Gimli

I just got new tires. I bought Toyo Open Country 255/55r18 because they were readily available. This is approx 1/2” wider and 1/2” shorter than the stock 245/60r18....

all I have to say is WOW. what a difference. This really woke up the 4.7l. No more having to use tow/haul mode to zip around town. Plus I picked up 1 mpg. Now I now you’re thinking that it only seems like I went from 16 to 17 miles per gallon because the tires are shorter so it’s showing more than actual miles... I drive the same amount of miles every week. It is 56.7 miles round trip for me to get to work and it is 12.3 miles round trip to my Girlfriend’s house. So I took the number of trips I made and divided by the amount of fuel used and came out to 17.2...

so did Jeep mismatch the tires on the 2008 Grand with the 4.7l from the factory?

dave1123

I think what you are seeing is the engine is running at a slightly higher rpm and pulling more efficiently with less throttle opening, thus getting more fuel economy. On the hilly, twisty country roads I drive on mostly, I find keeping my WJ 4.0 in 3rd gear gets me better throttle response and more power at the higher rpms and saves fuel and the trans constantly shifting on hills. This is running at 45 mph at around 2000 rpm. The Grands are designed to run around 1500 at 55 mph, which is not really into the power band yet, but is fine for long distances on flat ground. The 4.0 is known to build up carbon on the valves if operated below 2000 constantly and to burn valves in the 99-04 models because they have smaller exhaust ports and cast iron manifolds that run hot. Pulling a load at 1500 is bad for a 4.0. I firmly believe this added heat in the center of the head is the main cause of the 0331 heads to crack. I'm not familiar enough with the 4.7 to tell you if this is the case with them.

You should check your speedometer and odometer over the measured mile on most highways. 60 miles per hour should be exactly 60 seconds with a stopwatch, clicked when the milepost crosses your A pillar. I've done this with every vehicle I've ever owned since 1960. I don't trust GPS to be that accurate...yet, because of the time lag for satellite communications. Of course I'm a dinosaur!

Gimli

It’s a 2% difference. That’s within the margin of error allowed by the federal government on odometers. When showing 70 on the speedometer I’m showing 70 on gps.

mom still trying to figure out my gearing... I’m guessing because I have a 4.7l that I have 3.73? If that’s the case the to get to a 35 I need 4.56 to keep the same performance.

dave1123

If the same hold true with later years, the standard gearing is 3.55 with 3.73 being used on tow package models. However, my 2000 XJ with tow package is still 3.55.

My WJ came equipped with 225/75R16 tires and I am running 245/70R16 for the past 8 years. My speedometer reads 2 mph faster than police stationary radar and holds true at that difference up to 70 mph on the measured mile. That's good enough for me.

When it comes to speed calculations, I remember back in the very early days of drag racing, the big question was "Will it do 60?" which means to us now a 15 second 1/4 mile. That seems very slow to us today.

Gimli

I have factory tow package with (I think) either a 6600# or a 7000# towing capacity.

EZEARL

Gear ratio looks to be 3.73 with the 4.7L:
http://www.wkjeeps.com/brochures/2008_WK_Brochure.pdf

dave1123

The whole point of this is 3.55 and 3.73 are so close to each other, it really doesn't matter much in horsepower and torque at highway speeds. 3.73 will give you a little more torque for towing, but the same can be had by dropping down a gear or out of lock-up. My 62 Chrysler 300 had 3.23s and it was a dog until you got some speed up, even with a 413 cu in (6.76L) engine. However, my 67 Camero SS had 4.10s and it ran at 4000 rpm at 70 mph! Amazingly, it got 17 mpg at that speed!