Hi all,
My brother in law told me this last weekend that if I put on smaller tires on my 2006 4Runner, that I would get better fuel economy. Any truth to this?? I am currently using blizzak P265/65 R18. If I go to smaller tire, will this do any damage to truck suspension etc. Appreciate it, Ben.
What kind of MPG are you getting now?
I like to do everything I can to keep my MPG up there but you also have to have realistic expectations. There are other factors besides tire/wheel size that will affect MPG like where you live, how you drive, whether your vehicle is a DD or not, etc. Hils and slow, bumper-to-bumper traffic or stoplight-to-stoplight city driving will kill your MPG, it's just a fact of life.
Another question would be: How are you calculating MPG? The most accurate way to do it is to always fill the tank the same way, preferably at the same station and around the same time of day (although IMO that's taking it a little too far...) Divide miles driven by fuel needed to fill the tank and that will give you your MPG. Of course, that only works if your odometer is accurate.
The odometer calculates your miles driven by the number of times the driveshaft or transmission output shaft turns. When you switch to a smaller tire, the odometer doesn't know you've switched to a smaller tire, so it's going to over-report the number of miles driven (the smaller diameter means the shaft turns more for each mile driven.) This will mean that when you hand-calculate your mileage, you will say "woo hoo! My mileage went up."
Because lets say there's a 3% error: You drive 200 miles but the odometer shows that you drove 206. You divide 206 miles by the amount of gas used, say 12.5 gallons ,and you get 16.48. In reality, though, you only drove 200 miles so your actual MPG was 16.
It works in reverse if you go to bigger tires. Because the speed sensor doesn't know you are running bigger tires, it will under-report the number of miles driven. Same example as above, except that you've gone to a bigger tire. The odometer shows you've gone 200 miles but in reality, you've gone 206. To get the proper MPG what you would have to do is compare the odometer reading to something that measures distance more accurately like a GPS.
(of course, some here might say "how do you know your
GPS is accurate?" but at some point you just have to stop being so picky and just go with what you have.
: )
Once you've got the multiplication factor, just figure it whenever calculating your MPG. Fill up at 300 miles (as shown on the odometer) with 16.5 gallons? 300 / 16 = 18.75 MPG.
But let's say you've figured there's a 4% error due to running bigger tires. So you should multiply that 300 x 1.04 = 312 miles, divided by that same 16 gallons = 19.5 MPG.
Finally, Ignore the silly dashboard "computer", it's worthless for calculating MPG. If you want real-time MPG data, you can get a Scangauge or Ultra-Gauge. I use both of them, the SG is in my T4R and the UG is in my DD Mazda. (Nice thing about the SG is that you can program it to show your automatic transmission temp as well.)
