Lower Gear ratio better mpg?

[PK]

I drive on 255/55 18's. I dont know exactly why I drive in 2wd only since it was that way when purchased. I jumped the check wires a while back and it indicated a problem with a control switch or unit or something under the tranny lever in the middle. I tested it in the dirt and sure enough that is exactly how it works. 2wd in 4H and 2 rear wheels lit up on the TOD display. 4LOW and I get 4LOW with all four wheels lit up on the TOD display. I've been driving it this way for a year or two and seems to have had no problems other than that dang blinking light. So I refinanced the house and now I can do some other things like put a turbo kit on my MR2 and try some new gears for the VX. I checked some sites and found that the ring/pinion is a 8.5 10 bolt as used on some other Jeep models and GM models. Man you guys ask alot of questions.

Don't worry about the questions.
Remember, on this site you are a newbie, and we have no idea of your mechanical expertise, driving habits, long term goals etc.
So when you ask for our advice, we need to ask questions so that the advice we give you is accurate and relevant.

For instance, we now know you are driving on one of the smallest diameters tyres that could be fitted to the VX, and possibly the easiest, cheapest options for you would be to buy larger tyres. Especially if your tyres are getting worn anyway.
Your tyres are about 29" diameter.
Most of the people here run 31 - 35" dia.
Is this a feasible solution??

FWIW anything over about 32" you have to do a little bit of trimming to the front wheel wells.

Anyways, sounds like you know what you are doing.

PK

[Ascinder]

I hate to burst your bubble, but I think what you are referring to is the GM "corp" 10 bolt. It is not the same as the isuzu Corp 10 bolt. They aren't interchangeable. Also, our 10 bolt is up front, so you'd be trying to hook up 4wd in that case to save gas mileage..... As Triathlete posted, our rear is a 12 bolt isuzu. You really won't save a ton of money(mileage) going to lower gears, unless you have either excess torque or horsepower either. If you could get used to the VX driving like an anemic shadow of itself, I'd say go for it, but you'd be pulling it out of the powerband it was designed for. To get the same speed using higher gearing, you'd be lugging the engine. It was mentioned earlier in this thread that we don't know your specifics, so maybe you've already thought all this out, so if I were you, I'd see what gears were made for ISUZU Corp 10 and 12 bolts and then take a look at both the pros and cons before I went and did it. It was also mentioned that we don't know your degree of mechanical skill, but I'll just throw out there that setting up a ring and pinion isn't done with a set of wrenches and a lazy Sunday afternoon. Shops charge a decent amount of coin to set them up because they require some special equipment. As I said, you may already know all this, but then agian, maybe you don't. Just throwing in my two cents.

[Riff Raff]

Thanx PK, you're right on target. The main problem is the tires-- you're running 255/55R18 (29" O.D.) which are smaller in diameter than the OEM tire of 245/60R18 (29.5" O.D.). Thus, your current smaller diameter tires are rotating 1.8% more than OEM causing your VX to consume more fuel (less MPG) for the same distance traveled.

Instead of changing the differential ratios, a better solution for more Highway MPG is to go to a tire that meets three(3) criteria:

1. Must be "tall" in O.D. between 32" -- 35" O.D. to act like a transmission Overdrive thereby reducing engine RPM's (saving fuel) for the same given forward speed of travel.
2. Must be "narrow" in width to create less drag/rolling resistance. The skinnier, the better.
3. Must be "lightweight" to reduce unsprung weight and use less fuel from standing start/launch.

Here are some optional tire sizes that meet the above criteria (tall, narrow, lightweight):

P255/70R18 (32.1" O.D./35 lbs) - bolt-on perfect with OEM rims.
P265/70R18 (32.6" O.D./44 lbs) - slight trimming required.
P275/70R18 (33.0" O.D./46 lbs) - more trimming required. (recommended tire: BFG - Rugged Trail)

To minimize fitment problems, utilize your stock OEM 18x7 rims for the above listed tires. Use only "P"-series tires (aka Passenger/P-Metric) which are lighter in overall weight, and stay away from heavy "LT"-series (Light-Truck) tires.

For the absolute extreme gas saving tire, you could use:
35/10.0-15 (35" O.D./48 lbs) - lifting required. (recommended tire: MickeyThompson - Baja Pro, which is a Sand-Rail tire and not DOT approved for the street. Use 15" super lightweight CenterLine brand wheels to keep weight to a minimum)

Focus on changing to a taller O.D. of your tires, not the gearing in your differentials. Then drive your VX like you are hauling very fragile eggs; be featherlight on the gas pedal, and let off the gas pedal early before coming to a stop. You'll be amazed at how much your MPG will improve.

[Triathlete]

(recommended tire: BFG - Rugged Trail)

Everyone I know that has bought these tires has complained about how quickly they wear. Just a FYI.

[VX KAT]

We all know this vehicle isn't cheap on fuel. Is it just me, or are others surprised how some are willing to invest serious coin to modify something or buy a different vehicle to get a few more MPGs...have you ever calculated the break even point on that cost?

Let's assume you do some modification that costs $1,000,and it increases MPG from 16 to 20 MPG....a full 25% increase, which is doubtful.
16MPG x 22.5 gallons = 360 miles per tankful
20MPG x 22.5 gallons = 450 miles per tankful
If fuel is $2.95/gallon, you gain 90 extra miles per full tank
90 miles ÷ 20MPG = 4.5 gallons to travel that far
$2.95/gallon x 4.5 gallons = $13.27 saved on a full tank
Another way of looking at it is you save .029/mile.....

$1,000 ÷$13.27 = 75.35 tankfuls....for most, filling up every 2 weeks, that would take 3-4 yrs to just break even.....and only then would you begin saving any money

Maybe for tracking MPG, you might look into registering at fuelly.com. It's kinda fun. Here's my VX:
http://www.fuelly.com/driver/vxkat/vehicross

[Riff Raff]

Everyone I know that has bought these tires has complained about how quickly they wear. Just a FYI.

Billy--- The BFG - Rugged Trail only has 10/32nds tread depth when they're brand new off the shelf. So compared to say a General - Grabber AT2 which has 17/32nds brand new-- then yes, the BFG - Rugged Trail will have a short life right from the start.

What makes the BFG - Rugged Trail so unique is it's the ONLY "P"-series tire made in size 275/70R18 with a tall O.D. of 33" and only weighs a featherlight 46 lbs each. Take a peek at TireRack.com for all tires made in 275/70R18 (33" O.D.) and you'll quickly discover that all of the rest are much heavier "LT"-series tires easily weighing 10 lbs more in the ballpark of 56 lbs each.

Remember; for best MPG we want tall O.D., narrow width, and super lightweight all combined in one tire. Class dismissed.

[Scott Harness]

Nice thread-info...have you guys drank too much coffee?

[Ldub]

OK. Thats it! I'm getting a new ring and pinion set. Yes I know I will need two sets but mine is only 2wd unless put into 4wd manually. I get 15-17 mpg city. I'm thinking down to 3.20's any suggestions?

Thanx PK, you're right on target. The main problem is the tires-- you're running 255/55R18 (29" O.D.) which are smaller in diameter than the OEM tire of 245/60R18 (29.5" O.D.). Thus, your current smaller diameter tires are rotating 1.8% more than OEM causing your VX to consume more fuel (less MPG) for the same distance traveled.

AND...

Unless you're already factoring in 1.8 - 2% less miles on the number of miles driven per tank, you're not getting 15-17 mpg...