MGB & GT Forum

Believe me, I've read a ton and the problem is just weird or I'm a moron.

I used a speed app on my 'phone' to discover that my Nigel with 195/65R14 tires shows just under 2 mph slower on the speedometer than actual at 50mph.

If the original tires (I have a 165SR14 in the trunk) are really 165/82R14 (Which I've read here and elsewhere.), shouldn't my speedometer show more than actual due to the 195/65R14 being a shorter tire?

Am I reversed or is the app, er, crap?

You're lucky the speedo is even that close since these units are 45-55 years old. Plus those apps aren't 100% anyway.

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jb

Too soon we get old, too late we get smart!

At the same rpm the smaller tire covers less distance.

Ken

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MG: Transforming gasoline into Fun!!!

Justin,

I believe you are thinking the right way around. I did the calc manually and came up with 24.65 in. dia. for the 165 and 23.98 in. dia. for the 195. So, I got about the same as your calculator. Maybe your speedometer or phone app is just off.

Maybe ask the guy standing next to you for an opinion.

Charley

Think about it. The car doesn't know how big the wheels are. When the engine is turning at 2800 rpm the speedo will read 50 mph. If your wheels are smaller than standard the car will be covering less ground consequently the actual mph will be less.

The app is correct.

Ken

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MG: Transforming gasoline into Fun!!!

Trust GPS. Your speedo is accurate to 5% so the 2% you are seeing is within spec.

Also the 65 aspect tire is smaller than the 82 aspect tire regardless of the 165 vs 195.
If you went to the 185/70 I bet those 2% disappear.

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Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.

Tread thickness, inflation, and expansion due to high speed rotation will vary a little, and if you look at tire rack specs, you will see for the same size tire across brands some variance in revolutions per mile.

2%, not bad. My Mini Cooper ( BMW) read about 61 when going 50. BMW said " so what, as long as it does not read low it is legal" Now my GTI reads 78 when I was going 81, which a Va. county trooper informed me of. VW also refused to take action. ( see below, I thought I was actually going 77) I can't find the letter from BMW stating the federal law so I did not push it. Both on OEM tires at OEM inflation. My Sonoma is almost dead on, but I have one size small tire. BMW, VW, GM can't get it right, yet my Honda's and Sabbs were all within a mile.

On the GPS. Strange. My first one was dead accurate. Then it did a software update and read 1 MPH slow at all speeds. I assumed this was to be legal as a car speedo and never read low. But my newer ones, again read right on. No idea what phones do. I had a GPS HUD my GT and it matched my Garmins exactly, just was slower to respond.

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Cogito ergo sum periculoso

I have a 77 GT with 175 wide tires on 5.5 inch wide alloy wheels. This seems to work well. Wider tires really need wider wheels to work well. So be careful when choosing. If you want to fit wider tires that's up to you but don't be surprised if the combination isn't what you hoped for. It is worth remembering that the MGB in its original form probably arrived on 4 inch wide steel wheels fitted with 155 section crossply tires. So the clearances are still largely based on that size combination.

As far as the speedometer reading is concerned my one read 60 mph when the car is doing about 65. It is easy to get caught because of this. On some cars you can replace the driven cog which spins the speedo cable but you need to also replace the housing in order to get the correct clearance or the cog will either jam or do nothing.

As a suggestion, I found an app which gives an accurate speedometer reading so I use that as the 'working' speedometer. It is large and clear as well as being accurate. Of course it depends on the GPS signal so there can be problems if the sky is covered by tall buildings etc.

I had 195/65/14 on my 67 GT when I first got it, they work fine and looked good. They were on the car when I got it with the chrome spokes, than I converted the car to bolt on wheel, and used the same size, I was never fond of the black wheels on this car. Later I changed to VTO Lemans in the 15x6 with a 195/60/15 tire, that was my favorite wheel, and tire package on that car.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Changed mine from 175/70/14 to 185/70/14. I never paid much attention to the speed since I have GPS. Speedo reads the correct speed now, but jumps around a bit.

Not so in my BMW Mini. Might as well not have a speedo, it's that far off. GPS in that one too.
Seems like they could have gotten that right since that car as a speed sensor on every wheel for the traction control.
Speedo does not even work in my 67 Mini. I use a smartphone speedo app on my Android phone. Works great.

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https://73-mgb.blogspot.com
https://71mgbgt.blogspot.com

IMHO, YMMV, and any other applicable acronyms apply...

Precisely the reason I will always drive 5 to 8 MPH over the limit.... And never get a ticket!

Most modern cars register a few miles higher than your actual speed. I know my Accord does. 35 is 32. 80 is about 74. The spread gets higher with higher speeds. I am pretty sure this is done purposely. Which makes complete sense.

"Your speedo is accurate to 5% so the 2% you are seeing is within spec."

Also the 65 aspect tire is smaller than the 82 aspect tire regardless of the 165 vs 195.
If you went to the 185/70 I bet those 2% disappear.
[/quote]

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Ha! Yeah, that guy...he just bounces around smiling, waving, and giving a thumbs up. I think he's an extra from an '80s movie about ghosts.

Check with the manufacturer for the actual diameter specs of the tire. What is imprinted on the side wall is the same across the board while the actual diameter can vary from make to make. Not by a huge amount but, you might be surprised.

I believe the US Federal spec is +/- 10%, +/- 2 MPH but must never read lower than actual speed. At least this was the letter BMW sent when they told me to basically get lost on the MINI speedometer.

Well, it was more accurate than the fuel gauge. 5 carrots and you were out of gas! ( half tank). 2 pages of defects. They fixed half. I will NEVER buy another BMW product. Not only due to the defects, but do to BMW's customer attitude. It is not we had a lowly Mini, they treat their 7 series owners the same way. I voted with my money and bought the GTI.

Paul: There are many threads on tire clearance. It seems 195-65-14 works on some cars and not on others. (fine on my 74 GT) It may depend on rear axle centering as well as the two different Rostyle offsets. ( or aftermarket wheel) I have seen cars with 205's but do not know what else was changed or if they rub.

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Cogito ergo sum periculoso

Thanks for all the replies everyone. I drove my Buick to work today and tested the app against its speedometer and both matched. I know the Buick has the stock size tires at proper inflation, so that makes a good baseline.

Also, can't really argue with math if the original tire on the 1977 MGB was the equivalent of 165/82R14, then 195/70R14 is a near perfect match.

My preference is to fill the wheel well a little more anyway. (See https://www.mgexp.com/phorum/read.php?1,3811755)

Jerard Basmagy's point about this speedometer being 40+ years old is probably the safest bet, so I'll focus my OCD there.

Thanks again!