By Pete Snidal, Cheapskate Mechanic (C)2000

Calculating Your Mileage

Having been in the business of trying to get people interested in some products which really do increase mileage in every case it has yet been my privilege to examine, I have found a startling fact: Many People Don't Know How To Calculate Their Fuel Mileage.

Now I'm not interested in being judgemental about this - a lot of us have been out of school for some time, and a lot more of us never learned much while we were in there, so I've prepared a file you may want to click on: How To Calculate Your Fuel Mileage

Once we're past that, let's discuss a few of the parameters that affect fuel mileage, for any gas engine, whether a single cylinder motorcycle, or a multi-cylinder car, truck or bus application.

Checking Engine Condition

If you are to get good mileage and power, your engine must be in good condition. Valves and piston rings must be sealing well - leaky valves allow expensive fuel to wheeze out before it's burnt, and bad ring sealing will do the same. So you should check your engine's condition by doing a compression test, using a reliable compression gauge. If your engine's in poor condition, some of the fixes described here may help, but none as much as doing the rebuild that is really necessary. And while you're at it, be sure to change the camshaft and lifters - a worn camshaft will do awful things to power and mileage, and most factory-stock cams are bad comprimises, trading things like low emissions at idle speeds for economy at running speeds, stuff like that. Look around the aftermarket parts sources for a camshaft that is more tailored to your specific needs.


1. Ignition Timing - The Flame Front

Ignition Timing is all-important to getting decent mileage and life from your engine. The topic is covered in detail in the file PINGTIME.HTM found elsewhere on this site.

2.) Exhaust Tuning

Most race engine builders have discovered the importance of Exhaust Tuning. Your exhaust system has a resonant frequency, just like a radio antenna or a trombone, and there is a proper length for your exhaust system for any given rpm, or pulsation frequency. There are formulae to determine this, which factor in pipe diameter and length, rpm, no. of cylinders on the pipe, and other factors. I've always been a "thumbnail tuner," myself, and I found what's best for the average V8 is dual pipes with a cross-over, and a pipe length of 11 to 12 feet. My 391 powered 36 foot Ford converted schoolbus, weighing in at a conservative 28,000 lbs., running a Holley 4 bbl carb and Allison 4-speed automatic, consistently delivered just under 7 mpg running up and down the west coast between B.C. and BC. This is with pipes that were not crossovered, and at a length of 12 feet. - That may not sound like much, but in our travels we met a number of incredulous owners of Ford 3/4 tons, carrying campers, and powered by the very similar Ford 390, who seemedto have a lot of trouble believing me. "That's more than I get! they'd say!


So much for the "cheap" stuff - after all , you need an engine in good condition, timed one way or another, and an exhaust system in any event. Now, let's look at some mileage "frills" - things you can get by without, but maybe, in the interests of good economy, you shouldn't.

A good place to start is your ignition system. If you're still running points in your distributor, you should look for an alternative high-energy electronic ignition system. Since these have been stock equipment on just about every engine made since the early '70's, auto wrecker parts can do this for you.

There are also "high performance" ignition systems on the market designed to replace stock electronic ignitions, claiming even better mileage, power, and even longevity. But, like most other things for sale, this is a good time for caveat emptor - buyer beware. One thing of which to beware is the cost vs. the projected savings. For example, I recently checked out an ad for an ignition system that comes with a 30-day guarantee that it'll give me 15% better mileage - but the price is over $500.00 US! Wheeee! The payback time, even if it works, will be quite a stretch! (On the other hand, using Pro-Ma GT5 gas treatment, and MBL8 Oil Treatment in a various healthy engines has shown me 25% or more in every case. Price to try: under $30.00!)

An Extremely Good Idea in ignition systems comes on many new vehicles, but the aftermarket version is, so far, something I've heard about but never yet seen. I heard that Carter made it, or even maybe still do. It's for electronic ignitions only, and it's basically a black box that fits into your ignition system, and electronically lags your timing JUST ENOUGH to stop the ping. It's input is in the form of a sensor or two that bolts onto your cylinder head(s). These sensors sense ping, send the info back to the box, which lags the spark just enough to stop the ping. In the installation, you set the distributor 'way too advanced, and the box makes sure the ping doesn't happen. Ideal, and I've been looking for one. All I've been able to find is that Bosch supply such a system stock on later VW's, Saabs, and Volvo's, but I don't think it'll bolt on to my Ford 391 and 460's.

There was an ad on the Art Bell show a few years ago which sounded like it, but by then I wasn't into RV's, and my cars and bike do very well anyway, just keeping them properly tuned, and of course using my favourite fuel and oil additives. But I'll be trying to track this thing down for my Flxi for sure - rear engine, no possiblitly of hearing ping, so I'll have to set my timing conservatively or find one of these gadgets. The price I heard was around $200.00. If you run into one, please let me know. Click Here To Mail Me (Editing note: I finally tracked it down - it's the $500.00 one. Later.)


Ignition timing is not the only timing variable in an engine. There is also camshaft timing - the opening time, closing time, duration, and lift of the valves can make huge differences in the performance characteristics of an engine. Such characteristics as power band - the range of rpm in which good power is developed, maximum horsepower - generally, you trade big power for a narrow power band, wider band gives a torquier, lower-rpm engine with greater range of roadspeeds in any given gear, and yes, mileage; all these can be traded around for one another by choice of camshaft.

Different manufacturers, like Iskenderian, Crane, etc. make after-market cams, ground off the normal range, for specialist apps like high-end power (race cars) mid-range torque (us) and torque/mileage. the latter are often referred to as "RV cams." RV doesn't necessarily mean just for RV's, by the way - we used one in a 460 Ford-powered dirt track stock car for years - but it does mean wide mid-range, and hopefully better mileage. Although I've tried one in a race car - we wanted wide-range torque and low rpm with tall gearing for motor life, and it worked well against all those wheezy little small-block Chevys - I have yet to put one in an RV engine. Next time, for sure. If my 391 isn't satisfactory in my new Flxi, I'll build a 460 with RV cam for certain.

Changing cams does cost money. Not a lot extra if you're rebuilding the engine, but it's a bit of a hassle to do midway through the engine's life, unless you're got it apart for something like a top end or freshening. Definitely a thing to consider in these cases particularly, since a worn cam will give you really weird timing, with definitely detrimental results to power and mileage. I understand that smallblock Chevvies respond particularly well to cam replacement - they're only $35.00 or so, so you should always replace them when rebuilding.

Additives, You Say?

Finally, you can spend just a little money trying additives. There are thousands of them on the market, and you can spend a lot of time trying different ones until you find one that works. I did this for quite a while, and as you know, found one that works very well. I use Pro-Ma oil and gas additives in everything I own; wouldn't dream of doing otherwise, and it saves me a bundle. You can read all about that elsewhere. By clicking on the link, for instance.)

Don't be a Hick! - Click the pic!