I know its possible to change to alternate tire sizing and change your final drive ratio (the Jeep and 4wd guys do this quite often). My question is how?

Heres what I want to do: I want to alter the final drive ratio on a vehicle that has been swapped to a higher gear ratio trans and larger wheels than stock so that I may improve gas mileage. Details are as follows.

Stock ratio/tire sizing:

3.545 FDR with 215/50VR16 wheels/tires.

Modified ratio/tire sizing:

4.44 FDR with 225/45VR17 wheels/tires.

So, currently it is running the 4.44 ratio with the 17's. The goal is to reduce (?) the gear ratio for better gas mileage, while retaining as close to stock speedometer readings as possible.

My question is: What size 17" tire would I use to do this? And with the 4.44 FDR, if I were to go back to stock wheels, what size 16" tire would I use to achieve my goal while maintaining accurate speedometer readings?

If it weren't for the transmission swap to the 4.44 FDR from the stock 3.545 FDR, I could figure this out. But having that swap puts this task in a whole 'nother realm of understanding that I can't comprehend

Anyone help?

See my cave, gear ratio in trans section.

Covers tire vs "final" ratio and how to calc new ratio

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 10-03-2011).]

Your speedometer will be wrong.

quote Originally posted by FriendGregory: Your speedometer will be wrong.

I realize this, which is why I want to find something that will get me closer to a correct reading. But since this vehicle also has a custom trans control unit installed, I believe it may be programmed to compensate the speedometer reading for the 4.44 FDR.

Basically, I'd like to get the FDR down to around 3.83-4.11 range if possible.

I've been doing this for years by using 20" in the rear.

quote Originally posted by madcurl: I've been doing this for years by using 20" in the rear.

Not an option in this case Its an AWD vehicle

I'd like to find out how to do this because winter is around the corner and the car currently has Bridgestone RE760's, and PA winters probably won't agree with those tires

[This message has been edited by Xerces_Blackthorne (edited 10-03-2011).]

Correcting speedo and gear ratio is two different things in many cases....

Correcting speed on most OBD2 cars is just a programing change. One of feature aftermarket PCM tuners... Changes Trans shift points, speedo, and more. ODB1 or carb setup just change speedo gear(s) in trans or try same aftermarket thing Fiero uses to fix speedo (use PFF search), or use aftermarket electronic speedo, or etc

Final Gear ratio, need math from cave and major work to cars....
Traditional inline 4WD... Need to change both axles or maybe change xfer box.
Changing ratio in Subaru or Kia 4WD is much harder... They doesn't use separate xfer box and front axle final... both are built into trans itself.

[This message has been edited by theogre (edited 10-03-2011).]

quote Originally posted by theogre: <snip> Changing ratio in Subaru or Kia 4WD is much harder... They doesn't use separate xfer box and front axle final... both are built into trans itself.

Ding ding, we have a winner This is why I want to alter by changing the tire size. This is being done on a Subaru that has had its trans swapped out with a stronger (but taller? geared tranny). And its pre OBD II (runs the SelectMonitor protocol).

Use math in cave.... first formula... but need trans final # or math won't work. (4wd likely need different # for front and back)

In general... bigger tires = higher effective final, cut power but maybe give some MPG.

Strong can mean a number the things...
Strong = Strong
Stronger usually = lower final and/or main gears (1st 2nd 3rd, higher gears can be overdrive gears)

But note... Many thing will cut MPG... Including tires design. Tire ads says tires make a difference is not just marketing speak.

FIT THE BIGGEST TYRES THAT WILL CLEAR

but you areNOT going to get near 3.5 from 4.4

jeep and 4WD trucks can jackup to clear big tyres with solid axles
I donot think you can get that on a subie without alot of work