Speedo off 8 mph

thecantaloupeman said:

So I did some digging online and read that I may need to change the gear size for my speedo, but I'm wondering if my issue is with the gauge or cable rather than size of gear. The original tires were 9.5x16.5s and I have 8.75x16.5s. Someone told me the 9.5s are slightly taller, but I was wondering if that slight of a tire size change could cause my speedo to be 8mph slow.

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C10MixMaster said:

8.75R16.5
Diameter : 29.5"
Width : 8.8"
Wheel : 16.5"
Sidewall : 6.5"
Circum. : 92.6"
Revs/Mile : 684

9.5R16.5
Diameter : 30.6"
Width : 9.5"
Wheel : 16.5"
Sidewall : 7.1"
Circum. : 96.1"
Revs/Mile : 659

Original Tire Size
8.75 x 16.5
New Tire Size
9.5 x 16.5
Speedometer Reading
65
Actual Speed
70.57

you are actually going 5.5 miles per hour FASTER than what the speedometer says by calculation based on 65 mph . Due to variations in tires that's usually a little off. so 8 mph is probably rite.

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bucket said:

That's backwards, but good info anyway.

It can also be the speedometer itself is no longer calibrated properly. My '78 is a good example of that. With the original tire size, the odometer reads dead on to the tenth of a mile, but the needle itself reads several mph high while going interstate speeds.

So with the incorrect tire size and a potentially slightly incorrect speedo, it could easily be off by 8 or more at higher speeds.

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