Best tire size for highway fuel economy
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. |
Originally Posted by yjay
(Post 1025330)
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. |
I did it for 100 miles so it should be really close, plus it made the math easy.
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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%.
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