The Ford Barn

Henry Hopper · 11-05-2017, 03:13 PM

I'm contemplating fitting 7" x16" tyres to the rear of a 36 Ford....I would really like to maintain an accurate speedometer due to today's speed cameras all over the place.

Is there a chart to show which gear drive wheels required to fit different application? I only want to do it once and not be "trial and erroring" with different gears.I can't be the only one this swap so I'm guessing the info is out there somewhere, or is it not possible with the gears available?

The car is totally factory stock.

TIA.

drolston · 11-05-2017, 03:37 PM

What is the current rear tire diameter and what will be the new tire diameter?

A 1" increase in tire diameter would cause the speedometer to read about 4% low; like indicating 48 when you are driving 50. You probably have a 19 tooth gear in the speedometer turtle. If you changed to an 18 tooth gear that would bring the indicated speed up to 51.

It is an easy enough change, but probably not worth the trouble.

JSeery · 11-05-2017, 03:42 PM

Gps?

TagMan · 11-05-2017, 03:54 PM

If you have GPS or a "smart phone" with a GPS app, it will show your exact speed and you can compare it to your speedometer reading.

Henry Hopper · 11-05-2017, 04:45 PM

I've thought of using a gps to see what the speedo reads but I would like if possible to dial it in using mechanical parts.

The tyre size chart shows a 3" increase in diameter when fitting 7" tyres over the 6""versions.....any brainiacs able to crunch the numbers with this info? 30.94" and 27.91" respectively.

drolston · 11-05-2017, 05:11 PM

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That is a very large increase in tire diameter. Do you know what speedometer gear is in your car. It is cast into the top of the speedometer gear housing (turtle). It is probably 19 but could be 18, depending on what rear end gear ratio the car was delivered with.

The tire diameter change you are making would lower the indicated speed by 9%, which is substantial. If you have a 19 tooth speedometer gear and change to the smallest available (17 tooth), that would bring the indication back up by 11.8%. So instead of reading way low, the speedometer would indicate slightly fast.

Changing to a 17 tooth Model A gear would require an 18 tooth turtle, which Mac's sells.

jake197000 · 11-05-2017, 06:42 PM

you could get a speedo gear box made up to go between cable and turtle if you had to.

34fordy · 11-05-2017, 07:43 PM

There are numerous internet sites that calculate speeds for tire changes. Most have a small percentage increase in tire size before serious effects on the stopping power of the brakes. I would think mechanical brakes would be affected more than newer hydraulic shoe or disc brakes.

Talkwrench · 11-05-2017, 08:32 PM

Without going to too much trouble, get a GPS and figure three speeds . I do 60 - 80 -100 kph [for Aust] and then I stick little arrows on the speedo. This should get you close enough at the speeds you need.
You guys have speed cameras??!! I though we were the only place that ripped people off!

Dave Castle · 11-05-2017, 09:32 PM

I just use my wife's smart phone, downloaded a speedo ap 4 free.

Henry Hopper · 11-06-2017, 04:18 AM

Yes it seems a big jump in diameter for an extra inch in width.....I can't check anything as yet as the car is in transit to me.I intend changing the rear tyres purely for asthetic reasons..I've had several early V8 Fords and the skinny tyre in the rear arches always bugs me.If I need to buy turtles / gears it means sending to the U.S. for them so I want to get it right from the off, otherwise it entails mailing parts and tha can get expensive real quick.

Torque wrench, we have the stationery GATSO cameras that you can slow down to pass and have the sneaky camera vans that park way up the road to zap you but the latest is average speed cameras...they measure your average speed between two cameras set quite a distance apart..they make hundreds of thousands of £s on the motorways where they place them through roadworks and we have just had some fitted out on the back roads where the Sunday motorcycles ride.....all done in the name of safety of course lol.

weemark · 11-06-2017, 05:59 AM

I have 6.50s on my car and even though i've got a 18tooth gear in the speedo its out by about 5mph. I have a 17 tooth gear sitting on my desk I need to fit to but I think the reading is too far out to be corrected by installing the 17 tooth gear. I tend to use my phone for an accurate speed when I can be bothered.

I got my gears from miles @ jackhammer, I didn't want to pay postage for a single item to the UK so got the gears via miles.

papanick · 11-06-2017, 08:18 AM

There is one way to set your speed in side the speedo. You have to brake the speedo down and there is a little slider that put more spring on the shaft. The more the spring the slower the reading the lest the spring the higher the reading. If you mach you speedo to a gps and see how far your speed is off then mack your speedo to that reading. What i do is put the speedo on my tester and if the speed was off 12 mile to slow i would set the speedo to run 12 miles faster.

11-05-2017, 03:13 PM	#1
Henry Hopper Senior Member Join Date: May 2010 Location: N.W.England Posts: 439	Speedo accuracy.. I'm contemplating fitting 7" x16" tyres to the rear of a 36 Ford....I would really like to maintain an accurate speedometer due to today's speed cameras all over the place. Is there a chart to show which gear drive wheels required to fit different application? I only want to do it once and not be "trial and erroring" with different gears.I can't be the only one this swap so I'm guessing the info is out there somewhere, or is it not possible with the gears available? The car is totally factory stock. TIA.

11-05-2017, 03:37 PM	#2
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Re: Speedo accuracy.. What is the current rear tire diameter and what will be the new tire diameter? A 1" increase in tire diameter would cause the speedometer to read about 4% low; like indicating 48 when you are driving 50. You probably have a 19 tooth gear in the speedometer turtle. If you changed to an 18 tooth gear that would bring the indicated speed up to 51. It is an easy enough change, but probably not worth the trouble.

11-05-2017, 03:42 PM	#3
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	Re: Speedo accuracy.. Gps?

11-05-2017, 03:54 PM	#4
TagMan Senior Member Join Date: Jul 2011 Location: Alvaton, Kentucky, USA Posts: 952	Re: Speedo accuracy.. If you have GPS or a "smart phone" with a GPS app, it will show your exact speed and you can compare it to your speedometer reading.

11-05-2017, 04:45 PM	#5
Henry Hopper Senior Member Join Date: May 2010 Location: N.W.England Posts: 439	Re: Speedo accuracy.. I've thought of using a gps to see what the speedo reads but I would like if possible to dial it in using mechanical parts. The tyre size chart shows a 3" increase in diameter when fitting 7" tyres over the 6""versions.....any brainiacs able to crunch the numbers with this info? 30.94" and 27.91" respectively.

11-05-2017, 05:11 PM	#6
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Re: Speedo accuracy.. Sponsored Links (Register now to hide all advertisements) That is a very large increase in tire diameter. Do you know what speedometer gear is in your car. It is cast into the top of the speedometer gear housing (turtle). It is probably 19 but could be 18, depending on what rear end gear ratio the car was delivered with. The tire diameter change you are making would lower the indicated speed by 9%, which is substantial. If you have a 19 tooth speedometer gear and change to the smallest available (17 tooth), that would bring the indication back up by 11.8%. So instead of reading way low, the speedometer would indicate slightly fast. Changing to a 17 tooth Model A gear would require an 18 tooth turtle, which Mac's sells.

11-05-2017, 06:42 PM	#7
jake197000 Senior Member Join Date: Sep 2010 Posts: 349	Re: Speedo accuracy.. you could get a speedo gear box made up to go between cable and turtle if you had to.

11-05-2017, 07:43 PM	#8
34fordy Senior Member Join Date: Dec 2014 Location: Fort dodge, Iowa Posts: 821	Re: Speedo accuracy.. There are numerous internet sites that calculate speeds for tire changes. Most have a small percentage increase in tire size before serious effects on the stopping power of the brakes. I would think mechanical brakes would be affected more than newer hydraulic shoe or disc brakes.

11-05-2017, 08:32 PM	#9
Talkwrench Senior Member Join Date: Aug 2012 Location: Perth, Western Australia Posts: 2,687	Re: Speedo accuracy.. Without going to too much trouble, get a GPS and figure three speeds . I do 60 - 80 -100 kph [for Aust] and then I stick little arrows on the speedo. This should get you close enough at the speeds you need. You guys have speed cameras??!! I though we were the only place that ripped people off! __________________ *"Came too close to dying to stop living now!"*

11-05-2017, 09:32 PM	#10
Dave Castle Senior Member Join Date: Jun 2017 Location: Carmel, Ca Posts: 269	Re: Speedo accuracy.. I just use my wife's smart phone, downloaded a speedo ap 4 free.

11-06-2017, 04:18 AM	#11
Henry Hopper Senior Member Join Date: May 2010 Location: N.W.England Posts: 439	Re: Speedo accuracy.. Yes it seems a big jump in diameter for an extra inch in width.....I can't check anything as yet as the car is in transit to me.I intend changing the rear tyres purely for asthetic reasons..I've had several early V8 Fords and the skinny tyre in the rear arches always bugs me.If I need to buy turtles / gears it means sending to the U.S. for them so I want to get it right from the off, otherwise it entails mailing parts and tha can get expensive real quick. Torque wrench, we have the stationery GATSO cameras that you can slow down to pass and have the sneaky camera vans that park way up the road to zap you but the latest is average speed cameras...they measure your average speed between two cameras set quite a distance apart..they make hundreds of thousands of £s on the motorways where they place them through roadworks and we have just had some fitted out on the back roads where the Sunday motorcycles ride.....all done in the name of safety of course lol.

11-06-2017, 05:59 AM	#12
weemark Senior Member Join Date: May 2010 Location: uk Posts: 367	Re: Speedo accuracy.. I have 6.50s on my car and even though i've got a 18tooth gear in the speedo its out by about 5mph. I have a 17 tooth gear sitting on my desk I need to fit to but I think the reading is too far out to be corrected by installing the 17 tooth gear. I tend to use my phone for an accurate speed when I can be bothered. I got my gears from miles @ jackhammer, I didn't want to pay postage for a single item to the UK so got the gears via miles.

11-06-2017, 08:18 AM	#13
papanick Senior Member Join Date: Mar 2012 Location: gouverneur n,y, Posts: 2,756	Re: Speedo accuracy.. There is one way to set your speed in side the speedo. You have to brake the speedo down and there is a little slider that put more spring on the shaft. The more the spring the slower the reading the lest the spring the higher the reading. If you mach you speedo to a gps and see how far your speed is off then mack your speedo to that reading. What i do is put the speedo on my tester and if the speed was off 12 mile to slow i would set the speedo to run 12 miles faster.