Gitout.com » Tech

Bigger Tires and Gas Mileage: How to determine actual gas mileage?

Peter Stewart — Fri, 16 Oct 2009 01:51:16 +0000

Bigger tires are an excellent upgrade for off road driving but they decrease gas mileage on road. It’s not uncommon to experience a 2-4 mpg reduction in gas mileage with your off road tires. Why is this?

1. They are HEAVY. Most weigh 20+ lbs more than stock tires.
2. Increased Rolling Resistance from aggressive tread.
3. Incorrect odometer & speedometer. Larger tires travel further per rotation.

Tire Rotation Diagram - Distance Traveled per Rotation

There’s nothing that can be done about the tire weight or rolling resistance of your tires (beyond proper air pressure). But you can do something about the odometer.

There are 2 simple ways to learn how to calculate your gas mileage after increasing your tire size.

1. With GPS: Drive your vehicle at 50 mph (gps speed). Read your odometer speed. Take your odometer speed minus 50mph and multiply by 2. This will tell you the % your odometer is off.

2. Without GPS: Measure the diameter of your old tires and new tires. Divide your old tire diameter by your new tire diameter. Subtract from 1. This will be the % your odometer is off.

Odometer Calculation Chart

If your vehicle odometer registers 250 miles and it takes 16 gallons to fill…

Multiply 250 miles by 10% (amount odometer is off calculated above) = 25 miles. Add 25 miles to your odometer reading = 275 actual miles traveled. Divide 275 miles by 16 gallons = 17.18 MPG

Your non-adjusted gas mileage would have been 250 divided by 16 gallons = 15.62 MPG

That’s nearly 2 MPG better! It’s also good to know your “actual speed” on the highway and in school zones to avoid getting tickets. We will talk about the other expensive alternative alternative (regearing) another time.

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Leaf Spring Tech: Shackle Length & Shackle Angle

Peter Stewart — Mon, 13 Apr 2009 06:49:06 +0000

Leaf spring suspension is one of the oldest forms of vehicle suspension. You’ll find leaf springs on the first production of Jeeps, Land Cruisers and Scouts. Today, most trucks and many SUVs still use leaf spring technology.

Leaf Spring Shackle Length

It’s good to understand the basics of leaf springs when it comes time to modifying or lifting your truck for off road use. Replacement leaf spring shackles are an inexpensive method for increasing ground clearance.

One of the most commonly asked questions is: How much lift will longer shackles yield? Every 1″ increased shackle length will result in 1/2″ of lift. For example: if your truck has 3.5″ factory shackles and you installed 5.5″ long shackles, you’ll gain 1″ of lift (2″ longer shackle x 50% = 1″ lift). Why do you only gain 1/2″ lift for 1″ increase in shackle length? Simple, you’re only lifting 1 side of the spring (ie 50%).

The other common mistake when replacing leaf springs is proper shackle angle. Increasing or decreasing your shackle angle will have a dramatic effect on how your suspension functions including: spring flex, compression, handling and ride quality.

Leaf Spring Bad Shackle Angle

The above picture is an example of poor shackle angle. As you can see the shackle angle is very low which has several negative effects: 1. Springs bottom out easily over bumps. 2. Soft ride handling on corners. 3. Somewhat limited spring flex.

Leaf Spring Good Shackle Angle

The above picture is an example of proper shackle angle. Correcting the shackle angle drastically improved the ride handling.

Leaf Spring Shackle Angle

How do you know the correct shackle angle? This can be achieved by drawing an imaginary line through the middle of both spring eyes and making a 90 degree angle. The angle can be increased or decreased slightly to change spring rate.

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Off Road Traction : Tire Air Pressure & Airing Down

Peter Stewart — Sat, 28 Feb 2009 22:08:36 +0000

Four wheeling with your tires at highway air pressure results in extremely poor traction off road. This was one of the first lessons I learned 7 years ago on my first off road adventures. A fellow 4WD driver took the time to explain the advantages of reducing air pressure for increased traction.

The basic concept is that as tire pressure is decreased it allows the tire to flex more; as the tire sidewall flexes it increases the tread contact surface area. The tread on the tire will wrap around rocks and terrain to grab a hold and give the vehicle traction.

Tire Air Down Increase Traction

Increased traction off road enables the driver to finesse over obstacles rather than using speed or spinning the tires. The increased control keeps the vehicle moving forward in the proper direction. This will help limit the possibility of damaging your vehicle or breaking parts.

How much air is enough? In most cases, less is more; meaning that it’s best to run with the lowest pressure possible. When airing down your tires wait until you see the tire sidewall start bulging out while sitting on level ground. This will be a good pressure to start with; target range is 6-15 psi. Then test it and air down further as needed.

Some tires with thick sidewall construction will need less air pressure than tires with thin sidewalls. For instance, Super Swamper TSL or Bogger tires (thick sidewalls) may perform best at 6-8 psi; while a BFGoodrich AT (thinner sidwall) may perform well with 12-15 psi.

Tire Air Down Sidewall Flex

Will too little air pressure be an issue? Yes, it certainly can be. The primary concern is that the tire may come off the rim if the pressure gets too low. This generally happens when a rock or stump pushes the tire sidewall away from the rim. You’ve probably seen the Extreme Off Road Vehicles with beadlock wheels. The beadlock wheels bolt the lip of the tire to the rim keeping it from coming off the rim at low tire pressure. In most cases beadlock rims are not necessary. I’ve had my tire come off the rim once in 7 years of 4wheeling. Just exercise a bit of caution.

Tire Air Down Off Rim

Quick note: Be sure you or someone with you has an air compressor and carry a spare tire. Sometimes the unexpected happens and having the proper recover equipment is essential.

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Toyota 4runner Seat Replacement

Peter Stewart — Wed, 25 Feb 2009 01:00:33 +0000

One of the common issues with older vehicles is worn out seats. This is especially common for Toyota trucks which often see heavy use and have thin cushion to start with. The seats in my 1986 Toyota 4runner had seen better days. The driver seat had no lumbar support remaining, unless you count the steel support bar which was protruding through the padding. The back seat was a full bench seat with no head rests. Overall, the fabric was in excellent condition consider the age but the seat cushion was worn out.

4runner Driver Seat Swap

There are many types of seats that people have swapped into their Toyotas; common seat choices include Honda Civic, Acura Integra, Subaru Impreza and other unusual ones like Mercedes. I chose Toyota. Oringally, I was looking to install some 3rd gen 4runner seats (1996-2002) but I found that extensive modification would be required.

Instead, I chose some 2nd gen 4runner seats (1990-1995). Why? Several reasons, including the wrecking yard had blue seats that matched my interior color and they were relatively cheap. The seats I purchased were out of a 1994 Toyota 4runner SR5. These are the fancy seats with driver side adjustable lumbar support, side support, bottom adjustments and adjustable headrest.

4runner Passenger Seat Swap

The seat installation couldn’t be easier. The driver seat is an exact bolt in; no modifications whatsoever. The passenger seat is another story. My passenger seat had a foot lever to slide the seat forward for passengers to enter and exit the back seat. The newer 4 door 4runners do not. To make it work properly you need to swap the seat frames. While this may sound like a daunting chore, it’s actually quite simple. It took about 30 minutes to get it done. The seat bottom comes out with 4 bolts; removing the seat back requires lifting the seat fabric up to access 4 bolts (2 on each side). Once again, it’s 100% bolt up; no drilling or modifications required. After the seat frame has been swapped it bolts perfectly into the truck.

4runner Back Seat Swap

The rear seat back bolts in without modifications. Note: I replaced my solid bench seat with this split fold bench. The seat bottom required 4 holes to be drilled and bolts installed. Your 4runner may or may not need holes drilled depending on the year (1985-1989).

This is one of my favorite modifications. It makes driving the truck enjoyable especially on long road trips and 4wheeling off road. Be sure to click over to the forum to post comments and questions.Similar Posts:

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New Website!

Peter Stewart — Fri, 20 Feb 2009 01:39:45 +0000

I’m in process with creating a new website for Gitout.com. Please check back to see the progress over the next week or so. The forum may be online as early as this weekend. Be sure to get on it and post up your thoughts and comments.

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