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Date:         Fri, 25 Apr 2008 04:55:44 -0700
Reply-To:     Mark Drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mark Drillock <drillock@EARTHLINK.NET>
Subject:      Re: Vehicles with "Overdrive" equals  fuel economy??
Comments: To: John Rodgers <inua@CHARTER.NET>
In-Reply-To:  <4811B2E4.1070607@charter.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Vanagon trannys already are overdrive. The axle ring and pinion ratios more than counteract this fact though.

Overdrive refers to a transmission where the output shaft turns faster than the input shaft, hence is "overdriven". This is the case for all Vanagon manual trannys in top gear.

An add-on overdrive for a car is an additional gear set fitted between the transmission and the axle differential to allow a increase in shaft speed. The output shaft turns faster than the input shaft, again, "overdriven".

Some car manufacturers have sold models over the years that had an additional "overdrive" added to the output end of a standard tranny to allow for lower engine rpms without changing the tranny internals themselves. The ones I have seen had a provision to keep the overdrive from being engaged while the tranny was in the lower gears. This was found in some old Volvos and before true 5 speeds became common.

Units of this type are sold for RVs and some are reversible in that if you install it turned around the other way it becomes an "underdrive". This feature is helpful for constant heavy loads and towing.

Since Vanagons have a combined transaxle there is no place for such an add-on to fit. You can change their overall gearing with a taller r&p ratio as many people do with engine conversions. You can also change the ratios for 4th and 3rd which is even more common. Both ways require a tranny rebuild as well as pricey parts.

In any case modern engines with electronic FI and OX sensor feedback get similar mpg across a wider rpm range than old carbed engines and relatively small decreases in rpm from an overdrive can't help as much.

Mark

John Rodgers wrote: > Back in the day I remember my Dad's favorite car was a Studebaker. He > drove the life out of three in a row. He traveled on the road a lot, and > he needed a car that got good gas mileage even back then. His choice was > a 6 cylinder Studebaker with a manual transmission with over-drive. Now > I don't even pretend to know what over-drive is or how it works, but I > do know that it made a significant difference in the gas mileage. I > know that you could just cruise along in over-drive, with the engine > just barely sipping fuel, but if you needed to pass or go up a hill a > hard press on the pedal would kick it out of overdrive and additional > power was then available. I also know that if you could ever get it > going fast enough in second gear - you could get it to kick into > overdrive while in second gear and eventually move that puppy up to over > 100 mph. As to how I know - well, I was a teenager back then - so let's > not go there! ;-) . > > So, my question is, what things might be done to a Vanagon transmission > that could increase the fuel mileage obtainable? Are add-on over-drive > units available? I know using a tranny from a diesel Vanagon with > gear ratio changes is one way to go if you can find one. One can also > change the gearing a little bit in the standard manual tranny. But what > other alternatives might be available, if any? How about subbing a > modern auto-trans from any vehicle at all into the vans, and coupling it > with a late model engine of some type - say a late Subie engine. I am > curious as to how our list Guru's feel about these possibilities. Daryl? > Dennis? Anyone? > > Just speculating this morning. I don't plan to give up my Vanagons just > because fuel prices have gone out of sight, but I may have to alter my > use of them to be able to stay in budget. > > Regards, > > John Rodgers > 88 GL Driver > Chelsea, AL. >


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