The tach may have been reading 6500, he may have really been turning 5700.
Well, the other night I was driving to Wal Mart and I noticed my tach was reading 1500rpms in 4th gear at 35mph. I knew that had to be wacked.
So I did this.
1500 rpms would equal 1500 driveshaft revolutions in 4th gear because 4th gear is 1:1. Since I have 2.73:1 rear end gears, that would be 1500/2.73. Now that was a little hard to do in my head while I was driving down the road. So I multiplied by 4. 6000/11. That equaled about 540. So the wheels were rotating 540 times per minute. How far is that? Well, I have stock sized rubber, so 25.6" in diameter ((225x.55)(2)/25.4)+16. The circumference is therefore (2pi)(d/2). So that's like 3.14x2x12.8. I decided to round this to 6x13, or 78". That's 6.5ft. 6.5x540 = real close to 3500. 1 mile = almost 5300ft. So I was covering ground at 3500/5300 x 60mph. Now 3500/5300 annoyed me to reduce, so I chose 3400/5200, or 2/3. 2/3 of 60mph = 40mph. That means my speedo was reading 5mph fast compared to the tach. Now there was a lot of rounding involved, so when I got home, I checked my quick math. I was off by a whole 1.8mph (I was low) which equals 4.5% I know... I could have done better. I was just preoccupied.
