As I’m going ahead with my street/track N/A build, I got to thinking about rear ends. My GXL has a 4.1 ratio and I always thought this was the highest available ratio til I learned of the GTUs with the 4.3.

my question is, will I notice a difference jumping from the 4.1 to the 4.3? Or am I better off buckling down and putting a made 4.7 unit in it? Obviously the 4.3 is going to be far cheaper..,

 

yes, it is noticeable. you can also run shorter tires, going from 205/60/15 to 205/50/15 makes about the same difference

 

The gear ratios of the stock transmission work far better and you get that much more torque multiplication when you go with the 4.78. It eliminates the use of 2nd gear in all but the slowest of corners. The 2nd to 3rd gear change has the larger gap and drops the engine out of powerband. The 3rd to 4th is the smallest gap. This is for an 8k rpm redline engine. So the car from 60 to 120 uses only two gears.

My calcs always showed the 5.12 was too tall.

 

I would love to drop 2nd. Is the 4.7X really the way to go? I have no problem ponying up the cash if that's the best R&P Im gonna get.

 

My boss always handles tire stuff, it goes over my head. When you say shorter on the aspect ratio, do you mean a small assembly as a whole, or that a wheel+tire assembly would benefit from a larger rim, shorter sidewall? Cause I've got some dinky *** wheel+tires on it right now, but the tires have a LOT of sidewall. I was planning on going up in size as a whole, but with larger rims, shorter sidewalls...

 

the main thing is the overall diameter, smaller acts like a lower gear.

the stock size is a 205-60-15, which is has a bigger diameter than a 205-50-15...

https://tiresize.com/calculator/

 

I like the 4.3 but it's all I know. I would question how well the 4.7xx would be for a car that still sees street use. On the highway/ interstate it's really going to be spinning. I'd guess 4k rpms or more at 70mph. I'm at 3.5K rpm@ 70mph. Being a GTUs I have the shorter .756 5th gear so your results may vary slightly due to the different 5th gear ratio. Flywheels I'm slightly concerned with how I'll like the 4.44 I'll soon have on the highway.

P.S. I should have my old 4.3 up for sale in a month or two.

Edited to add. I found this site and my numbers were off. https://spicerparts.com/calculators/...rpm-calculator
According to them I'm going 79.79mph @3500. I think my speedometer reads wrong. Of course I haven't driven my car in a few months 'cause it's in pieces.
Going by the .697 5th and the 4.778 gear Mazdatrix sells you'd be going 77.88mph @3500rpm. So possibly livable depending on your 5th gear ratio.

Since you may not be able to pull in 5th gear top speed may be in 4th which goes from 137.87 @8000rpm with the 4.3 to 124.08 with the 4.778.
Food for thought.

 

https://www.blocklayer.com/rpm-gear.aspx

at 7000rpm in 5th with the stock tires and 4.1 diff, you're going 176mph....

 

I have a manual conversion power rack on the car right now, it’s already unfit for freeway use. Besides I almost never drive on the freeway, and if I do it’s for very short spurts, so going 55 in the right lane is ok.

 

I added to my original post and corrected some numbers. Found a mph calculator and I feel better about my future gearing now.

I would think for a future track car you'll have to cure any issues that prevent you from going over 55mph comfortably. With the 4.778 that site says 3rd tops out at 90.65mph @ 8000rpm.

P.S. I used 24.9" which is the diameter for my current brand of 205/55/16 tires.

 

I think I like your site better. Less number entering once initially set up. My car would do what the speedometer says. IF I had enough power to pull 5th.

 

Well my joke about the "unusable for the highway" stems from the manual rack having almost no play to it, and not returning to center by itself. Its fine for short bursts, but when youre talking 20+ miles on the freeway having to constantly be adjusting your wheel because its drifting all over the road? no bueno. Its a 14:1 ratio rack too so its quick.
Im sure itll be better when I get my coils in it with some caster/camber plates and I can put a little caster in it. Maybe I need a bit more toe out? Havent checked it since I dropped it in...

 

I actually have two kia sportage 4.78, 7" gear sets ready to go to the machine shop. There are a few shops that do them. Rebello racing has beeen mentioned in the past. If you're interested and we can find a shop that has done a few of these you're welcome to the 2nd set.

 

How much you looking to get for the set?

 

i like zero toe, it felt the best. response was best and it was not darty at all. adding toe in or out made it feel less sharp.

 

do you have a manual rack? The biggest issue is there’s no natural return to center around 45*. So you have to physically move the wheel back to center. Combined with the twitchy steering, this makes it very unstable at speeds

 

I plan to buy a 4.78 and reach out to Rebello racing this week. If Les doesn't take it, I most likely will.

Hell, we wouldn't even have to send them up here, right to Rebello.

 

the last NA i had was an 87 GXL, and i had a really good play with the alignment on that, and it drove great. it had power steering.

current car is manual steering (for a bit, going to put the pump back on), and it was a big project, so have no clue where the toe is. playing with the alignment is just about the next thing on the list though

 

I'm the original owner of a '90 GXL with MT. It came with open differential and 4.100 R&P. In Jan 2003, I installed a new Mazda S4 clutch-type LSD and 4.300 R&P. I bought the GTUs 4.300 R&P (p/n M066-27-110A) new from Ray Crowe at Malloy Mazda. Basically, the performance increase going from 4.100 to 4.300 is 4.9%. So, slightly faster acceleration off the line and slightly lower ET, but nothing to write home about. So, if you're driving at 90 mph (or whatever the speed would be) at 4000 rpm with the stock 4.100 final drive. Changing to the GTUs 4.300 R&P, at 4195 you would be going the same speed. So, that's only about 200 rpm difference. IMO, not enough difference to experience the WOW! factor. I think the 4.300 was just a Mazda marketing gimmick to promote the GTUs model. Personally, the only difference I notice is that I can drive on a flat road in 5th gear at about 45 mph whereas with the stock 4.100 I need to be up near 50 mph in 5th. If you can pickup a used 4.300 R&P at a good price, than I'd say to go for it.