My 99 has the stock size tires (235/85/r16) with a 5 speed and 3.55 gears. At 70mph the rpm's are at 2100. Would I benefit from going to a larger tire like 285's to drop my rpms more and get more in the middle of the torque range? Anybody done a similar swap and seen mileage benefits or decreases?



On a highway run this weekend I got 21mpg hand calculated on a 200 mile trip going 62 mph (1850 rpm). For comparison I drove 70mph for 120 miles and got 17mpg (2100rpm).
Is this due to rpm or simply the fact I was going faster ie wind resistance?



Note. I was driving on a highway that had mile marker signs and noticed that my speedo is off by 4% and used that in my calcs so they should be perfect.

 

On these trucks, the lower the RPM's the better. To a certain extent of course, once too low of RPM's the engine will lug. With my tires, aprox 34.5", I spin at 1500rpm at 62 mph with my Auto. Gets pretty good mileage doing that.

The larger the tire, the lower the rpm. If you go wider it will have a reverse effect as it is more resistance to roll. A 285/75 would be good, or 285/85 even better for mpg's.

 

That will not get you as close as you might hope, generally speaking. DAMHIK. A GPS is about the only real true mileage you can get.

 

I did it for 100 miles so it should be really close, plus it made the math easy.

 

Margin of error is still pretty high at 100miles.

1800-1900 for an unloaded truck is great. just slow down unless you hate your tires (highway rib tires will have the least rolling resistance)


Assuming:
$4 per gallon
fuel savings of 4mpg for the 8mph difference in speed. 62 and 70mph (13% gain)
saving 7 minutes per hour of travel time (driving 70mph instead of 62mph)

70mpg to go 60 miles = 3.53 gallons =$14.12
62mph to go 60 miles =2.86 gallon=$11.42

cost of $2.70 per 7 minutes = $23.70 per hour saved of driving 62mph instead of 70mph

Unless you work on an all cash basis there are taxes on earned wages:
FICA (SS, disability etc) 12.4%
Federal 28%
State 9%
= nearly 50% of wages paid to govt. A dollar saved is TWO dollars earned

The question to ask is it really worth over $47 an hour saved in drive time?

With towing this # is greatly increased.

 

Makes sense. I do drive 60 or so to save fuel, but it sounds like some bigger e rated narrowish tires will let me pick up the pace and still get good mileage, a win win. Going 60 does suck, even if it is only 13%.