73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Wheels & Tires => Topic started by: Sodium Duck on January 08, 2012, 08:56:08 pm
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I need some help figuring out the math on this one.
My speedometer is off by 5MPH. When I'm doing 35MPH, I'm actually doing 40MPH - roughly.
How do I figure out how far off my odometer is? I'd like to calculate what my actual MPG is. Just topped the tank off today.
I have 35" BFG Mud Terrain's. I believe the stock tires were 28's?
I think what I do is figure out what percent bigger my new tires are. so (35-28)/28=.25. So they're 25% bigger.
So whatever I get for mileage until my next filllup, multiply by 25%. Then do the miles driven divided by how much gallons I put in to get the MPG.
But 35's aren't actually 35" tall. And I'm not sure if the truck did originally have 28's. And I'm just guessing I'm about 5MPH off on my speedometer.
Did I just confuse myself? Yes.
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My 2000 GMC crew cab 4x4 has bigger tires on it. What I did was use gps and determine how far off my mileage was ( on my truck no matter what speed, I am going 3 mph faster than what my gage says ).
Next I determined a mile distance on the road driving and clocked my truck, it was spot on for some reason, even though my mph is wrong, my odometer is correct in showing miles driven. ( My work truck, 02 f350 diesel is the same way, it shows me going 2.5 miles faster than I really am, though the odometer clocks the miles driven exactly ).
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what i would do is use mapquest and find out how far the gas station is from your house lets call it point "G" your house being "H" ="MPG" drive to the gas station fill up dont just drive from gas station to home drive around town but keep a simple root or the long way home then drive back the same way use and fill up see how many gall it took.use this rout when your finding the distance on mapquest or if you live a good distance away go to the gas station fill up drive home G+H=X. then go to work from home H+W=Y then W+G=Z. then X+Y+Z=MPGS simple lol but anyway just use mapquest to track your millage then go from there
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If all you did was change tire size then it will be off by a percentage of that circumference change on the tire. It isn't off a certain MPH it is off a certain percentage and as you go faster the MPH difference gets bigger. If you say that it is off about 5 MPH at 35 MPH on the speedometer you are actually doing 40 MPH then that is reading slow by 12.5% (1-(35/40)). That would mean that if you only changed tire size and you have roughly 35" diameter tires now that it previously had about 31" tall tires (1-(31/35))= 11.4%...(1-(30/35))=14.3% (which is about right for one of these trucks).
Your mileage will also be reading 12.5% less than actual. So if you times your mileage by 1.125 you will be pretty close (assuming all of the information you provided is correct).
If you actually use a good GPS to track your speed and mileage and compare it to your speedometer you will find that it is not an exact science on vehicles with large sidewalls. The tires grow at speed and consequently the circumference increases, slowing your speedometer down slightly. Also, tire inflation pressure affects the speedometer too. If you compare your odometer to the GPS over various driving conditions you can arrive at the best compromise. A GPS is a great tool for this, but if you try to get it exact you will pull your hair out because the GPS will reveal all of these little variables.
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You did not say what year your truck was ? If it has a cable driven speedometer Change the gear on the end of the cable , 1 tooth smaller should speed up the speedometer just about 5 MPG. Ralph..
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I do have a cable driven speedo. I was thinking about regearing it, but hadn't researched how to do it yet.
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You could just get a Mechanical Speedometer Ratio Adapter like this one:
(http://www.transmissioncenter.net/RatioAdapter.jpg)
Very simple to install! I have one on my '77 K20.
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Yes, I come up with the same 12.5% correction factor here. Just use that to correct the odometer reading to find your true miles traveled. Or use a GPS type app in your smart phone.