Date: Thu, 16 Jul 2015 09:36:06 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Crazy Idea On Mismatched Tires
In-Reply-To: <CAH32RNY=rA16UTOzKr2qpa-zts88_xpgHP9-bQBivz3wMtRKSw@mail.gmail.com>
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For a number of reasons I would go with larger tires all around. 215/70-15 should work on all fours. Your wheel offset may make an issue up front. That tire will get you a 98 load index though. Just be sure the tire choice is one that is rated to be inflated to 51 psi. This will give you a tire almost 27" diameter as compared to the 25.6 you have now. From 817 revolutions per mile to 744 revolutions per mile. You can use your head and GPS to compensate for the speedometer error. The Vanagon odometer reads high so this change will also correct it.
This is one of the reasons I like to go with 16 or 17 inch wheels. The 225/60-16 and 225/55-17 tires are 26.5 inch. The ideal compromise in size. Both of these sizes are readily available in extra load ratings (101 or 102), and there are so many high performance and winter tire options available.
While slowing the engine down makes for more comfortable cruising the fuel economy savings is minimal with the water boxer. You still are accomplishing the same work which is pushing that box through the air. As RPM's go down for a certain horsepower requirement the torque or stress has to go up. This begins with longer term increased combustion pressures in the engine resulting in higher peak flame temperatures. The result is increased emissions of NOx, (why these engines were low compression and geared to spin fast), and increased tendency for detonation, (knock and ping). The detonation usually goes unnoticed until you have to deal with the damaged pistons, rings, or burned valves.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Loren Busch
Sent: Wednesday, July 15, 2015 11:54 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Crazy Idea On Mismatched Tires
Okay, this wacky idea popped into my head while driving today.
I now have a 2.3L WBX with a lot more power in my '90 Westy with automatic. I could (but cannot possibly afford) go to a higher 3rd gear in the auto tranny, adds say it would drop the RPM at cruising speed by 800 RPM. And thus save some gas on long highway runs.
But what if I did this? Go to larger tires on the rear? Why only the rear? To keep the speedometer and odometer accurate.
Right now I am running 205/65 R 15 all around. If I were to go to a tire with larger circumference on the rear, although loosing a little power at the ground I should drop my RPMs at cruise.
So, is there a fault in my logic? And would my gain be so small as to not be worth the effort? BTW, I haven't tried to do the math on this one yet, just thought I'd throw it out.
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