Rear Axle Replacement 3:90 or 4:10

[sunvader]

Hi Folks,

I plan on changing the 5 bolt rear axle on my 1980 18' Sunrader (1978 Toyota Chassis). In my search for a rear 6 bolt dual wheel axle I have the option of a 3:90 or a 4:10. Does it matter which rear axle I choose. The reason for the change is obviously the recall, however I do need the capacity to tow a boat that is about 1200 lbs (boat motor and trailer) with 100 lbs tongue weight.

The questions:

3:90 or 4:10 (does it matter)?

Does it matter which year the doner truck is, given that it is a 1987 - 1993?

Pending the year, will this axle just bolt on?

Any tips?

Thanks in advance!

[turtle]

I plan on changing the 5 bolt rear axle on my 1980 18' Sunrader (1978 Toyota Chassis). In my search for a rear 6 bolt dual wheel axle I have the option of a 3:90 or a 4:10. I do need the capacity to tow a boat that is about 1200 lbs (boat motor and trailer) with 100 lbs tongue weight.

The questions:

3:90 or 4:10 (does it matter)?

Does it matter which year the doner truck is, given that it is a 1987 - 1993?

Pending the year, will this axle just bolt on?

Simple questions, complex answers........

The most compatible axles will be 1986-1988. Later axles will use wider springs and different spring mount plates and require some extra fiddling.

Your vertical shocks will not work with the new axle U-bolt plates as from 1984 on the shocks were mounted front and rear of the axle not directly above to the cross member. What I did was to purchase some 1988 4x4 upper shock mounts (they go on the top of the frame) and weld them on to the frame (I have a 1981).

I went with 456:1 gearing (not a Toyota option so no point looking for this as a standard gearing). 410:1 would be best if you have that choice. The lower the gear ratio the better.

I doubt that you will be able to adequately pull a boat unless you change out your engine for a V-6 or better yet a US made V-8 the 22R (and yours is probably a carb engine) is underpowered to pull the motor home and when you add a lot more weight to drag around you will over tax things more. You may get away with it for a little while if you have a 4 or 5 speed gear box but an auto will over heat on you and leave you, your motor home, and boat somewhere where you don't expect to be.

If you feel that you must pull a boat I would highly recommend that you look into one of the Chevy V-8 engine/trans refit kits. http://www.downeyoff-road.com/EngineCompon...ersionKits.html

[sunvader]

Simple questions, complex answers........

The most compatible axles will be 1986-1988. Later axles will use wider springs and different spring mount plates and require some extra fiddling.

Your vertical shocks will not work with the new axle U-bolt plates as from 1984 on the shocks were mounted front and rear of the axle not directly above to the cross member. What I did was to purchase some 1988 4x4 upper shock mounts (they go on the top of the frame) and weld them on to the frame (I have a 1981).

I went with 456:1 gearing (not a Toyota option so no point looking for this as a standard gearing). 410:1 would be best if you have that choice. The lower the gear ratio the better.

I doubt that you will be able to adequately pull a boat unless you change out your engine for a V-6 or better yet a US made V-8 the 22R (and yours is probably a carb engine) is underpowered to pull the motor home and when you add a lot more weight to drag around you will over tax things more. You may get away with it for a little while if you have a 4 or 5 speed gear box but an auto will over heat on you and leave you, your motor home, and boat somewhere where you don't expect to be.

If you feel that you must pull a boat I would highly recommend that you look into one of the Chevy V-8 engine/trans refit kits. http://www.downeyoff-road.com/EngineCompon...ersionKits.html

I have a 4 speed. I'm not big on engine upgrades, I would have bought a ford or GMC motorhome if that was the case. Maybe a 22RE, but I think I'll see how it goes with the axle upgrade. So what you are saying is that even an 88 axle will require some modifications to install it? Would a 410 have a lower top speed then a 390?

[turtle]

I have a 4 speed. I'm not big on engine upgrades, I would have bought a ford or GMC motorhome if that was the case. Maybe a 22RE, but I think I'll see how it goes with the axle upgrade. So what you are saying is that even an 88 axle will require some modifications to install it? Would a 410 have a lower top speed then a 390?

Axle from a 1988 will use the same spring mounts as yours. If you use your old U-bolt cap you can use the same style of shocks as before. Going to the later style shocks you get a beefer shock then the early kind but it will require you to weld on upper shock frame mounts and use the U-bolt caps from the new axle/truck.

You may want to consider adding an additional ride spring when you have everything apart. I have done this on both the 81 and 91 motor homes I own and it does make a difference. I got both of my extra ride springs from Downey off road: http://www.downeyoff-road.com/SuspensionCo...ts/1979-95.html

4.10:1 gearing will turn less times per engine rev than a 3.90:1 so the top end will be less but I doubt that you will notice the difference in top speed. You will notice better pulling power with the lower gear ratio. Your stock gear ratio may be 4.10:1 now because you have a stick shift. 3.90:1 were normally put into vehicles with automatic transmissions

[nolan]

Axle from a 1988 will use the same spring mounts as yours.

4.10:1 gearing will turn less times per engine rev than a 3.90:1 ]

If I may make a couple of corrections, the 78 chassis is narrower than the 80 and up chassis so the spring mount pads will need to be cut off and relocated inboard on the axle housing aproximately an inch each side. The lower Ubolt/shock mount is too narrow for the 1 ton floater axle so new lower mounts will need fabricated or OE ones found.

The 4.10 gears will actually be turning faster than the 3.90 gear set , for every one revolution of the rear tire the driveshaft will turn 4.10 times.

Personally with the motorhome weight, I would opt for the 4.10 gearing, it'll give better power with a small sacrifice to top speed.

[turtle]

I have found that 4.56:1 is a better ratio for these motor homes. Top speed is still easily 65-70 but hill pulling is done in 3rd gear (45-50 mph) instead of 2nd gear (with a 5 speed). I will be changing out the dif gearing in the 91 Sunrader to 4.56:1 this next winter for next years adventures.

Tire size will change the effective gearing also. Putting on larger tires is the same thing as going to smaller gearing as it takes less shaft revs per tire rotation. So if you have a 4.10:1 and step up to 195 or 205 tires you are effectively changing the gearing to 3.95:1 or 3.80:1 by using the larger tires instead of the stock 185 tires.

Here is a nice online calculator that will tell you what you are doing when you change gearing or tire size http://www.4lo.com/calc/gearratio.htm