Hi, so I'm just wondering if there is any way to get a lower Rear end gear ratio? Can i swap the gears out of a Mazda truck or something else? The car i have is a 87 Base model N/A

Thanks
Matt

 

You've got a 3.9 rear end already, which is the lowest ratio of any FC rear end. There's not going to be any bolt-in rear end w/ lower ratio that wouldn't require a custom driveshaft, mounts, etc...

Why would you want a lower ratio anyway? You'd be trading acceleration for top speed, which means nothing since an NA motor will never run out of gearing.

 

Sorry to say, it may be the lowest number, but it's actually the highest ratio available.

A GTU-s would have a 4.3 ratio, other than that, Mazdatrix sells 4.3 R&P for around $600. Pretty steep for the amount of performance improvement.

 

Ahhhhh, you're right. That terminology is so counter intuitive. The ratio would be larger, but yet it's called a lower ratio.

Lots of 4.1 open diffs and LSDs available. 4.3 LSDs will be tougher to find.

 

Well I was told that I had a 4.10 in the rear. The only reason why I'm wanting to change it is because I don't think the 4.10 gear ratio will be to friendly on fuel with a Small Block 350 and a 3 or 4 speed auto in it. But your saying my rear end has a 3.9 Gear Ratio?

Matt

 

Well, I'm 0 for 2 unless you have an automatic, which did have a 3.9. With a manual trans, you'll have a 4.1 open diff stock. There's a sticky thread at the top of this section that lists all standard features by year/model.

I take it you actually would prefer the 3.9 since it would reduce cruising RPMs and save fuel? If that's the case, '88 convertibles and NA automatics had them.

 

Yea mine was a standard. I would defiantly prefer the 3.9. Is that gear in all NA Autos?

 

Well, if you think changing from a 4.1 to 3.9, which is actually higher gearing, will be a fuel saver, it will only drop the RPM .048%. If you're spinning at 2500 changing from a 4.1 to a 3.9 will save you about 147 RPM and that small of a change won't do anything that is noticeable for putting money in your pocket. Also the small block certainly likes the lower gearing.

 

also on gm 4 speed autos 4th gear is over drive and are actually pretty good for conserving fuel. Depending on your setup, you should be able to get as good as or better gas mileage.

you may be able to run a a larger tire too.

 

I think you will definitely want a transmission with overdrive or your motor will be turning at a high RPM on the highway.

I would suggest a T-56 if you can find/afford one. I *think* an LT1-style T-56 works with an older SBC. If you can find a T-56 with a 0.50 6th gear you will be turning at about 1800 RPM at 65 mph, even with 4.10 rear gears.

If you want to look into using something like that I would suggest checking out www.v8rx7forum.com. They will be able to give you some other good ideas. I have an LT1/T-56, but I don't know what the SBC guys usually run.