Need some mathmatical help.... - Page 2

junior3382 · 08-20-2008, 09:32 AM

electronic.... there is just signal wires to tell the guage what to do.... So I would think it would be mare adjustable that way?

And 75.... the last formula in that chart,(thank you BTW) indicates that I take the new tire size.. 35" and devide by old tire size... 28" and then multiply by old gear ratio... 3.09 to get the new one.... amd I understanding that right? If so then the new ratio only has to be 3.87? seems wrong to me... and if this is right, then with 4.56 my speedo would read the other way? meaning when it says 65 I would be going slower? I am just lost.... perhaps I should just leave this to the professinals....lol... I will get it eventually.

75K30 · 08-20-2008, 10:04 AM

Dont get true MPH, and indicated MPH mixed up. The indicated value is what youe are trying to correct, but it is easy, and requires less math than you think.
With your new tire size (35") and 4.56 gears, you will be right on the money, and turning approx 2845 rpm at 65 mph. Given this formula, you can equate the differences in MPH from one gear set to another.
Here is a chart and the formulas. By knowing the rpm's at 65 mph, you can input both varibles, to determine how much the speedo (indicated) will be affected. You will have to know whether or not the speedo is correct now.
4Lo.com :: Gear Ratio & Tire Size Chart

gmctrucks · 08-20-2008, 05:01 PM

That's it you got it now. HUH ..........................

junior3382 · 08-20-2008, 11:37 PM

75... when you say RPM... you are tlaking engine RPM right? cause I have seen that chart too, but I thought it was to regulate the differences between gas mileage and horse power when increasing gear ratios. But let me see if I understand..... right now.... if I cruise at 65mph, my tac is reading almost 2900 rpms.. so I assume, about 2875? and that is on a 3.09 gear ratio on a rough measurmant of about 28" for the tire.... which is not on the chart. Lowest it goes for 28" tires is 3.31..... to increase form there to 35's at 3.31 would be truning at 2065, but assuming i had 3.31 the 4.56 would get it as close to factory as I could.... but I have 3.09..... does the same concept still apply to my tranny? Meaning when I set it at 4.56 it will stay closet to factory, or would I be better off going with 4.88's or 5.13's..... or would I go higher to 4.11's? Or am I just not getting this at all?

08-20-2008, 09:32 AM	#11
junior3382 Senior Member Join Date: Mar 2007 Location: Currently, I live in Lancaster county, Pennsylvania. Posts: 3,170	electronic.... there is just signal wires to tell the guage what to do.... So I would think it would be mare adjustable that way? And 75.... the last formula in that chart,(thank you BTW) indicates that I take the new tire size.. 35" and devide by old tire size... 28" and then multiply by old gear ratio... 3.09 to get the new one.... amd I understanding that right? If so then the new ratio only has to be 3.87? seems wrong to me... and if this is right, then with 4.56 my speedo would read the other way? meaning when it says 65 I would be going slower? I am just lost.... perhaps I should just leave this to the professinals....lol... I will get it eventually. __________________ *The Blazer is dead, so I pumped her with lead, laid her to bed, and bought a FORD instead!*

08-20-2008, 11:37 PM	#14
junior3382 Senior Member Join Date: Mar 2007 Location: Currently, I live in Lancaster county, Pennsylvania. Posts: 3,170	75... when you say RPM... you are tlaking engine RPM right? cause I have seen that chart too, but I thought it was to regulate the differences between gas mileage and horse power when increasing gear ratios. But let me see if I understand..... right now.... if I cruise at 65mph, my tac is reading almost 2900 rpms.. so I assume, about 2875? and that is on a 3.09 gear ratio on a rough measurmant of about 28" for the tire.... which is not on the chart. Lowest it goes for 28" tires is 3.31..... to increase form there to 35's at 3.31 would be truning at 2065, but assuming i had 3.31 the 4.56 would get it as close to factory as I could.... but I have 3.09..... does the same concept still apply to my tranny? Meaning when I set it at 4.56 it will stay closet to factory, or would I be better off going with 4.88's or 5.13's..... or would I go higher to 4.11's? Or am I just not getting this at all? __________________ *The Blazer is dead, so I pumped her with lead, laid her to bed, and bought a FORD instead!*

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08-20-2008, 10:04 AM	#12
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	Dont get true MPH, and indicated MPH mixed up. The indicated value is what youe are trying to correct, but it is easy, and requires less math than you think. With your new tire size (35") and 4.56 gears, you will be right on the money, and turning approx 2845 rpm at 65 mph. Given this formula, you can equate the differences in MPH from one gear set to another. Here is a chart and the formulas. By knowing the rpm's at 65 mph, you can input both varibles, to determine how much the speedo (indicated) will be affected. You will have to know whether or not the speedo is correct now. 4Lo.com :: Gear Ratio & Tire Size Chart

08-20-2008, 05:01 PM	#13
gmctrucks Senior Member Join Date: Nov 2006 Location: Houston,Texas Posts: 4,030	That's it you got it now. HUH ..........................