I'm looking to get more MPG. I'm actually getting about 12-14mpg city. Right now I try to avoid driving on the freeway, mainly because I have to rev over 3k RPM to get around 60-65 MPH. My truck is a '76 SR5 and has a rebuild 20r engine, Weber 32/36, original 5 speed and according to the Toyota reference website my truck is equipped with the 4.375:1 differential. My tires size are 205/60R15 and the idea I have at the moment is to swap the axle from the newer trucks like an 84+ models and look for a decent diff ratio that will work for me. what I really don't know is what to look for, do I need the whole axle? just the diff? are they compatible? custom drive shaft? and finally how to check for ratios on the differentials, numbers, labels etc. thanks!

PS. I know that playing with gear ratios has consequences, but I'm not worry about that. I mainly use the truck to carry my bicycle when I go mountain biking.

 

If you have the w50 transmission, you're going to want to shoot for gearing between 3.73 and 3.4 for optimum results. Even a 4.10 gear would help. You should be able to get most or all of these ratios out of the 79-83 trucks.

If you don't haul loads, I can say a 3.73 gear is a reasonable mix between takeoff speed and gas mileage. In a hilly area with a mix of driving that alternates between mild city driving and 70mph cruizing speeds, you can reliably get 25-27 depending on your driving style (I've heard of 30 in this type of driving if you don't have many lights). If you're a flatlander with 55-65 cruising speeds all day for long even stretches (40+ miles each way or so), there at least one dude who's gotten in the 35mpg range.

Having the weber mated to the excellent flow of the 20r helps. Bumping up your compression a tad would help a little more. 4 into 1 long headers into a 2" or 2.25" exhaust would help a little more without robbing your torque or pushing your powerband too high up the rpms. A 3.43 rear would help a little more (and get you a 70mph cruising speed below 3k). Add enough things together, and you end up with one heck of a gas mileage.

 

Play with this:

Interactive RPM and MPH Computator Program

Also, the first and simplest adjustment to better gas mileage is "change the nut behind the wheel". Driving style makes a big difference.

 

You should be able to get the centres out of any G series diff. I think supras had 3.5's. Some people say they had 3.0 but I've never laid eyes on them.

 

toyota's have a 3rd member, so just the diff.

there is a build plate in your truck on the firewall, looks something like this.

the important bit here is the axle code (letter)(3 numbers) that one is from a 75, it has a G code diff (8in) with a 4:38 ratio

i used this site to decode Toyota Differential Gear Guide

toyota refrence is AWESOME, but always check your build plate to confirm.



real big question is do you have a F or G code diff (7.5 or 8in), go to the junkyard and get a new donor third, ALL SAME CODE TOYOTA DIFFS ARE INTERCHANGABLE, except newer stuff +89 or IFS 4x4's. you can transfer the guts around but there are plenty of bolt in options for 3rds.

 

Thanks for the help guys. Yes. I already looked at the plate and it is a G102 axle and I've been playing with the calculator that Scomber posted and I think I will go with the ratio that Tuhd recommend me (3.73) according to the calculator it will jump from 61MPH@3000RPM to 72MPH@3000RPM I think that's going to be fine.

 

So, if I'm not mistaken I need to look for a truck with the G302 number to get the diff (3.73) I need and just swap the pinion and ring from that diff to mine right?

 

No need to swap the pinion and ring or even take the new (to you) center apart, unless you want to rebuild it before install or its too "sloppy". The Toyota diffs are like the Ford 9inch in that the whole front "center" will come out as one unit. Just remove it from the donor truck, check it for clearance and install in your truck. (With all appropriate sealing of course)

 

Also if you were to change the entire rear end, the width is wider as you get newer gen by gen. My old '78 20r with a weber and a header got 20-23 city and 25-28 highway. Hit 31.5 doing 60 on a road trip once. A good ignition coil helps burn the fuel to get all the power out of your fuel too. Also, check for fuel leaks and see what size jets are in the weber if you got it used.

For gearing I would say a 3.5ish is perfect. 4 speed trucks usually got around a 3.08. 5 speeds got the 4.10s and 4.38s. V6 5 speed usually got the ones you're after. I have a beefy motor so I plan to see how a 4 speed 3.08 works out. Might be a tad long on gears but should get nice cruising rpm and mileage.

 

I have a 4 speed and 4.10's, to my knowledge.. G082 axle code

With my weber 32/36 and 4speed I get about 20mpg mixed driving, and I don't exactly have a light foot

 

I get around 20 with the 22r I have now. I'm not light foot though. I think something may be wrong on his truck if its only getting 12-14. Search for a leak. Speedo could also be off throwing the mileage way off.

 

Thanks for the advice everyone. I'll keep my eyes open for a diff next time I go to the junkyard. as for the fuel consumption I've tried almost everything, new tune-up parts, gaps, distributor advance, checked for fuel leaks, brake check, rebuilt stock carb, didn't make any difference, bought the Weber carb brand new and It was a little better, with the stock carb I was getting 9-11 mpg. I don't have a heavy foot. I even try to shift as soon as possible to avoid reving the engine too much, as a fact of the matter I feel like I'm driving a semi because I shift into 5th gear in like a half of a block, sometimes I even have to double check if I'm actually in fifth gear... As for power I once hauled two quads going through mountains (Escondido-Ocotillo Wells) and I didn't had any issues with it, just the temp went a little higher than normal but it was almost nothing.

 

I have a g082 4.11 in mine, I have swapped from 4 to5 speed w55 box. First gear is almost pointless and it still revs higher than I want at highway speeds. Does anyone have experience with swapping to the g662 3.071 rear pumpkin? I don't want to go too low on ratio, but the 3.07 is all I can find locally at the moment.

 

I haven't run a 3.08 but I think the stock engines can handle it. Lots of 4 speed trucks came that way. Sometimes you have to make a small notch in the housing for the ring gear but that's the only issue I've ever heard of with swapping diffs

 

So I went to the junkyard this last weekend and for some strange reason, all theToyota trucks I found didn't have their third member, are those things in high demand or what!? I just found a couple of complete f302 axles but that's gonna cost me more money...

 

So... yes or no on swapping the whole axle?

 

The later axle is wider. So if you want the back rims pushed out a few inches on each side its cool. Just swap the diff, there never was much changed. Only issue I've ran into is the driveline flange pattern changes, so drilling new holes or swapping the driveline and/or ujoint flange and it should work. I swapped the pinion flange but that requires holding it from spinning and setting the preload. Easier to change the other side.

 

Never stick an F code into a Toyota pickup unless you're hauling it in for scrap or its going in a celica. They are too light duty, so unless you have one stock that hasn't blown, stay away from them. G codes are better.