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Posted by: jim_hoyland Apr 26 2014, 05:59 PM

What's the speed of a typical 914 in 5th gear at 3,000 rpm ? My speedo have been under reporting for a while.

Posted by: 914itis Apr 26 2014, 06:08 PM

I would say 60-65 mph
My speedo is also 10 mph advanced

Posted by: Tom Apr 26 2014, 06:15 PM

Jim,
That would depend on your wheel/ tire combination. I run 195/55/15's on mine and I am doing more engine RPM's at any given speed that if I were running 195/75/15's.
I believe there is a tire size/MPH calculator on this site somewhere.
How did the mirror joystick work out?
Tom

Posted by: jim_hoyland Apr 26 2014, 06:34 PM

I'm running 16" wheels. My iPad speedo app say 3,000 rpm in 5th is 65 mph. Just wanted to see if that's common...

Posted by: struckn Apr 26 2014, 07:01 PM

If you have, or can barrow a GPS, you can read the MPH at 3000 in fifth.

Posted by: jim_hoyland Apr 26 2014, 07:06 PM

I did. My question is whether 65 mph in 5th @ 3,000 rpm is normal

Posted by: mepstein Apr 26 2014, 07:27 PM

Mine was around 75-80 (GPS) on my way to Hershey at 3K but my spedo was reporting about 4 under. The tires are skinny but tall.

Posted by: Hine62 Apr 27 2014, 05:53 AM

My speedometer is also off because of tire size. I used gps on mine and found that the speed advance changes as the car goes faster...

Posted by: toadman Apr 27 2014, 08:11 AM

If the tires and transmission on your car are completely stock, and you have the owners manual, you can look in the manual for this information. I have a 1976 car and the owners manual, too. In my manual there is a transmission diagram that shows engine rpms and ground speed information for each of the five forward gears. The diagram specifies a pinion to ring ratio of 7:31 and a tire size of 165R 15. The ratios for each gear are also listed. According to this diagram, for my car, 3000 rpm in fifth gear equals a ground speed of 70 mph.

Changes to tire size or transmission modifications affect the accuracy of the diagram. If this is the case for you, I think using a GPS to measure ground speed is a good method.

Posted by: jim_hoyland Apr 27 2014, 11:11 AM

Just checked my manual. Says 3,000 rpm in 5th = 70 mph, for 165 R 15 tires. If I have 205 R 16 tires would 65 sound right ? The tire calculator wasn't working. In any case, the GPS on my iPad reads 65.

Posted by: r_towle Apr 27 2014, 11:47 AM

165r15 are 25.4" in, diameter
205 60 r 16 are 25.685" in diameter.

It matters exactly what tire you are running.
205 50/16 are shorter and have a smaller diameter than 205/60

Once you know the diameter, use a calculator to determine how far off from stock your speedo is.

So, 25.4/25.685= 0.988
About 0.9% off.

If you have found a tire you like, north Hollywood speedo, or Palo Alto speedo can redo your speedometer to be correct based upon your tire diameter.

165r15 rotates 207.8 times per miles
205 60 16 rotates 205.5 times per mile

You are very close to correct, so if your speedo is reading wrong, it's just wrong and may need to be repaired.
I bet they were all off....

Posted by: jim_hoyland Apr 27 2014, 04:01 PM

Does the car go faster with smaller tires, given the same rpm ?

Posted by: Big Len Apr 27 2014, 04:07 PM

No, you would be going slower.

Try this - http://www.tacomaworld.com/forum/tirecalc.php

Posted by: r_towle Apr 27 2014, 04:11 PM

Think that one through based upon how many times a larger diameter tire goes around over a mile versus how many times a smaller tire goes around.

So with larger diameter tires, you go faster at the same rpm.
You are effectively changing the final drive ratio by changing the tire diameters.

This is why we put 205/50/15 tires on autos cars.
Lower gear ration, quicker acceleration.
Down side is 4500 rpms on the highways....

Posted by: pvollma Apr 28 2014, 07:34 AM

I've attached the chart I created for my 914 with 195/65-15 tires.

 914PerformanceChart.txt ( 1.88k ) Number of downloads: 84


Let me know the complete size of your tires (width/section-diameter) and I'll run one for you.

Posted by: Elliot Cannon Apr 28 2014, 10:56 AM

It doesn't matter what size tire you use or what size wheel you have them mounted on. When it comes to speedo accuracy, it is based on the outer diameter of the tire mounted on the wheel and inflated. I have always had 17" wheels on both my 914's but I chose a tire that gave an overall diameter of just over 25 inches. I found a chart on the Tire Rack web site that gives combinations of wheels and tires and what their overall diameter is. It helped me a lot when deciding on what tires to order when I went to a five lug on my car.
Cheers, Elliot

Posted by: jim_hoyland Apr 28 2014, 06:19 PM

Fantastic ! I began the same today using my iPad. Our figures for 5th gear above and below 3,000 rpm are very close. Mine would round up to the next whole number.

At speeds below 45, my speedo and iPad app were in agreement . My speedo reads 5-10 mph after 65 mph; can a speedo be re calibrated when the accuracy is mostly in the higher speeds ?

Posted by: ndfrigi Apr 28 2014, 11:05 PM

Drove my 75 1.8 F.I. last weekend with 185x65x15 tire size and it shows 70 mph at 3,000 rpm. But my phone apps speed show 66 mph. I believe the difference of 4 mph is from the current tire size which is not the factory size. Well hoping when I got my new tire 195x60x15 or 205x55x15 will have more accurate speed.

Posted by: dlkawashima Apr 28 2014, 11:53 PM

You really need to use the tire size calculator because you're setting yourself up for disappointment.

Use 165/80-15 (or 165/82-15) as the original size tire and input the three tire sizes above to see the differences.

http://www.tacomaworld.com/forum/tirecalc.php

Posted by: ndfrigi Apr 28 2014, 11:56 PM

Yes sir I already did use that tire size calculator and thanks again!