# What would my actual MPH be after gear AND tire change?

Discussion in 'The Garage' started by y5mgisi, Jul 14, 2006.

1. ### y5mgisi1 ton status

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I went from 3.08's to 4.10's and from 31's to 36's anyone know how to figure out my actuall MPH?

2. ### smokkey11/2 ton status

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3. ### goldwing2000Guest

Man... that's some crazy damn chart.

Just take the truck on the freeway and count off milemarker posts. Write down your odometer reading at one post, drive for 10 miles, record the reading at post #10. Divide the number of miles your odo clicked off by ten and divide the mph on the speedo by the result. That eliminates any effects of torque converter slippage or minor differences in tire diameter.

It sounds complicated but it's really easy.

For instance, when I had 3.42 axles and 35" tires, my odo clicked off 7.9 miles for every 10, which gave me a factor of .79 for every mph on the speedo. I made a chart to stick on the dash so I would know how fast I was going.

7.9/10=.79

20/.79=25.31
25/.79=31.65
etc.

Speedo Correction
IndicatedActual
20=25
25=32
30=38
35=44
40=51
45=57
50=63
55=70
60=76
65=82
70=89
75=95
80=101
85=108

4. ### 4X4HIGH1 ton statusPremium MemberGMOTM Winner

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I don't think you can calculate like that since the odometer is a fixed and not a variable, what i mean is the speedo cable has 1000 turns per measured mile whereas the speedo works on a magnetic force which varies by cable rotation speedo.

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6. ### goldwing2000Guest

Right. The odometer is fixed but it's still based on revolutions of the speedo cable and therefore revolutions of the transmission output, not revolutions of the engine.
I never said to use the speedometer for calculations, only for indications. Same as if you're driving.

Trust me. It works. I've checked it against other people's speedos.

7. ### goldwing2000Guest

That still uses engine RPM and theoretical tire size (a 35x12.50-15 tire is not exactly 35" in diameter, especially if it's installed on a narrow wheel) and doesn't account for variable T/C slippage. It also does not take factors like different speedometer gears into account and doesn't tell you what your speedometer error is.

Short of asking a cop to hit you with his radar, comparing your measured miles to actual miles is the most accurate way of figuring your correct speed.

8. ### Chaddy1/2 ton statusGMOTM Winner

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I went from 3.08's to 4.10's and from 32's to 35's and my speedo reads 4 mph over.

9. ### 1985_K5_Silverado1/2 ton status

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If you have a GPS unit (they are pretty cheap these days), you could use one of those to read your speed. I think a few forum members use them for that purpose, in additon to navigation.

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11. ### pauly383Daddy383Staff MemberModerator

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Mine was under 100 bucks , and is USB into my laptop . I am 5 mph off at 55 with 3.73's and 33's in 4th on a 700r4 .

12. ### y5mgisi1 ton status

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good idea.

13. ### colbystephens1 ton statusGMOTM Winner

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since speed is determined by ratio, i would find it hard to believe that you would only be off by 4 mph over the whole range of your driving speed. as an example, take a look at goldwing's first post in this thread. the error starts out relatively small, but as his speed increases, the error increases much more.

14. ### 88Silverado1/2 ton status

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15. ### goldwing2000Guest

Yep. You're always off by a percentage, not just a number. Unless your speedo is computer-controlled, that is. The speedo in my 2004 Mazda reads consistantly 2-3 mph high, throughout the range. That's a "safety margin" that the manufacturers like to put in to keep you from speeding. Can't do it with a cable, though.

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