Date:         Thu, 15 Feb 2001 14:39:41 -0500
Reply-To:     Derek Drew <derekdrew@RCN.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Derek Drew <derekdrew@RCN.COM>
Subject:      How Much Hosepower Is Lost In Wheel Size?
Comments: To: Syncro@onelist.com
Content-Type: text/plain; charset="us-ascii"; format=flowed

As some of you may know, I am preparing an article about changing to larger
sized wheels and tires on the Vanagon, particularly with reference to Syncro.

Among the issues that has come up is the limited amount of horsepower that
the 2.1 litre motor has to push the vehicle, and how much of this
horsepower gets eaten up driving larger tires. I have seen reference in
various places to "increased rolling resistance" of larger tires, and the
consequent apparent decrease of power to the driver of using larger wheels.

The situation I am looking at right now is one where we are switching from
26" tall tires to 31" tall tires, and the question arises as to how to
quantify the apparent horsepower percentage loss of using these larger
tires. I can compensate for this apparent horsepower loss by making sure
the gearing stays within a certain range or in a range similar to the stock
range in order to stay conservative, but I have no data to go on as far as
what the correction factor should be.

In 4th gear and 4,000rpm the stock tire travels 70.95mph wherease the BFG
27 x 8.50 / 14 tires travel 74.32 mph. One situation I am looking at is
whether to set the 31" tall tire so it is traveling in 4th gear at
77.27mph, which seems like a nice welcome bump up in effective top cruising
speed on the highway. But if the rolling resistance is much higher with
these larger tires on it, it is possible that it would be better to stay
conservative and set 4th gear at 72.56mph.

To put this into perspective one can chart:

70.95mph is stock
74.32mph is BFG
72.56 is the Conservative Option (better for having power to push the vehicle)
77.27mph is the Aggressive Option (better for higher top speed)

Can I have a discussion from somebody knowledgeable about the issue of
trying to quantify the additional rolling resistance in terms of
horsepower? I am not sure if the sheer additional weight of the larger
wheels and tires also degrades the apparent horsepower as well, and needs
to be calculated separately. Possibly neither of these two elements of much
effect and we can forget them. And can I also hear opinons about whether
the 4000rpm-in-4th speed for this scenario should be set at 72.56mph or
77.27mph?

I can say from personal experience that the BFGs seemed to tucker out the
motor a little bit to my experience, and so I did not experience the
mileage per gallon gains one might have expected from bringing the rpms
down. That is, it is my experience that the BFGs are toward the top end of
a good user experience. If you disagree I need to know this now because it
will influence further calculations. Ideally, I would have set 4th and 4000
at 81mph, but after thinking about it, I sadly feel that the stock motor
cannot handle this very well and will poop out the benefit that would
otherwise be achieved.

I am speculating that it is possible one can make up for some or all of the
hosepower losses by means of the ratio rocker/ECU chip and other Lilley
mods, but I can't even address that question until quantifying the loss
from the larger tires.

Needless to say, if you already installed your subaru motor or TDI or
whatever, you are less sensitive about this issue, and would likely go for
the 77mph gearing or even higher is possible. Depends how much those motors
like to be run above 4000rpm and what their power is so it is an individual
case there.

For the record, the situation here involves either 7.50 x 16 tires or
235/85 x 16 tires running 6.17 ring and pinions and a 4th gear of either
0.82 or 0.77 which are alternate to the factory 0.85 4th.
_______________________________________________
Derek Drew                                            New York, NY
CEO & Co-Founder
http://www.ConsumerSearch.com/
===========================
"Best Expert Review Site"
for product reviews on the Internet
Jan. 2001, PC World Magazine
==========================
80 South Street, 2nd Floor
New York, NY 10038
derekdrew@rcn.com
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