Contour Enthusiasts Group Archives Forums Modifications Transaxle Final drive ratio change

Well I did some calling around today too fine part numbers and pricing on the 3.84:1 final drive gears. I can get the ring gear no problem. The final drive pinion gear purchase option has been changed by ford (according to the dealership). Now the only way to purchase the pinion gear is with a complete output shaft set - for 1000 dollars.

At that price the gas savings will never payoff.

Any suggestion, or does anyone have a 3.84 set they want to sell or trade for a 4.06:1 set?

You think the change from 4.06 to 3.84 is going to net you any discernable gas mileage improvement?

Anything within a couple hundred rpm is going to net about the same mileage return on the highway.

If you want to get technical you will need to apply more gas pedal in city driving to attain the same power needed to accelerate.

Either one will be miniscule changes up or down in the grand scheme of things.

Since the 3.84 gear set was stock with the manual trans on a Contour 4 cylinder, there should be an abundance of them, especially in the wrecking yards.

I wouldn't bother unless you have done something to dramatically increase torque, such as doing the 3.0 conversion.

Yes, I agree... when my shift tower broke, it would take itself out of 5th gear; meant I had to drive like 450-500 miles home in 4th gear... Still got 27mpg

Alternatively, you can just use a larger circumference tire/rim combination. This will have the same effect as changing your ratio.
From my experience with an automatic Mazda Protege with tires larger than stock....
acceleration was reduced
gas mileage was the same.

If ya want to do this, Terry Haines got ahold of me and wrote:

Originally posted by HMS Motorsports:

---

re The final drive ratio post.As we have stocks of the
parts needed to convert to a Zetec FDR we can do the
swap for a parts cost of around $300 (not counting
labor and any other parts needed for repair/upgrade)
Vs the cost of parts from Ford at around $1,000.This
is only for shipped in transmissions.We do not sell
these parts as mail order but keep them for our own
customers.

---

So if ya send in the tranny to him (for some upgrades, possibly some repairs also), he can change them out for ya.

Ok here is my calculation. I know that this is all theoritical and could be different depending on traffic and weather conditions. I did the calculations based on a continuous 60mph. That is the average speed on the freeway in the Seattle area. At 6am most days that is the speed that I travel. I figured 25 miles per gallon average.

The current final drive ratio of the CSVT in 5th gear is 3.12:1. I know in the manual it says .77, but that is just the 5th gear itself and not including the ring and pinion gear. With the zetec ring and pinion the final drive ratio would be 2.95:1. With a 23" diameter loaded tire diameter you get a 72.26 tire perimeter.

63360in per mile * 72.26 = 876.83
876.83 tire rotations per mile.
876.83 * 3.12 = 2735.7 engine rotations per mile (Original)
876.83 * 2.95 = 2586.6 engine rotations per mile (New)

2735RPM = 1 Mile per minute = 60MPH
2586RPM = 1 MPM = 60MPH

148 engine rotations difference
25 miles per gallon = .04 gallons per mile
.04gpm/2735 = .000014625 gallons per engine rotation
148 * .000014625 = .0021645 gallons per mile savings

76 mile round trip to work each work day - 2 weeks vacation
250 workdays per year give or take.
19000 miles per year

.0021645 * 76 = .164502 gallons per 76 mile trip
.164502 * 250 = 41.13 gallons saved per year
41.13 * 2.50(average price/gallon in WA) = 102.81

So with the change I would save approximately 102.81 dollars per year.

Then I would also have the added benefit of being able to go 61.3MPH in second gear at hard limit.
I know I would have a little slower acceleration, but I plan on doing some engine upgrades in the future.

Enlarging the wheel diameter causes increase rotational Inertia, which means slower starts that are not there with just changing the FDR.

I feel that it is an all around better option to change the final drive to the higher ratio.

You can't calculate it like that though.

MPG is based on the engine's efficiency and MAF reading at said rpm range.

Not how many tire rotations per mile.

You can't even use the same engine rpm for a direct computation because said car is now going faster and incurring more wind drag.


Also what you are trying to calculate is steady state cruising MPG.
What about the rest of the time when you are using more accelerator pedal to speed to car up?
What about stop & go traffic?
What about city driving?

If definitely takes more fuel to accelerate then it does at steady state cruising. Exponentially more actually. The cruising fuel demand between a 300rpm range is negligible to none. I can post MAF numbers to show that if you like. If the MAF is reading the same airflow it is delivering the same fuel trim. Period.

So even factoring out engine efficiency at said rpm and wind resistance your final numbers are still incorrect.

I would guesstimate the real final number would be within a meal at your favorite fast food store apart. That's even if you stay out of the throttle and accelerate slower.

Then factor in the price to change out the gear and you would never recover those costs in a decade. ($300 just for parts)


~~~

The difference in final gearing is roughly 5.1% which is 2941rpm vs 2782rpm at 70mph. That's just a 159rpm swing. You will not see a MPG difference in that small amount of rpm change.

is it a good upgrade if you are doing a 3.0L? and why?

You can usually handle taller gears better with more torque. The 3.0 conversion provides more torque.

All of those wonderful calculations above did not account for engine load, which increases with taller gears. The actual improvement would probably be about half of what was calculated.