Tire size and speedometer

[big mike71]

If my new tires are about an inch taller than the old ones, and I'm going 55 by the speedo will I be going faster or slower than with the old ones.

[handsomerob]

Faster

[big mike71]

That's what I thought, But I drank so many beers while I was putting them on I wasn't sure. Thanks

[handsomerob]

No prob, I'll give a brief explanation although you may not care,

say with your normal size tire you are traveling @ 55mph and the engine is running @ 2700 RPM now say you stock tire was 30 inches tall thats a circumferance of 94.25 inches so for every rotation your truck moves 94.25 inches.

Well now take that same RPM and a 31 inch tall tire, that tire would move you 97.4 inches per rotation.

so your speedo says it 55mph cause its calibrated for the stock tire size, but in reality you are moving faster.

hope that makes sence.

[big mike71]

Originally Posted by handsomerob

No prob, I'll give a brief explanation although you may not care,

say with your normal size tire you are traveling @ 55mph and the engine is running @ 2700 RPM now say you stock tire was 30 inches tall thats a circumferance of 94.25 inches so for every rotation your truck moves 94.25 inches.

Well now take that same RPM and a 31 inch tall tire, that tire would move you 97.4 inches per rotation.

so your speedo says it 55mph cause its calibrated for the stock tire size, but in reality you are moving faster.

hope that makes sence.

Good explanation.

[grs]

If the city has a radar sign. Drive your truck, and see how fast it says you are going. Then see what your speedo says. When the radar sign says I am going 35 mph my speedo says I am going 30 mph. They move these radar signs around town, so look for one, or call the police dept. and ask where one is.

[Quackaddict]

Also another way to check is if you have a friend with a GPS everyone I have ever used will also tell you how fast you are going..........I am about to have this issue as I am going to be running either 37's or 39's when I am finished so I am gonna have to figure out how to change the speedo gear.......yeah

[D-Rat]

what size tire was standard on a '70 2wd CST, and what is the metric equivelent, height wise? I know the tires I have a shorter than stock and I'm not going as fast as the truck says, also meaning, the odometer is rolling faster than it should.