14.5 Avg MPG?

[Robert Dedrick]

Ok so I recently got new tires...265.75..16 AT2 Grabbers...and put on my ARE Topper and I went from about 19 on the Interstate to 14.5...Is this normal or should I start looking at my engine? It had 133,000 on it but I keep up with oil changes, air pressure, filters.

 

[Richard L]

Robbie,

There are too many variables to say exactly why your gas mileage dropped, but here are some things to consider.



Toppers and tonneau covers do not improve gas mileage....in some cases they may even hurt mileage, as well as adding weight.



New tires might have more roll resistance, may be heavier, and may be inflated to a lower pressure. Generally, 33-35 PSI is the optimal pressure for MPG by minimizing roll resistance while still providing ride comfort and maximum tire wear.



If you have not changed the sparkplugs and plug wires, it's past due.



The factors that most often effect gas mileage are your driving habits that you may not realize you are doing, or things that you have no control over.



Traffic, weather, wind, etc are things that can have a big influence on your gas mileage and you may not even be aware of these slight changes and their impact on MPG.



A really big factor is if you have a lot of traffic on the Interstate and you constantly have to change speeds...ie: slow down for slower moving traffic and then accelerate when the traffic clears...That slow-down and speed-up driving really kills mileage on heavy vehicle like the Sport Trac. One strategy that I have tried and it seems to work, is to drive just a wee bit slower than the average flow of traffic. That means you can drive in the right hand lane with your cruise control on and rarely have to slow-down or speed-up, and it's the speed-up part that really kills mileage.



Your new tires might be a slightly different diameter which could be throwing off your speedometer/odometer. If you don't mind spending about $60 and buy an UltraGauge. This gives you an instantaneous MPG read-out and lets you see exactly what kind of driving saves gas and what driving styles waste gas. Just using the UltraGauge can gain you a couple MPG. It also will insure that your speedometer/odometer is properly calibrated for your tires by allowing you to set the UltraGauge speedometer/odometer correction factor so that you are reading a true and accurate speed and MPG.



...Rich

 

[Jerry Gerner 2]

It's from the added weight and bigger tires. Also remember your speedo/odometer is slightly off now with the taller tires.

 

[Robert Caffyn]

+1 to Jerry. Your tires are taller than stock, hence your speedometer will read lower. So you are still covering the same distances and using the same amount of fuel. But the miles recorded on the odometer will be less. The math tricks you by giving you a less than normal MPG. The only solution is to correct your speedometer for the larger diameter tires. Its less about how much the tires weigh or that you have some sort of covering on the bed. Its just the math. Fix your speedo and you should be back where you started. Bob

 

[Richard L]

Bob C,

Reread Jerry's post. He was not refering to the "Weight" of the tires, but the added "Weight " of the bed topper. But the larger tires are probably causing an inaccuracy in the speedometer/odometer reading as you said.



I perfer to use of the UltraGauge since you can set the calibration of speedo/odometer correction factor to be extremely accurate which cannot be done with the OEM speedometer...since they have a very limited adjustment range of about +/- 5%. I think going from 235/70's to 265/75's probably increased the tire circumference by more than 5%??



...Rich

 

[mike burns 3]

Going with a larger tire will impact power, mpg, durability of your running gear, and probably handling. The weight is a part of it but the overall gearing change and rolling resistance hurts as much.



But before doing anything I would put some miles on them. Lots of things change mpg some of them have nothing to do with the truck. One trip or even one tank will not tell you much. I would keep track of the mileage over 5 or more tanks.

 

[John G]

The effect of tire size on odometer calibration can not cause a drastic effect like 19 mpg changing to 14.5. Such a large effect would require replacing the tires with new ones that are a whopping 31% larger in diameter. Not a chance that could explain the OP's result.

 

[Les]

Use this link to determine the change from your old to new tires. Will also give you information on how the speedometer is affected. The bed cover will not help with milage.

 

[EddieS'04]

The bed cover will not help with milage.

True. MIT did a test a few years ago. They used several types on a chevy colorado. The only change they found was. Some of the heavy covers, made the milage worse, by a small amount. Due to the weight. A conclusion that larger trucks with more motor. Would not be affected by the heavy covers.

Over all no milage increase.

 

[Robert Dedrick]

Thanks guys.Yea iv replaced the plugs( not the wires)..I wasn't expecting anything from the cover as far as the milage goes and the tires are only one size bigger than my old ones but they are a different class so they weigh a lot more. I knew it would go down but not like it did.I never really thought about the speedometer throwing it all off though. Im about to make a trip this weekend and ill it again so this will be the 3rd time checking it on a 250 mile run.

 

[George T]

Robbie,



Hey man, I have the same exact tires and the same exact size. Trust me, it did the same thing to my MPG. Mine fell to about 14 in the city though, and about 16/17 on the highway though. But considering I was getting about 20 on the highway before. Check your tire pressure. I left my speedometer the same, I just do the math in my head when I fill up.