GTs never had 3.18 diff gears all were 3.44 both manual and automatic versions. This means that there are no GT diffs with anything but 3.44s in them so you would have to rebuild one - bearings, set-up etc. at considerable cost. Just get a late model GT diff with the conventional wheel bearings and use the money you save to buy the tiny bit of extra gas you may - or may not - use!
When your gearbox needs replacing consider a Getrag five speed with overdrive top gear.........
I disagree, Jim, and I use the 3.18 rears in pretty much all my Opels. Of course it helps that all my Kadett 1.9s are autos, so they came that way already. I would argue that finding a good Kadett Wagon 3.18 rear might be the easiest option, and come with the rear sway bar to boot.
I can't really comment on the mileage difference, but I figure 27mpg for trip planning puropses driving my Kadetts and always average slightly better on the highway. I have to believe a manual tranny Gt with the same rear would do at least as good.
Guys, a 3.18:1 may not get the gas mileage a 3.44:1 does. Note I said may. My reasoning is, the motor may not have the HP to push the car fast enuff at a lower throttle setting that would equate to the same speed with the 3.44:1. Now for an example, my Honda SL-90 with stock gearing of 14:56, that is a 14 tooth counter shaft sprocket out of the tranny to a 56 tooth rear wheel sprocket got a top speed of 55 MPH on a flat road. The same bike with a 13:70 a week later got 70 MPH. The lower gearing got the engine to turn higher RPM which put it closer to the power band for more HP, thus the power to overcome wind resistance and went faster. I hope I explained it well enuff to understand.
Gearing is a big part of optimum mileage. I agree with Ron in that it will be very "iffy". It's all about RPM and MPH and how you drive the car. This is why I have to agree with Jim that the first thing to do is put the five speed in. It allows you to keep your RPM in the best range for either power or mileage. To sidetrack slightly, notice that GM just introduced a new six speed automatic transmission for this reason... When you think about it, it's not unlike why big trucks have 18 speeds, it keeps you in the best powerband regardless of the situation!
Face it. More gear selection is most important, final ratio is just a part of tuning it all in. Other huge factors are engine tuning, like your camshaft choice, fuel management, exhaust system, even your tire selection and front end setup!
There's just a lot to it, and few cars from the early seventies were designed to be extremely fuel efficient... GT's were only supposed to fun, right? Ultra comfort and economical are terms used together only when describing the brand new plastic cars.
If your idea of fun is making your GT get 40 miles to the gallon, a lot of us are with you but you got your work cut out for you! And it won't be pristine stock any more.
The "mileage" poll is neat. It would be fun to verify the cars with the best reported mileage and see just how it is done. It would also be cool to see the end results of all the fantastic engine swaps that are daydreamed here.
Where's the hybrid GT? The diesels? The Ecotech rubber waster? Oh, they were too much work and a new Geo is only $12,000...
Enjoy your sweet little German version of transportation every way you can, while you can, you only live once...
Darn Jeff, did I see a little dig in the ribs there? Honest, I'm still working on Willit? I think it would be neat to see what mileage I'll get with it when I'm done. That is after I get the fun driving out of my system. An interesting aside here, I got better mileage in my 72 Dodge full size one ton van after I replaced the stock 318 with a full blown go fast 340 out of a Road Runner built by Chrysler Racing in LA.
Oh, Ron, you know yours will be the most intensely modified GT yet. And it will get decent mileage as well as great performance. But how many guys can and will go through with what you have? Some will even snub it as being a "hacked GT".
My S10 gets pretty darn good mileage if I want it to. It can also get bad mileage and eat rear tires if I'm in that mood...