got to thinking… which is scary in and of itself… but I thought I would share my thoughts anyway.

Speed.  Or lack there of.  Fuel efficiency.  Or lack there of.  Scheduled maintenance.  Hmm…

Like many folks reading this, I’ve fitted larger tires on my truck.  Like a lot of you, I run 265/75 16s.  I’m sure you’ve noticed your speedometer is off.  If you haven’t, it is. 

Running three different GPSs over several semi-controlled experiments, I found my speedometer/odometer to be about 11% off.  If I am going an indicated 65 mph on the speedo, in reality I am going just over 72 mph.  No biggie, I just run around an indicated 60 mph which is equivalent to 67mph and forget about it. If my odometer says I have gone 100 miles, I have really gone 111 miles.

So what’s the big deal?

Let’s take a look at the components of the term “mph”.  Miles Per Hour.  If your mph is off 11% like mine, guess what’s really off… your miles. 

An hour is an hour is an hour.  It’s constant.  It doesn’t change, but your miles do.  If you are going 65 mph that means in 1 hour you will cover 65 miles.  If your speedometer is off 11% like mine is, you really have covered over 72 miles in the same hour.

Now, think of the total time you have driven your truck. If you've been behind the wheel 1,000 hours... that is a constant.

For example: if you have 67,500 miles on your odometer and you have had larger tires on the truck over these 67,500 miles, that means in reality you have 11% more miles than the indicated 67,500; your truck really has nearly 75,000 miles on it!

And, how frequently do you change your oil?  If you do it according to Land Rover, you change it every 7,500 miles.  If you do it according to common sense, you more than likely change it every 3,750 miles (half of the 7,500).  And some of you change it every 3,000 miles.

So let’s talk about a few things…

I already talked about speed.  If your speedo is off by 11% like mine, here’s a chart of how fast you are really going…

Back to oil.  How frequently do you change yours?  If you said 7,500 miles per Land Rover’s recommendation, then taking into account the 11%, you’re really not changing the oil until it has been in your engine over 8,300 miles!  If you go with the 3,750-mile interval, in actuality you are changing it at just over 4,150 miles.  And you 3,000-mile folks are changing it after it has been in your engine for 3,333 miles.  And beyond oil… think of all of the other maintenance items that are based on mileage…

Now, let’s say you wanted to buy a used rig.  Let’s say you found one with 82,000 miles on it.  That’s not too bad.  Good price?  You decide to buy it.  If this truck has had larger tires on it it’s entire life, and if the speedo/odometer was off 11%, the 82,000 miles indicated on the odometer is really over 91,000 actual miles on the truck!  Would you pay the same thing for a truck with 82,000 miles verses a truck with 91,000?

So, what are you saying?

Watch your speed.  If you have larger tires, check your odometer against a GPS.  Find out if it is off.  If it is, calculate the difference by using the following method and calculation:

Reset your odometer and GPS to zero.  Drive 10 miles (preferably in a straight line… interstate… on a clear day… so GPS isn’t affected).  How many miles does your GPS say you went?  Mine would say 11.1.

The difference I found is about 11% with 265/75 16 tires.  Your truck may be different.  It may be 9% off… it may be 13% off… it may not be off at all (but I bet it is though if you have larger tires).

Oil changes? If you truly want to change your oil at 3,750-mile intervals and your speedo/odometer is off by 11%, then you have to change it at 3,380-mile intervals using the odometer. And for those of you who let it slide to a 7,500-mile interval, you would change yours at a 6,760-mile interval if you used your odometer.

A silver lining?

On a positive note, say you fill up your tank with 25 gallons of fuel at an indicated 300 miles on the trip meter, doing a quick calculation, that's 12 mpg. But, that's also off by 11%... so in reality you're really getting 13.3 mpg! WOO HOO!!!

Another positive note is that you can sell your rig for more than it should be worth to someone who hasn't read this article...

I guess you could always get your speedo/odometer corrected and avoid all this...

As always, if you have any questions, feel free to ask!

-- Bill Mallin