Bigger tires?

[Longshot270]

On the torque, do the math for the tires you plan to get. With the help of Pegasus's gear ratio calculater and some torque formulas Jack has posted on older threads on the same topic, you can use that to help figure out how much you are actually losing. If you are only going up a size or two like I did, that quantity of lost torque is not something you really worry about.

If you only go up a few sizes like I did then the power loss is not as much as you'd think. Yeah there is a SLIGHT decrease and your stop and go traffic mpgs MIGHT take a hit, but you are also upgrading for looks and you pay to play no matter the game. My stop and go city mileage didn't really go down (from 13-14ish), my 45-65 mph mileage went up some (17-21 to 18-23) and my 75-95 mph highway mileage went down a bit but I dont have numbers there because I'm not usually on the highway for more than 20 miles. These all have to do with my driving style though. I plan ahead so I'm never in a hurry.

Depending on the engine I can vouch for the gotts mod. I (independently) calculated up to 9 hp based on quarter mile time comparisons and MAF sensor gains on my 4.6L. I'm also using a drop in K&N filter. Funny thing is that K&N says I should get about 9 with their intake. If you have your doubts test it yourself.

If you want a good idea of your real horsepower without a dyno, go to a local drag strip. Make a few runs that are really close to each other and then weigh your truck. There are tons of formulas that will give you a general number including the one in the pegasus program.

The avg economy on any programmer has a tendency to read high or low. It is one of those things that makes perfect sense on paper but the real world makes it lose accuracy. I'd expect the same effect of the mileage on the truck readout. Similar formula but with different computations.

The vacuum guage is an old school mechanical device that tells you how LITTLE throttle you are using. It hooks up to a vacuum line after the throttle plate and reads the amount of intake manifold vacuum pressure. I think most of the ones you buy in the U.S. and such are measured in "Inches of Hg (mercury)", the original standard in vacuum operated devices. It does not tell you your mpgs but the philosophy is make as much vacuum as you can. The more vacuum means less air is getting past the throttle plate and into the engine...Less throttle = more vacuum = less air to the engine = less fuel usage = better mpgs. You could possibly put estimated mpg readings on it but it will be a multi tier field like on cheap volt meters. Then you'd be spending more time figuring out which number is applicable rather than looking at the road so I don't recommend it.

[Jackpine]

Nice reply, Longshot!

I would not be surprised to see a fuel economy increase in the 50 mph range with slightly bigger tires. I hardly ever drive at that speed in Arizona with our 75 mph interstates and 65 mph country roads (everyone runs about 80 mph and 70 mph on each of those).

About the only time I'm at 50 is in the city, where the speed limit is 45. (I'm keeping up with traffic).

- Jack

[Longshot270]

My programmer makes an electrical average of 15-17ish when I'm either going to or coming back from Cabela's. That includes jumps from 70 to 95 and maintaining that speed. I'm usually anxious when I go there and pissed off when I leave so I dont have any "normal" driving numbers.

[ticopowell]

Great posts all! I just have a little to add, I put some bigger tires on my truck, 315/70R17's and I did notice a difference in mileage, I expected mileage to go down overall, but I drive a lot between 45 and 75mph and while there wasnt a large bump, it was noticable . I also enjoy the look, I put a 3" block in the front so my truck is a little "over leveled"(2" would make it level), but I love the look and the tires seem to fill the void quite nicely. I have some pics posted on my profile here but if you want to see any specific areas I can go take a pic for ya .
I did notice a large difference in off the line performance, and I use more gas to go, but once I am up to speed I didnt notice any difference. I have 3.73 gears btw, and if anyone wants to donate to me to put 4.10's or 4.56's in feel free!

[jmwilso2]

Thanks for the great replys everyone. Its really given me a lot to think about. I'll see about getting some time at the local track come spring and then do some math. The vac gauge sounds a lot like a manifold pressure gauge from an airplane. I'm a commercial pilot by trade, and on many light aircraft you use a combination of manifold pressure(actually manifold suction), engine rpm, and fuel air mixture controls to set power. I have an aircraft mechanic friend that may have an old manifold pressure guage around that cant be used in an aircraft anymore. It would be pretty simple to plumb it in. As for trips to cabelas, I'm sorry to say we dont have one around here yet, but we do have a bass pro shop a couple hours away. I've found my fuel economy is usually better on the way there than it is coming home. I have attributed this to an increase in vehicle weight on the return trip as a result of exchaning light portable money for heavier items like guns and fishing gear.

Thanks again guys

Joe

[Longshot270]

That sounds like the exact same guage so it should work. The guage I use measures manifold "pressure". You'll need one that only reads to about 25-35 inches of Hg. Regular driving I see 20" on idle, 25" if there is a bit of engine braking. Probably will be the same exact guage and your already trained to use one.

[Jackpine]

Ah, the joys of recips. Throttle, mixture, prop - and all for just ONE engine! (Too damned many levers in my opinion.)

As for your mileage to and from the shop, I suspect it is "downhill" from you. You could confirm this with a GPS that shows elevation.

- Jack