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Cruising speed?

Posted by Parky

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Cruising speed? #1
Parky Richard Park bolton-by-bowland, United Kingdom	Posted: April 03, 2012 05:35AM Registered: 4 years ago Posts: 82

I've been driving my 1977 1500 now since about january and absolutely love it. However! i find i'm only really comfortable cruising at sort of 50mph? and even then its at about 4000 rpm? i'm running 13" alleycats with 155 by 50 I think. Is something wrong?

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Re: Cruising speed? #2
btinsley Bishop Tinsley Jr GA, USA 1976 MG Midget 1500 "Dino"	Posted: April 03, 2012 05:59AM Registered: 2 years ago Posts: 293

Sounds like something is amiss...at 4000 rpms, I'm doing 80mph. At 3500, I'm dead on 70mph. Standard 145/80/13.

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Re: Cruising speed? #3
AmishIndy Seth Jones Wheaton, IL, USA 1971 MG Midget MkIII "Guenevire" 2007 Mazda 3 "Porco Rosso"	Posted: April 03, 2012 06:22AM Registered: 2 years ago Posts: 3,069

I'm only comfortable cruising at 55 in my '71 but its for other reasons (like noise).

Seth Jones

1971 MG Midget

www.SpridgetGuru.com

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Re: Cruising speed? #4
Parky Richard Park bolton-by-bowland, United Kingdom	Posted: April 03, 2012 07:09AM Registered: 4 years ago Posts: 82

Could it be due to my rolling radius? or is it more likely what diff i have? can't imagine the gearbox ratio's differed did they?

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Re: Cruising speed? #5
AmishIndy Seth Jones Wheaton, IL, USA 1971 MG Midget MkIII "Guenevire" 2007 Mazda 3 "Porco Rosso"	Posted: April 03, 2012 08:00AM Registered: 2 years ago Posts: 3,069

Did you say you're running 155/50R13s? If so yeah that might be it. I run 145/80/13 tyres on mine. Course I also have a 5 speed that lets me cruise at 2500 at 55.

Your rear axle ratio was supposed to be 3.9:1 and yor top gear is a 1:1 ratio so overall thats 3.9:1 in 4th. That means that at 4000 rpm your rear wheels are spinning at 1025.64 rpm. If you know the diameter of your wheels then you can figure out whats going on. I've come up with the following equation (simplified for ease of use I can explain if need be). Where D is diameter in inches and R is engine RPM in top gear

Speed=(D x R x 4.833219)/6336

That should give you tools you need to figure this out. I found a neat tire data calculator here. not sure how accurate their diameters are but lets give it a go.

http://www.1010tires.com

Stock size was 145R13 which had an overall diameter of 22.13 inches giving 67 mph at 4000 rpm

155/50/13 tires have an overall diameter of 19.1 inches giving 58 mph at 4000 rpm

Something else happens though when you change the diameter of the tires. It screws up the speedometer. Even though you're now only going 58, your mechanical speedo should still be reading 67. Our speedometers were always a bit optomistic and smiths electric tachometers arent terribly accurate so that may also play a factor here. You may need to recalibrate your tachometer. On mine at least there is an adjustment screw on the back. A good way to check is to try and get a hold of a timing light with a tachometer built in (amazon has one for about 64 dollars american). Then use a GPS to measure speed.

Seth Jones

1971 MG Midget

www.SpridgetGuru.com

Edited 1 time(s). Last edit at 04/03/2012 08:14AM by AmishIndy.

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Re: Cruising speed? #6
Dell WA Delbert Golightly Spanaway Wash, USA	Posted: April 03, 2012 08:18AM Registered: 1 year ago Posts: 272

Parky........are you keeping up with traffic? Then no matter what yer speedo or tacho sez, yer ok. BTW.....NEVER....lead the "pack" you'll gitt nailed everytime. (frown) My 1976 MGB speedo is TU (tango uniform...or...toes up as in DEAD) I have a windshield reflected GPS speedo sold for motorcycles that I lay atop the dash and works well. (switchable between MPH and KPH) Since my MGB is painted "arrest-me-RED) I haffa travel incognito. ........leadfoot Dell

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Re: Cruising speed? #7
76mgdriver Gone Drivin' Beautiful downtown Wanatah, IN, USA 1976 MG Midget 1999 Chevrolet Camaro 2001 Ford Mustang Bullitt	Posted: April 03, 2012 11:02AM Registered: 4 years ago Posts: 409

I noticed the same thing when I first got my Midget. I drove behind my son on the highway and told him to do exactly 60 mph--my speedo showed 67 mph. Try it and see what your speedo shows.

I don't have stock tires--it makes a diference.

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Re: Cruising speed? #8
AmishIndy Seth Jones Wheaton, IL, USA 1971 MG Midget MkIII "Guenevire" 2007 Mazda 3 "Porco Rosso"	Posted: April 03, 2012 11:17AM Registered: 2 years ago Posts: 3,069

In reply to # 2030242 by 76mgdriver I noticed the same thing when I first got my Midget. I drove behind my son on the highway and told him to do exactly 60 mph--my speedo showed 67 mph. Try it and see what your speedo shows.

I don't have stock tires--it makes a diference.

Yeah and conteporary road tests showed they could be off almost that much even WITh the stock tires.

Seth Jones

1971 MG Midget

www.SpridgetGuru.com

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Re: Cruising speed? #9
TheFlyingMG Johnny Quest Skyloft, Mexico 1974 MG Midget	Posted: April 03, 2012 07:15PM Registered: 1 year ago Posts: 85

So my personal land speed record of 80mph on mine may really be 70?
Hahahaha that was a scary, noisy and bumpy ride..
60 is my comfort top zone but 40-50 is just heaven...

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Re: Cruising speed? #10
rrllff Richard Fisher Mountain view, CA, USA	Posted: April 03, 2012 07:32PM Registered: 2 years ago Posts: 103

I owned a 1500cc midget back in the 1970s. I remember driving from Edinburgh to London, close to red line all the way. This put me at about 90mph. The car was fairly solid on the road compared with other cars of the same vintage. However, I do remember once going over an unmarked bump in the road. The car got very light and settled down at a slightly different heading on the road. Scared me a bit but it immediately went back to tracking true.

70mph should feel steady in a good car, even if very noisy.

Richard

Edited 1 time(s). Last edit at 04/03/2012 07:33PM by rrllff.

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Re: Cruising speed? #11
Tveditr Eric EN Indianapolis, USA 1971 MG Midget	Posted: April 03, 2012 07:35PM Registered: 1 year ago Posts: 371

Is it an original tranny or has someone swapped it for a dataun or something else?

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Re: Cruising speed? #12
metalhead Andrew F NSW, Australia 1970 MG Midget MkIII 1977 Mazda 3 1979 Ford F-250	Posted: April 04, 2012 09:49PM Registered: 2 years ago Posts: 414

4th is 1:1. If you actually have 155/50R13 tyres as you say (these are much smaller than stock) then at 4000rpm with a 3.9 rear end you will only be doing 58mph. If you have the earlier 4.2 rear end you will be doing just 54mph. In short, yes your tyres do make a difference. If you were running 175/70R13 (a common choice, slightly larger than stock) then your speeds would be 69mph (3.9 ratio) and 64mph (4.2 ratio) respectively.

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Re: Cruising speed? #13
Acropilot Ty Ty Frisby Newport Coast, CA, USA 1970 MG Midget "Ty's MG"	Posted: April 04, 2012 10:36PM Registered: 1 year ago Posts: 221

This helps

http://www.f-body.org/gears/

TyFrisby.com

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Re: Cruising speed? #14
7mg2 Andrew Hardie Calgary, Alberta, Canada 1969 MG MGC GT "Mr "C"" 1972 MG Midget	Posted: April 05, 2012 10:50AM Registered: 4 years ago Posts: 2,097

Parky'
Unlikely those tires are 155/50's.More like 155/60,155/70 (a good option to the original 145SR 13's the car came on) or even 155/80
You won't be with traffic at 4000rpm, but at 4500 you certainly should be, given that the original 3.9:1 or 3.7:1 is still in the rear axle. Swapping diff's in these cars is dead easy, so verifying the ratio might be useful. The numbers are stamped on the outside of the right front of the diff, on one of the webs. The numbers represent the number of teeth on the pinion and crown wheel. Some simple math will give you the ratio. (eg: 10/39 for 3.9) BTW, the 3.7 was only in a very small number of the last 1500 Midget's, and are like rocking horse poop these days, so if you have one, hang on to it.
If you find that a 4.22 or 4.55 from an earlier car has been swapped in, simply going on the hunt for the correct diff and swapping that back in will improve things for you. If the correct diff is in place, then I'd suggest either your rev counter or speedo are out quite badly, and you already have some suggestions for rectifying those problems.

Andy

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