# Piston Slap: The Fallacy of Engine Hour Meters?

By on July 16, 2015

Ryan writes:

Sajeev Mehta

It appears that you have picked up where Steve Lang left off. That is answering general automotive questions that puzzle automotive enthusiast.

My question, what average speed would be good or bad for a used vehicle? I am in the market for a used truck (3-6yrs old), and have noticed that these newer trucks have mileage and hours tracked in the dash display screen. It is easy math to calculate the average speed for the life of the vehicle. What should I look for?

Or run from?

Best and brightest?

I’ve been doing this Piston Slap thing for how many years now? (Hint: longer than Lang’s written Q&A columns.) Personal affronts to my fragile ego aside, you ask a good question.

With the advent of engine hour meters on modern gauge clusters, calculating average speed is easy.

But to make a purchase decision on that figure? No, considering the plethora of gearing choices and driving conditions that induce wear independent of your calculations. And the fact these meters can be reset … more on that later.

A higher average speed implies more highway driving, which is almost universally regarded as better than city miles. Vehicles with higher average speeds spend less time on the brakes, or accelerating from a standstill, or wearing engines out with cold oil, etc.

But here’s a problem: Average speed displays can be reset. Disconnect the battery for a few hours.  Or re-flash the computer for a performance tune to wipe that number clean while adding value to would-be buyers looking for more performance and/or fuel economy.

I care not for this metric. When buying a used vehicle, stick to the basics: inside/outside condition, service history, fluid age/quality and the lifespan of typical wear items like tires/brakes. If the engine hour meter and odometer also agree with your assessment, then hey, even more reason to open your wallet.

[Image: Shutterstock user Forgem]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry … but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

## 27 Comments on “Piston Slap: The Fallacy of Engine Hour Meters?...”

• PrincipalDan

Interesting assessment from one of the original priests of Panther Love, the first place I saw an hour meter in a car (instead of a tractor or other piece of industrial equipment) was in a late production CVPI.

So here’s a question: Why can the hour meter be reset unlike an odometer which cannot?

• sproc

Federal law prevents odometer tampering. I don’t think hour meters (if a car even has one) has any more legal standing than a trip meter.

• Scoutdude

Now I’m not sure about the trucks but the purpose in the P71 is to track idle hours, not total hours. Of course it is common for a p71 to sit idle on the side of the road while running radar, writing tickets, working a collision ect. So looking at the cars at auction I’ve seen them with 200hrs on the meter and 140K on the clock and cars with 2000 hrs on the meter and 100K on the clock. So at least in that case they exist for maintenance purposes. Presumably they have some sort of algorithm to determine when to do service based on the combination of miles and hrs since the last service.

• I know someone who has a Lincoln navigator with 110,000 miles on it .

if he hadn’t disconnected the odometer it would have over 250,000 miles on it .

• pacificpom2

If the manufacturers based oil changes and/or maintenance on the engine hours, like most military or plant equipment, the meter would make sense. Without a line like “Change oil every 500 eng hours”, the meter is senseless.

• LS1Fan

A meaningless number.Ive known folks who changed entire IP clusters and reset their vehicle computers in the course of regular mods and repairs.
To say nothing about deliberate attempts to conceal the history. If a car has an electrical issue the meter would doubtlessly say its barely been driven….because it is being disconnected from the battery rather frequently.

• On a lot of newer cars, the cluster no longer stores the odometer data. It’s in some module deep down in the chassis. But yes, this can and does happen, which is why it’s necessary to perform due diligence and ensure that the odometer statement is commensurate with the condition of the vehicle. Typically, a good sheaf of maintenance records would help account for the car’s history and rate of age.

• HerrKaLeun

why wouldn’t they make it as hard to reset as the odometer?

anyway, buy a \$20 brake fluid tester, a \$60 OBD dongle (via Bluetooth to your phone), tire tread gauge, voltmeter… and you can scare the shit out of the seller. Use the jacks to lift up the car in question to check brake line corrosion near the rear wheels (remainder usually plastic). buy a Haynes manual of the car in question (a good \$20 investment that helps you understand what you are looking at). Verify the service history mileages so you can spot if odometer was manipulated. Take the wheels off and measure rotor and inspect brake pads. Do the checks recommended int he Haynes manual and you will be in the top 95% of private buyers when it comes to avoiding a bad car, or negotiating a discount.

Every used car has problems coming up. EVERY seller sells the car because he/she knows some things (even if just some larger wear items) are coming up. NOONE invests a lot of money right before selling. You just need to find out what the upcoming items are and if you can live with them based on the price you pay.

Sign up for a forum for the car in question to learn about specific weaknesses to investigate. check via VIN if there are recalls / TBDs and check if it all is taken care of.

• When I made my first (used) car purchase in 1985, I gave all the cars I looked at compression tests. I was looking mostly at old (no newer than ’71) slant sixes, and none of those engines were in really good shape. I ended up with a ’77 Corolla that passed the compression test quite nicely, which I drove from 91k to 161k. It was indeed a good engine, despite my thrashing it. (1.2 liter!)

• Scoutdude

The hr meter is hard to reset just like the odometer. Sajeev was talking about the average speed readout in the trip meter section which is as easy to reset as selecting that reading and pushing and holding reset for a couple of seconds, just like the running MPG total or a trip odo reading.

• danio3834

I like me some engine hour meters. High engine hours but a low odometer reading indicates a lot of idle time and is a better representation of the wear on the engine.

• fincar1

My father once bought a GMC dump truck that had been used on a job in Alaska. It needed an engine rebuild way before it should have. Speculation was that it had been used during the winter and left running 24/7 for some period of time. This is about the only situation where I can see an hourmeter being of use for a vehicle.

• heavy handle

It’s just one data point, but it’s not completely useless when checking-out a used car.

Similarly, I always look at the average fuel economy and distance traveled to make sure they are somewhat reasonable. Mind you, a used car that’s been sitting on a lot will have poor recent fuel economy because it’s either idling, being moved around the lot, or getting short test drives (a few blocks of stop-and-go, a full-throttle merge onto a highway, no cruising, and some more stop-and-go).

• ...m...

…unlike automobile engines, engines used in industrial applications generally operate for long periods at constant load and throttle, which makes hours a well-suited proxy for engine wear…automobile engines operate under highly-variable load and throttle conditions, so the correlation between engine wear and hours doesn’t hold, but conveniently miles driven tend to scale proportionately with variable operating conditions, so instead we use mileage as a proxy for engine wear…

…in short: hours don’t tell us much about wear-and-tear to automobile engines, and miles per hour even less…i suspect engine hour meters are offered on automobiles mostly as an ego-stroke feature, because “industrial-grade” marketing appeals to certain demographics…

• danio3834

They’re mostly offered on vehicles that are sold heavily to fleets. Many fleets prefer to use that metric on service vehicles that accrue a lot of idle time.

• ihatetrees

Excellent post, especially the point about industrial applications that use engines under constant load and throttle.

Average speed could be very misleading – “average” paints a very weak picture of long-ish events.

Example: Consider – 3 rooms with a temps of 35, 70 and 105 degrees. Put 90 year old Grandma, dressed in PJs, in each room for 2 hours. She’ll probably die – from living with a 70 degree average temp over 6 hours.

A single metric that could point to (long term) engine and car reliability is TOUGH to find. Maybe an interactive graph, over years, showing revolutions per minute… And another long term graph with total revolutions… BUT – you’d need the ability to drill down into the data to find odd spikes – i.e. the odd day when the rental Camry was at the track and repeatedly red lined. And you’d have to compare these graphs to graphs of similar cars – and that data isn’t available (yet?)…

• If it is a diesel truck, the calculation is meaningless, since it has spent many nights idling under my apartment window.

• Hummer

The above picture is actually from an 06 Duramax.

The design was used in tons of GMs, the 5k redline gives away it’s a duramax, and the M on the shifter gives away it’s a 6 speed.
… or maybe I’m wrong and it’s the 8.1…

I felt so confident until I remembered the Big block. If only we could see the oil pressure mid-value.

• bball40dtw

Either way, the powertrain is virtually unkillable. Gimme the 8.1, all day though. I have a great fondness for the 2000s big block GM trucks and V10 Ford trucks.

Actually, the idling truck example is why hour meters are important. Lots of idling is causing engine wear that would not be reflected by the odometer reading.

In addition all diesel ECM’s record average engine load as a percentage which gives you a pretty good indication of how hard the motor has been worked. I have seen heavy duty trucks that averaged a 50% load and off road equipment that averaged a 95% load; that makes a big difference in service life.

• ezeolla

My Pontiac has an average speed indicator in there with the MPG computer. I reset it each time I get gas (along with the MPG one, I like to see how they correlate on each trip). The one thing I have learned is that your average speed is waaay lower than you think it is

• whynot

That is very true. My Golf records average speed, mileage etc over an “extended period” (something like the past 99 driving hours then it resets), since last filled up, and since start. My average speed is usually only ~19 mph.

• SWA737

Most modern (electronically controlled) outboard motors for boats have digital hour meters with the ability to download a breakdown of RPM range. For example; XXX hours spent from 500RPM – 1000RPM, XXX hours between 1000RPM and 1500RPM etc. It gives a fairly accurate indication of wear and tear on the engines. Without a similar function, automotive hour meters seem like a bit of a gimmick.

• I think the toughest thing on an engine is the time spent warming up. The colder the ambient temperature the tougher it probably is.

Years ago, Consumer Reports wanted to compare different oils–some synthetic, some dino. The test vehicles were NY city taxis, under their regular use–which have a very low ratio of warm-up time to fully warmed time running. They tore the engines down at 60k. They found virtually no wear, and couldn’t tell the difference between engines according to the type of oil used.

• fincar1

This is just an anecdote but worth considering. In the naval ship repair facility where I worked we were a 24/365 operation. The speed limit in the shipyard was 20 mph. The shipyard had lots of railroad tracks and crane tracks, so actual speed was even lower than that. The vehicle we had was probably started twenty to thirty times each dayshift, about half that often on backshift, trips were never more than 4 miles maximum and most often one mile or less. Under those circumstances the motor was most often not quite cold but also not quite up to normal operating temperature.

I consider that to be quite severe service. Under those conditions a 1978 Dodge slant-six pickup lasted at least 100k miles. We did replace several starters though.

IT note: All the cruft that runs on this site causes it to be slow in general and especially when one is trying to write a comment.

• sgeffe

Even when using AdBlock Plus on IE11, if I open several tabs to TTAC content, each tab starts sucking CPU like crazy, especially if I sleep or hibernate the computer; probably some sort of JavaScript code in some sort of loop.

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