the rear driver's tire. Two (2) revolutions of the tire turned the driveshaft just a little over 3 1/2 revolutions. So, can I safely assume I have a 3.7?
The rear diff needs to be rebuilt. Assuming I don't need or want to make any changes to the front diff do I need to keep the rear diff at the current ratio?
MOTOV8R
SILVER Star
the front and rear ratio need to match.
Stock '78 should be 4.11, mine is a manu date 10/78 and it is 4.11.
Perhaps I would have been better off starting a new thread. The FJ40 I'm asking about is a 1979.
Sorry was responding to Pinhi70, didn't he start out the thread? And listed his rig as a '78.................
