Taller top gear makes a real difference (10%)

[tjts1]

Great idea and excellent write up. I drive a 1991 bmw 318i that came equipped from the factory with a 4.11 LSD differential which runs about 3300rpm at 70mph.
http://farm1.static.flickr.com/231/5...dc947c06_o.jpg
Not only was the high RPMs on the highway annoying but first gear was virtually useless for a daily driver. I had to shift to second almost immediately after starting from a light. I found myself using 4th and 5th gear on city streets more often than I was using 3rd. Eventually I replaced the differential with an open 3.64 which made a world of difference. Fuel economy improved by about 10% in all driving situations and the cabin is a much quieter place to be.
http://farm1.static.flickr.com/190/5...13535d9f_o.jpg
On the E30 BMW offered virtually any diff ratios between 2.79 to 4.44 while they kept the gear box ratios the same.

[Red]

Sweet swap

[ebacherville]

barring a spedo being off wouldn't the simplest gearing change be a tire size swap... I was looking at old beetle tires.. 165/80-15's same tire patch size as my stock 165/70 13's but 15% taller..

For the speedo i was going to run a GPS.. most likely far more accurate than my 10 year old spedo.. lol

to calculate mileage just correct the miles by the tire size difference.

Best part is its easily put back to stock and only about 220 bucks to try a different gearing ratio with a new set of tires..

[rocket]

Originally Posted by ebacherville

For the speedo i was going to run a GPS.. most likely far more accurate than my 10 year old spedo.. lol

GPS is accurate, but likley less accurate than your speedo. the best you can hope for is 3m accuracy w/o dgps. my gps is 1-3mph faster than any of my vehicles speedos, inc. my '04 tacoma

[cfg83]

Multiades -

Welcome to EM! That's a great mod. I've done the same type of 5th gear swap on my Saturn, but I haven't published mine. I agree that it's a big boost. I basically "live" in 5th gear probably 90+% of the time. I'll publish mine soonish. Thanks for reminding me.

CarloSW2

[IndyIan]

Originally Posted by bhazard

I just wish my turbododges had overdrive.... 3 speed autos FTL!

I think my neon has the same transmission, does yours lock the converter? For pulse and gliding I think our 3rd is a benefit as it locks from 40mph. Most autos are hunting around slipping until 60mph with any kind of acceleration. For highspeed highway cruising I agree, although I've got a few tanks at 6.5L/100km averaging 110km/h with 130 at times.
I would trade the auto for a manual in a heart beat though.
Ian

[bhazard]

Yea the neons have pretty much the same trans.

The shadow has lockup, the spirit unfortunately does not. They didnt have strong enough lockup clutches and such back then to take the turbo motors power.

[Multiades]

Wow, thanks for all the replies guys! I monitored the thread for a while then forgot about it. Glad to see it was educational for so many people.

I noticed this thread on the front page of the site:

http://ecomodder.com/forum/showthrea... swap-2779.htm

That's a pretty big ratio change. 21% is amazing -- I wish they made such a thing for my car!

Right now I'm driving a miata which gets pretty bad gas mileage for its size. I suspect this is largely due to the engine being poorly optimized in the low range (intake manifold design and cams), so peak BSFC is at higher RPM and is a lower value there because of higher friction. The car really can't get out of its own way below 3000rpm despite being a couple hundred pounds lighter than the Protege LX and having the same engine.

It is possible to change the final drive ratio, but much taller ones are hard to find and making the speedometer read correctly again may not be possible.


BTW: It turns out the automatic protege had a taller top gear (4th).

-Chris

P.S. Speedo is not affected with changes to individual gear ratios. Final drive changes CAN affect the speedo, but in FWD cars generally do not.

[Daox]

Taller top gears in automatics are really very common. That is one reason why some of the new automatics actually get better highway mileage than the manuals. The reasoning behind not putting in taller 5th gear sets I believe is only to accomidate the lazy consumers who don't wish to downshift to pass. Pretty sad when you think about it. That change alone could net every manual transmission car another 5% at least on the highway without ANY increase in price in the vehicle.

[Multiades]

Originally Posted by Daox

Taller top gears in automatics are really very common. That is one reason why some of the new automatics actually get better highway mileage than the manuals. The reasoning behind not putting in taller 5th gear sets I believe is only to accomidate the lazy consumers who don't wish to downshift to pass. Pretty sad when you think about it. That change alone could net every manual transmission car another 5% at least on the highway without ANY increase in price in the vehicle.

So true. It's cool from an aftermarket standpoint, but such a waste.

And some of that is to prevent lugging and need for downshifts at speeds above 55mph, because everything is designed for a theoretical driver that isn't the norm.