[Wales Garage]

im over the fuel ecomony of my 08sti 16.5-17mpg is a joke. my plan is to get the car to be able to cruise at 80-85mph and return fuel ecomony in the mid-20mpg range. what ive figured so far is that in any gear over 3350-3450rpm the engine is going into open-loop and dumping fuel, since i dont want to lean the car out and burn up the pistons there has got to be a mechanical fix for this (gears). i have no need to race in 6th gear, so this is where i would like to focus my attention. the LGT Spec.B boys and girls are getting what i want in fuel ecomony and i have a strong feeling its due to their 6th gear ratio. what i need to know is what is the center diff ratio of both cars, and the ring gear ratio. this is what i have so far. if im able to get this right this could be the last thread on how bad the fuel ecomony is in our cars.

08 STI - Ratios - Speed
1st - 3.636 - 36.17
2nd - 2.375 - 58.84
3rd - 1.521- 86.46
4th - 1.137 - 115.66
5th - 0.971 - 135.43
6th - 0.756 - 173.95
Reverse - 3.545
Final drive - 3.9
Redline 6700
245/40-18


07 Legacy Spec B - Ratios - Speed
1st - 3.636 - 34.95
2nd - 2.375 - 56.86
3rd - 1.521- 83.55
4th - 1.137 - 115.66
5th - 0.891 - 142.63
6th - 0.707 - 179.75
Reverse - 3.545
Final drive - 3.9
Redline 6500
215/45-18

thanks
Ben Wales

[Rum]

interesting - subscribed.

[solovus]

The problem you will have is that by changing the gear ratios you are going to reduce the effective power at the wheels in 6th gear or every gear if you change out the final drive. To compensate for this reduced power you are going to have to rev a bit higher to stay in your 80-85 mph range. Nothing is free, if you want better fuel economy..stick to 55ish.

[Wales Garage]

effective power in 6th gear is a joke. the 1-2 seconds it will take to bring the power/revs up is not a concern, i can always down shift to 5th if i really need to put on a show for another car. florida is a long flat state that has 95% of its drivers cruising at 80 or more mph. as far as reving higher to attain my speed your not seeing the point. i want to drop the revs to keep the car out of open loop fueling to promote better fuel ecomony. changing the 6th gear ratio will allow the car to have lower RPM at my desired speed, thus promoting better fuel ecomony.

[solovus]

Originally Posted by Wales Garage

effective power in 6th gear is a joke. the 1-2 seconds it will take to bring the power/revs up is not a concern, i can always down shift to 5th if i really need to put on a show for another car. florida is a long flat state that has 95% of its drivers cruising at 80 or more mph. as far as reving higher to attain my speed your not seeing the point. i want to drop the revs to keep the car out of open loop fueling to promote better fuel ecomony.

Another thing to look at is how long will it take you to actually save money? Say you do get 25mpg....After 10,000 miles you will use about 400 gallons a difference of about 190 gallons which we'll assume a $4.50/gallon price will save you about $850 if and only if (a) most of your driving is highway; and (b) you actually can get 25mpg....

Actually, I just convinced myself that this may be a damn good idea ...let me know how it works out.

[SLVR08STI]

I would like to know if this could be done also..

[paikman]

Also , factor in the cost for the average person to have this done, it's not very efficient at all considering you'd be paying up front, several months to a years worth of gas to have it done correctly.

[Wales Garage]

ok, so does anyone have the info on the center diff and ring ratios, from what i can gather the 08 sti is 1.01 and the ring gears in the diff is anywhere between 3.60-3.90. the specb is the one i an unable to find any info on. i have the ability to do the swap i just need the info so i can get the right parts. i promise to post pics, give a detailed write-up, and post part numbers if this can be done easily.

[Wales Garage]

Originally Posted by paikman

Also , factor in the cost for the average person to have this done, it's not very efficient at all considering you'd be paying up front, several months to a years worth of gas to have it done correctly.

the price of gas is not getting any cheaper, i would assume a transmission shop would charge around $800-$1000 including parts and labor. for myself and others with mechanical backrounds it would just be the cost of parts in the neighborhood of $150-$300. i could be wrong but, im willing to be the test subject.

[Wales Garage]

please feel free to post any info, ill be back to talk on monday.

[1day]

The biggest difference I see from the info you posted between the 2 cars is the tire width. The difference between a 245 and 215 on all 4 tires is pretty large, especially in the relm of rolling resistance. If I were going to build a hwy cruiser and not race the car, that would be the first thing I changed. Needless to say it would be a lot cheaper-easier-and not void your warranty. Maybe get you a spare set and keep the 245 rim/tire combo for weekend/spirited driving??

But I do understand your stance on the open-loop after 3350 rpm. Possibly something that can be tuned for in 6th gear?

[Wales Garage]

Originally Posted by 1day

The biggest difference I see from the info you posted between the 2 cars is the tire width. The difference between a 245 and 215 on all 4 tires is pretty large, especially in the relm of rolling resistance. If I were going to build a hwy cruiser and not race the car, that would be the first thing I changed. Needless to say it would be a lot cheaper-easier-and not void your warranty. Maybe get you a spare set and keep the 245 rim/tire combo for weekend/spirited driving??

But I do understand your stance on the open-loop after 3350 rpm. Possibly something that can be tuned for in 6th gear?

rolling resistance has very little to do with the fuel ecomony of the car, when you figure in the overall diameter of the tire the sti is 25.72 and the spec is 25.62 which is not even enough to effect the speedo. warranty dosnt bother me, the sti trans is the last part ill ever break, i dont do donuts or attempt standing burnouts. as far a tuning the car for open loop in 6th gear its impossible the engine dosnt care what gear its in, it dosnt know, all it knows is rev range for fuel and spark, load for boost, and vacume for idle. leaning the engine out in an area where its supposed to have boost and extra fuel to support the boost would cause engine failure. so a specific tune for 6th gear is impossible. the only way to get the ecomony and cruise speed i want is mechanically through the gear ratios to drop the revs and keep the car out of open loop fueling.

someone please help with the ring and pinion gears and the center diff ratio's.

[twigg107]

Sell the car and buy a yaris.

[Wales Garage]

Originally Posted by twigg107

Sell the car and buy a yaris.

hello, thanks for your important information. im trying to do something that can make these cars more enjoyable to drive on long trips. it appears that you have no relivant information for this topic so please STFU and move on. if you have any issuses and want to further discuss this topic pm me your phone number and ill tell you in person.


Ben Wales

[solovus]

Originally Posted by Wales Garage

hello, thanks for your important information. im trying to do something that can make these cars more enjoyable to drive on long trips. it appears that you have no relivant information for this topic so please STFU and move on. if you have any issuses and want to further discuss this topic pm me your phone number and ill tell you in person.


Ben Wales

classic.